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[extract from Lamplugh Geology of the Isle of Man pp377 - 389]
East of the section shown in Fig. 91, p. 346, the slate slope is too steep for much drift to lodge ; and as the upper part of the cliff is grassy, there are few exposures in the little that does occur, which appears to consist of slaty rubble with interbedded streaks of red clay. The slipping of masses of this material down the dip-slopes of slate produces striations on the rock-face at the summit of the cliff, which resemble those due to glacial agency and have been assigned to that cause (p. 359, footnote). Although a Pre-glacial cliff no doubt existed in this locality such striations may be found on rock faces which cannot have been in existence in Glacial times. They occur here and there all along the east coast wherever the conditions are favourable for their production. That they should resemble glacial markings is to be expected, since both are alike due to the sliding of stony rubble under pressure in a definite direction over the rock-surfaces, the difference being that in the one case the slope is essential and always determines the direction of the striae, while in the other it is non-essential and the striae only accidentally coincide with it.
The presence of red boulder-clay on the foreshore outside the narrow rock-shelf between Ballure and Lewaigue has previously been mentioned (p. 347). In Port Lewaigue (Sh. 5) this rises along the cliff into a pre glacial hollow and is exposed in the disused brickyard on Tableland Point (Gob ny Rona of revised Ordnance map) in a section exhibiting up to 15 feet of red stony clay with streaks of stoneless clay and of sand and gravel.
Mr. Kendall, who figures this section (in Fig. 9 of his plate), notes the occurrence of a few shell-fragments in the lower part, and states the order of prevalence of the stones to be as follows:—" Red Sandstone, Granite (all Scottish), Silurian Grit (probably from Wigtonshire), Clay Slate, Vein Quartz, Flint." (Op. cit. p. 415.) Similar material lines the cliff-top up to Port e Myllin (Mwyllin of 6 inch map), and probably also again underlies the lower part of the foreshore there. [444]
Having once obtained entry at this low point, the red clay runs south-eastward up the depression between Maughold Head and Slieau Lewaigue (p. 347), occupying the whole breadth of the valley, but fading off into slaty drift on the western slopes, along the dotted line drawn on the map. On the eastern slope it is covered by or passes into a moundy sloping bench of gravelly drift, consisting of slate fragments with a fair sprinkling of foreign pebbles. In the valley-bottom (in which are a few old clay-pits) the present drainage, artificially assisted, goes in part north-west to Port e Myllin, and in part south-east to Port Mooar; and in two places level patches of peaty alluvial wash indicate old flood-basins. The incurving coast-line south of Maughold Head intercepts the valley again at Port Mooar ; and in the cliffs around this place and in patches on the foreshore the red stony clay, with interbedded sand and stratified stoneless clay, is here and there well exposed.
On the outer or northern side of the headland the slope is too steep to retain more than a little rearranged slaty rubble mixed with foreign stones. Glacial striae occur at two or three places on the crest of the ridge, and afford remarkable proof of the swerving of the ice-flow in rounding this corner of the Island (see List, p. 476). Numerous large erratics are scattered upon the headland and its shores, regarding which Mr. Kendall notices the remarkable abundance of slaty quartzose grit, which he identifies with the Queensberry Grit of Wigtonshire (op. cit. p. 415). These boulders have been studied and catalogued by Rev. S. N. Harrison of Ramsey, who discovered, among other rare and interesting rocks, a rounded block of Arran Pitchstone (identified by Professor Zirkel), 4 feet in diameter, on the shore at Port e Myllin, and one of gneiss at Ballure.1 A broken boulder in the east corner of a field 400 yards E.N.E. of Ballaterson East appears to be of Borrowdale lava, and Mr. Kendall records another Lake District rock, viz., Buttermere Granophyre, at Ballure.2 As usual, however, the majority of the erratics are of Galloway granite.
A narrow strip of Raised Beach lines both sides of Port Mooar ; and a broad rock-platform, slightly above high-water mark, indicates the old shore where the beach stuff is absent. As we pass southward from this inlet the solid rock rises in the cliff, and the red clayey drift becomes thinner and in places disappears. On the south side of Gob ny Garvain, where a little gravelly and red clayey drift rests im a slight hollow in the rock, the surface of the flags shows glacial striae in two directions, the older, N. 10° W.—S. 10° E., and the newer, E. 10° N.— W. 10° 8.; this is the more noteworthy as striated surfaces are of rare occurrence directly beneath the red drift. From this point to Port Cornah (Sh. 8) the drift, where present at all, is confined to mere traces of red clay mixed with local rubble and foreign pebbles, too impersistent to map. At the last-mentioned place a valley-terrace of gravelly drift, probably deposited by late-glacial floods, reaches the coast on the western side of the deep glen: and half a mile inland there are some fine sections in boulder clay in the glen (p. 450). For the next 1.5 mile southward there is no drift in the vicinity of the coast.
We then reach Bulgham Bay, where the profile of the cliff is unusual, an undercliff of slate, 30 to 50 feet high, forming the sea margin, with a great precipice rising to over 600 feet behind it, the two being separated by a gently sloping cirque-like platform which tapers out crescent-wise northward and southward. On this interspace some clayey slaty rubble containing a few blocks of Dhoon granite and a sprinkling of foreign pebbles has lodged ; this resembles drift, and if really im place and not simply material dislodged from the cliffs, it would indicate that the inner cliff is pre-glacial.
The platform is not easy of access, and is so cumbered with fallen blocks and overgrown with tall bracken in the summer that its exploration is diffi cult ; the remains of a carefully constructed ancient pathway may be traced in the upper precipice.
1 "Boulders around the Maughold Coast,' Yn Lioar Manninagh, vol. 1, pt. 8, p- 208; see also Mr. Harrison's " Report of the Geological Section," ibid., vol. i, pt. 11, p. 375; and Reports of Erratic Blocks Committee, British Assoc. Reps. for 1891, p. 297—9 ; 1892, p. 286—9 ; 1893, p. 521. 2 Op cit., p.415.
[445]
South of Bulgham, the inaccessible cliffs are composed wholly of slate, until we round Laxey Head, where the coastline is broken by the preglacial valley of the Laxey River, which is lined on both sides by banks of drift (p. 451). A triangular strip of Raised Beach is preserved at the mouth of this glen. From the abrupt cessation of the solid rocks on the foreshore, it is probable that the preglacial valley is excavated below sea level, and that its drift-filled channel underlies the beach, but no exposure was visible on the shore during my examination of the area. This matter may be of practical consequence if new harbour-works be constructed at Laxey. It is also not unlikely that drift may exist beneath the beach ex posed outside the rocky shelf at low spring tides for some distance south of Laxey, the conditions bearing much resemblance to those between Ram sey and Lewaigue.
Between Laxey and Garwick the slate cliffs frequently have a capping of rubbly drift, but being dip-slopes, rarely show sections at the top. The cuttings of the electric tram-line to Douglas, which hugs the coast, have however made good this deficiency. At Ballybeg the cutting which crosses an old burial ground with stone cists reveals 6 feet of slaty gravel-drift, becoming rather clayey in places towards the top. South of this locality the excavations show a more clayey material of variable thickness, contain ing a few blocks of Dhoon Granite.
In Garwick the drift is again deeply banked in a preglacial hollow, which has been partly re-excavated to form Glen Gawne (Sh. 11). There is a good section at the mouth of this glen in a cut-road descending the cliff on the northern side, showing 20 feet of grey till in which the majority of the stones are slate, but with a few foreigners. In the stream-bed a curious unidentified boulder of richly micaceous garnetiferous pegmatite was found by one of the cottagers at Ballig, from whom we obtained specimens.
Between Gob ny Stowell and Braggan Point, 0.33 of a mile south-east of the glen, an irregular strip of red pebbly clay 200 or 300 yards in length, con taining a few foreign pebbles, is lodged in an unusual position upon the steep slope just above the actual cliff. This has furnished the Laxey miners with material in which to stick their candles, and appears to have given rise to the name of the headland, which is not very appropriate con sidering the small extent of the patch and the bold rocky character of the whole promontory. Glacial striae. at three places on the headland in the vicinity of the cliff, along with other examples farther north and farther south, indicate a general ice-movement parallel to the coast (see List of Striae, p. 477).
On the southern side of Clay Head there is little or no drift until we reach the depression at Port Groudle (Sh. 14), where on the north side of the inlet a little platform of rubbly local drift, containing a few red sand stones and other foreign pebbles, with a depth varying from 8 to 20 feet, is exposed in the cliff section.
On Banks Howe (Sh. 14), as is so often the case throughout the Island, on the steep northern side of the hill is practically free from drift, and is scored in many places by glacial striae, sometimes in more than one direc tion (see List), while the less steep southern side possesses an irregular cap ping of till and glacial rubble, as may be seen in the cliffs to the east of Onchan Harbour. Fragments of the Groudle mica-diorite (p. 152) occur in this drift. Similar drift is exposed in the tram-line cuttings between On chan Harbour and Port Jack, and at the cliff-top west of Port Jack. All along the east coast the foreshore is sprinkled here and there with extra insular boulders ; a notable example occurs on the western side of Port Jack, where one of the blocks is a rounded mass of grey (Galloway ?) granite 6 feet in diameter. At the same place are smaller blocks of Dhoon Granite.
As we enter Douglas Bay (Sh. 13) the drifts thicken, but are for the most a part concealed by buildings. At Derby Castle a strip of Raised Beach is preserved at the foot of the slate cliff, and continues thence around the bold crescent of the Bay up to the Harbour. It is protected throughout by a gea-wall, and in the middle of the bay has a little blown sand upon it. This low terrace with its open front, backed by large houses and the steep bluffs of the old cliff behind, combine to give that imposing aspect which [446] characterizes Douglas as viewed from the sea. At Strathallan Park the top of the cliff shows a gravelly rubble of slate with a few foreign pebbles, becoming more clayey towards the surface. This is broken by Glen Crutchery, but is continued again on the south side, and near Athol House begins to show streaks of red clay. At Falcon Cliff, as the drifts thicken the presence of the Extra-insular material becomes more marked, beds of yes stratified sand, gravel and warp associated with tenacious red clay making their appearance. South of the last-mentioned place the preglacial cliff of slate recedes inland, and the glacial deposits which are banked against it occupy the whole of the high bluff south of the ravine behind Castle Mona Hotel. At the mouth of this ravine there are sand pits showing 10 to 12 feet of fine stratified sand, with bands of loam and lenticles of gravel ; and near the base, a bed of stiff red and purple clay with a few stones ; below which is clayey gravel ; the whole mass resting against, and perhaps partly slipped upon, a steep rock-slope. This section, like those in similar positions in localities previously described (Figs. 91, 92, and 93), repre sents the wedging out of the Extra-insular drift against the higher ground occupied by the slaty drift. There seems to have been a pre-glacial valley in this vicinity, perhaps a former outlet of the River Glass, into which the lower layers of the ice-sheet have entered. A gap a little over 100 yards wide in the rock-reefs at low water on the shore nearly opposite, which is indicated in the ornamentation of the ordnance map, probably represents the prolongation of this channel below sea-level. Some patches of indurated slaty gravel resting on a surface of decomposed slate were exposed in 1896 on the beach to the westward of this gap ; but recent beach-stuff hid the part of the shore which is most likely to be underlain by drift alone.
South of the section above described the slope of the old cliff becomes less steep and more encumbered with buildings, and there are no more open exposures. The character of the drifts in the upper part of the town was, however, revealed in drainage and other works carried on during our survey ; and the evidence thus obtained supplements that of the cliffs.
There is a small section in brown weathered rather gravelly boulder-clay in a cutting on the Peel Road at the south-western outskirts of the town, half a mile from the shore ; and similar material appears to occupy the surface over the whole of the rounded hill between the valley of the Glass south of Cronkbourne (p. 456), and Douglas Bay. The majority of the stones in this clay are of slate, but with a fair sprinkling of foreigners. Boulder-clay of this kind was exposed in Bucks Road and its prolongation up to Woodside Terrace, and also in Upper Derby Road, and in several places farther west. In the slight depression farther north, which perhaps marks the filled-in preglacial valley previously mentioned, the drift becomes more sandy, and on both sides of Ballaquale Road sand is the prevalent constituent and has been dug in several places. It appears to be continuous with that seen in the cliff near Castle Mona. It was well exposed in the tramway excavations and in the foundations fer the tramway engine-house. In a sand-pit opposite Ballaquale Cottage the section showed 3 to 5ft. of stony loam (perhaps partly wash), on 12 feet or more of sand with contorted warpy streaks ; gravel was said to occur below the sand. On the rising ground north of the sandy hollow the drift gradually resumes its previous stony-clay character.
In the lower part of the town bordering the harbour the Raised Beach fades off indefinitely in the alluvial river-flat. At the brewery on the north side of the harbour, 40 yards east of the Douglas Bridge road, I was informed that a pipe-well starting at about 17 feet above O.D. passed through 14 feet of made ground and loose shingle, and 26 feet of compact clay, to slaty rubble yielding fresh water of good quality. Another well, 9 yards north of the last, reached the water-bearing stratum at 30 feet, the compact clay being only 16 feet thick. Nuts are said to have come up the pipe from the bottom of the well ; but as the "compact clay" is likely to be boulder-clay and not alluvium, the evidence seems scarcely sufficient to establish the presence of vegetable remains at this depth.
[447]
At the north-west corner of this area, between Ballaugh and Kirk Michael (Sheets 4 and 7) we find the sandy drift-mounds sweeping up to the foot of the solid hills, and south of Bishopscourt mounting a little way on their slopes.The basins among these mounds bordering the massif appear , to have held temporary lakelets, of similar character to the Curragl Lake, bat of much smaller dimensions. The largest of these minor basins lies between Orrisdale and Bishopscourt ; it has been distinguished on the map by the 'Late-glacial Flood-gravel' colour and sign, but its level floor is for the most part composed of sandy or silty wash, with a little patch of newer peaty alluvium at the southern end. It probably persisted as a lake in flood-times until drained by the excavation of Glen Trunk (p. 425). A small pond still existing on the north side of the highroad half a mile N.E. of Bishopscourt appears to occupy a natural depression. South of Bishops court the railway passes through the mounds in a deep cutting, which is, however, now obscured ; it has consisted mainly if not wholly of sand and gravel containing shell-fragments. To the east of this place the slate hills have a more gradual descent than usual ; and here, east of Whitehouse, the gravel mounds, composed chiefly of slaty detritus, are found up to an elevation of 400 to 500 feet, resting in part on the solid rock and in part on clayey rubble-drift. They extend sonthward in a broken chain at about Micnagr the same level up to Glen Wyllin.
Cronk Urleigh, on the east side of this glen, is one of the most prominent of these mounds, while on the opposite side they line the crest of the valley and rise up on both margins of the dry rock-gully near Ballalheigh, as shown in the ground plan Fig. 102, p. 365. The railway intersects these mounds again near East Berks, in a cutting showing 30 to 50 feet of sand and gravel, possibly resting on slaty till almost concealed by talus. They also extend inland along the upper slopes of Glen Mooar for more than a mile, entering well within the slate hills ; as may be seen on both sides of the valley around Chester, where small pits and road cuttings reveal sand and fine slaty gravel containing many foreign pebbles (porphyry, flint, etc.), with some streaks of warp and redclay. In the lower part of Glen Mooar, just below the waterfall known as Spooyt Vane (Sh. 7), and extending thence for 250 yards on the east bank of the stream, there 1s a grand cliff of slaty till, 40 to 60 feet high, containing, like that at the mouth of the Glen (p. 428), a few foreign pebbles and shell-crumbs. Similar walls of local till are found in Glen Wyllin west of Cronk Urleigh, where the stream has cut a deep inner ravine in the glacial infilling of its old valley, the remnants of high-level fluviatile terraces here and there recording the progress of its re-excavation. Three large boulders of Galloway granite and one of coarse grit were noticed in this ravine. As previously mentioned, the 'Fullers Earth' which has been dug in this glen at a place to the southward of Erinville, marked Summer Ho. on the six-inch map (Sh. 7), forms part of this drift. In the upper part of the glen, near Cammall, the till is overlain by a gently sloping sheet of slaty gravel, which was probably formed at the same time as the great terrace of similar material at the mouth of the glen around Kirkmichael (p. 374), and dates back to the closing stages of the ice invasion. There are good exposures of drift of strictly local derivation at several places in Glion Kiark, between Sartfell and Slieau Freoaghane, up to its head at over 1,000 feet above sea-level ; and again in the north branch of the other feeder of Glen Wyllin, east of Ballalionney. The mountain ridge itself is peculiarly free from drift right up to its northern extremity in Slieau Curn ; nor, so far as could be discovered, does it possess any straited surfaces. This condition is probably due to the floods which have swept across the corner of the Island during the melting of the ice-sheet, as shown by the dry gullies on the lower slopes described in a previous chapter (p. 364, Figs 101, 102), and by similar indications which may be traced up to the very summit of the ridge. Thus, 200 yards N.of the boundary fence between Sliecan Curn and Slieau Freoaghane, a channel (briefly referred to on p. 363) has been excavated in the slate, with a floor 10 to 20 yards wide and walls 20 to 40 feet high, starting on the crest at an elevation of 1,058 feet and descending the slope for a short distance in a southwesterly direction.
[448]
Where Glen Dhoo opens out upon the gravel platform at Ballaugh it contains on both sides well-marked terraces conterminous with the platform ; and these rise with the valley from an altitude of 120 feet above O.D. at the mouth to about 200 feet at Ravensdale, a mile farther south. Above this place the valley-bottom contracts and the terraces can scarcely be separately distinguished.
The occurrence of foreign drift in this glen has already been discussed (p. 352); it consists mainly of yellow sand, with some gravel and red clay containing foreign pebbles. Found on both sides of the valley above the gravel-flat, it is best developed on the eastern side, where it extends up to the mouth of Ravensdale. Its relation to the local drift is shown in the following diagram (Fig. 106).
Fig. 106. Diagram of the Drifis on the eastern side of Glen Dhoo, north of Ravensdale. Length of section about 600 yards. 4 Late-glacial Fload-gravel. 3 Slaty rubble, with some striated fragments : = rearranged drift. 2 Yellow sand streaked with gravel with foreign pebbles, and with red clay; mixed towards the top with No. 3. 1 Slaty till. a Manx Slates.
The slaty rubble (3) of this section is a form of the local drift which is plentiful among the hills both on the gentler slopes and on level ground. it appears to represent the material left loose on the surface at the melting of the ice sheet, and has no doubt since been in part redistributed by subaerial agencies. When swept into the valleys, it probably furnished the principal portion of their terraces and deltas. It is generally, as in the present instance, sharply distinct from the till when this is present, but often merges into the stratified drifts.
Towards the head of Glen Dhoo, the drift is limited to a narrow strip entirely of local origin ; and the same is the case in its tributary, Ravensdale (Glen Shoggle of 6-inch map, Sh. 7), at the head of which there are good exposures near Nascoin. In a high-lying combe near the head of the main glen, on the northern side of Sheau Dhoo, there is a small crescentic mound, apparently of coarse local rubble, which has been shown on the map by 'boulder-clay' colour. It has somewhat the aspect of a valley moraine, but the hollow seems too small ever to have held a moving glacier. On the rock-surface below the entrance to this combe some doubtfully glacial striae were observed, the only examples found in this part of the island (see List, p. 476).
The high moorland ridge between Glen Dhoo and Sulby Glen is for the most part free from mappable drift, though loose rubble with a peaty covering lies rather thickly upon it S.W. of Killabraggah (Sh. 7). The short deep glen, tributary to the Sulby, to the west of Mount Karrin, has a thickish bank of local till in the bottom, which is well exposed on the east of the stream, opposite Earybedn (Sh. 4).
[449]
In passing up Sulby Glen from the mouth (p. 438), we find a narrow strip of local drift on the east bank, forming a definite feature, but not often seen in section. The river-alluvium is broader than usual, and probably somewhat ancient; and the stream has not reached its pre-glacial floor until we are two miles up from the entrance of the glen. Above this point there is very little drift in the valley, or in that of its tributaries, until we reach the head-waters. On the west bank opposite Druidale farmstead we have an excellent example of those effects of temporary post-glacial stream deviation described on p. 372, in the form of a deep rock-gully which gashes the valley-wall high above the present river. Other deviations are indicated 1 mile higher up the valley, where the river does not seem yet to have found its buried pre-glacial bed. In this locality high banks and cliffs of slaty till, rising in places up to 70 feet in height, form the southern side of the valley and continue up to the head-waters, at over 1,000 feet above sea level. In two places in these excellent sections I noticed dragged-out streaks of red clay and sand of Extra-insular aspect among the local till, but failed to find any foreign pebbles.
A narrow tongue of this drift sweeps across the col, at 1,100 feet, between Sartfell and Slieaw Maggle (Sh. 7), into the basin of the Neb River (p. 458), while the main mass spreads out in an irregular sheet eastward up to the slopes of Beinn-y-Phott and Snaefell ; it is well exposed in places in the upper part of Glen Crammag ; and less clearly in the glen west of Snaefell, where it runs up to 1,350 feet and crosses the col to the head waters of the Laxey River (p. 451). It is frequently covered by boggy hill-peat, which attains a depth of 6 feet or more on the northern slopes of Beinn-y-Phott, and here has been extensively dug. At the higher eleva tions the clayey till becomes more rubbly, and on the steeper slopes fades off into moundy slaty débris containing many scratched stones.
Similar material sweeps round the northern shoulder of Snaefell, and again crosses into the Laxey basin at an elevation of 1,380 feet; while northward and north-eastward it extends to the head of Glen Auldyn and of the eastern tributaries of the Sulby, where however it is thinner, less continuous and without conspicuous sections. The northern side of Slieau Monagh. on which occurs the glaciated quartz-vein of Creg Bedn previously described (p. 361 and Figs. 99,100), is almost free from drift ; as is also the greater part of the high moorlands between Sulby Glen and Glen Auldyn. The northern end of the ridge, above Ballamanaugh, differs from the rest of the northern bluffs in being apparently swarthed in drift to more than half its height, perhaps due to its sheltered position within the mouth of Sulby Glen. In the Narradale depression the drift is thinly spread, except in one portion of the lower part of the glen, half a mile E. of Cronk Sumark (Sh. 4), where the stream appears to have broken into an old ravine filled with till.
In Glen Auldyn the phenomena of Sulby Glen are repeated, the local drift occurring as a narrow sloping platform chiefly on the eastern side of the stream, with good sections, showing 20 to 40 feet of slaty till, near the head-waters. Its smaller tributaries descend over steep slopes of bare slate, but intersect this platform in the trunk-valley, while their gathering ground on the upland is generally more or less drift-covered. On the high moorland to the eastward which culminates in the long steep ridge of North Barrule and Clagh Ouyre (p. 139), rubbly drift, based in places on slaty till, forms a fairly continuous sheet covering the smooth slopes ; below the broken craggy crest, as may be seen in the cuttings of the oe mountain-road ; but this sheet splits northward into lobes at the head of the little glens which gash the plateau above Ramsey, leaving the inter vening ground for the most part free from drift.
On the ridge itself, scattered blocks of local dyke-rocks and quartz, with occasional glaciated fragments of slate in the thin surface-rubble, would suffice to prove that ice had swept over the crest, were other evidence wanting. But still more convincing proof is afforded by the glacial striae which are found at intervals along its whole length, from Sleau Lewaigue close upon the east coast - across North Barrule and Clagh Ouyre to Snaefell ; and thence across Mullagh Ouyr, Carn Gerjoil, Slieau Ree and the more westerly
[450]
spurs, up to the central valley. As the position and character of these striae are indicated in the List (p. 476), they require no further descrip tion. In many other places where no striae could be detected, the rock surface is distinctly moutonnee.
There is a little rubbly and clayey local drift around the northern end of North Barrule near Dreem e Lhergy, and this spreads in a thin covering southward and eastward around the flanks of Slieau Lewaigue, descending 'nto the Cornah valley in the one direction, and in the other to the western side of the Port e Myllin depression. In the last-mentioned locality, as it reaches the lower part of the slope a few foreign stones make their appear ance, at about 250 feet above sea level ; this is best seen in a aeep gutter which descends the slope from a house by the roadside near Ballasaig, 600 yards S.S.E. of Lewaigue, where there is a section showing about 10 feet of slaty-rubble drift with some extra-insular pebbles ; and a 4-ft. boulder of grey Galloway granite, and another of coarse granite not unlike that of Shap, 3 feet in diameter, hein the bottom. At the foot of the slope red clay is seen, as previously described (p. 444). Similiar conditions pre vail all along the eastern flank of the massif ; but south of Laxey the inter mingling of the foreign with the local material becomes more intimate and extends farther inland.
Between the hamlet of Cornah (Sh. 8) and the coast the valley-floor is flat and fringed with gravel terraces, except at one place just above the confluence of the Rhenab tributary, where the stream seems to have missed its preglacial channel. This part of the valley is thinly lined on both sides with local drift, in which a streak of red clay was noticed about 1/4 mile from the coast. The rocky glen above Cornah is clear of drift until we approach Corrany, where there is a little river-flat in which some recent changes have been made in the water-course. West of Corrany we enter a broad glen running back into the heart of the hills, which appears to be an older feature than the lower valley. Local drift of the usual hill-type is thinly overspread on the northern side of this glen up to 1,000 feet or more above the sea-level, but on the steeper southern side is more restricted. As is generally the case in upland valleys of this character, the present stream has almost everywhere reached the rock-floor, and the drift forms grassy banks somewhat above the valley-bottom which constantly tend to slip. On the high-lying slope near Park Lewellyn (Sh. 8) there is a group of large blocks of slate and greenstone which, if in their natural position, are rather sug gestive of a small valley-moraine ; they may, however, have been artificially brought together for some purpose in prehistoric times. If morainic, their occurrence is exceptional, as indications of valley-glaciers, so common in Scotland and Wales, are conspicuously wanting in the Island.
Returning to the mouth of the Renab tributary, we find for 200 or 300 yards on the northern side of that stream a high cliff of slaty till, while the opposite side is a bare rock-slope. Although the Dhoon Granite crops out on the crest of the hill, 3/4 mile to the southward, no fragment of it was found in this drift. Toward the head of one of the branches of the stream, which descends directly from the granite near Ballig, one or two boulders of this rock were, however, noticed. Another of its branches, 150 yards south of Trinity Church, exposes slaty till 16 feet thick, resting on decom posed red and yellow slate-rubble.
This material continues in a broadish strip northward to Corrany, being proved to a depth of 12 feet in a well at Ballacorteen 400 yards E. of the church: and also extends southward, along a shallow depression, across the granite outcrop, to the head of Dhoon Glen ; leaving Barony Hill to the eastward and Slieau Ouyr and Slieau Ruy to the westward practically free from drift.
To the southward of the granite, while slate still constitutes the main ingredient of the glacial deposits, the igneous rock has contributed freely, especially to the upper_or more rabbly portion. There is no drift in the deeper part of Dhoon Glen, but a thick mass fills its higher valley and is exposed in sections 30 to 40 feet deep on both sides above the upper high road. A mining adit 70 yards long, driven northward from the valley a few yards west of this road, is said to be entirely in till. On the slopes above Rhenny the drift becomes thin and yubbly, and in this form extends [451] over the western part of The Dreem at an elevation of 900 feet, descending thence into the Laxey basin. The eastern portion of The Dreem is without mappable drift, but boulders of Dhoon Granite are plentiful there. South of The Dreem thin drift covers the upland, but fades out on the brow of the Laxey valley.
Near the mouth of Laxey Glen, on the northern slope east of Minorca (Sh. 11), there is a well-defined terrace of slaty gravel with streaks of red clay, 50 to 60 feet above the present stream; and portions of a similar terrace are traceable on the opposite side, just above the confluence of Glen Roy. These are no doubt equivalent to the 'Late-glacial Flood-gravels' of the northern glens, and have probably at one time been connected with a delta outside the valley-mouth, which has been removed by the sea. Up to its junction with Glen Agneash the river possesses a terraced alluvial flat, and has not reached its pre-glacial floor ; but above that point its alluvium contracts, and solid rock is constantly seen in the stream-bed. Irregular strips of rubbly drift, in places covering hard slaty till, line the valley on both sides, generally a little above the bottom, up to its head, filling the high cols north and south of Snaefell to a considerable depth. A large boulder of Dhoon Granite lies at the edge of the alluvium 500 yards above the Agneash confluence ; but none was seen higher up the valley, and the fragments of 'contact-rock' observed by Mr. Kendall on the flanks of Snaefell 1 are of local derivation. This absence of Dhoon boulders west of the granite outcrop shows that the south-westerly current in the ice under the lee of the Island was subordinate to the general south-south-easterly movement of the main sheet which passed over the summits.
At the mouth of Glen Agneash there is a bank of coarse flood-gravel ; and the drift has been almost denuded from the lower part of this valley, but lies deeply in places on its upper slopes, as for example in the banks of the Glen Drink feeder, when there is slaty till 20 feet or more thick. It is to the eastward of this place that the Dhoon Granite blocks begin to be numerous. The patch of drift on the high moorland south-west of Slieau Lhean is chiefly of the rubbly type. Glacial strize on this moorland and on the high ground on the southern side of Laxey Glen are recorded in the List (p. 476). 3
In Glen Roy the drift is mostly confined to the northern bank where the side-ravines frequently expose deep sections in hard slaty till ; this is 40 feet or more in thickness near the head of Glen Roy itself at 800 feet above sea-level, and in the more northerly branch rises to 1,220 feet in the pass between Carn Gerjoil and Mullagh Ouyr, and descends the opposite slope along the Creg-y-cowin feeder of the Baldwin River (Sh. 10). No boulders of Dhoon Granite were observed in Glen Roy or its tributaries, though there are some derived from the elvan-dykes associated with this granite, which cross the heads of the northern feeders of the glen (p. 367).
South of Glen Roy, the little undulating plateau west of Christ (Lonan) Church is thinly overspread with drift, which apparently contains a few foreign blocks as well as Dhoon Granite, judging from the stones collected from the fields, and from the presence of a few boulders, including one of an co !) granite 2 ft. in diameter in a field 300 yards east of Hoanes
At Poolvilla, 700 yards W.S.W. of Christ Church, at 500 feet above sea level, there is a little dry gully, excavated partly in slate, for which the present drainage can scarcely be held responsible. The narrow moorland spur running eastward from Slieau Meayl to Barroose, a mile inland from Garwick Bay, is notched across by several dry gullies of similar character to those which have been described on the north-western flank of the massif. These are slightly developed in two or three places between The Skoryn and Cronk Garroo, at 800 to 900 feet above sea-level: a better example is-seen at Cawnrhenny, east of Cronk Garroo (Sh. 11), at a little over 700 feet ; and another, smaller but in some ways more remarkable, half a mile farther east, at between 500 and 600 feet, near the farmstead of Barroose. The last-mentioned, in which the slate has been quarried
1 Yn Lioar Manninagh, vol. i, p. 401.
[452] does not cross the ridge, but runs more or less paralle] with it, and ends off abruptly on the slope. A little below its mouth there is a lenticular mound of slaty gravel which may possibly be connected with its erosion. The ridge, from its position athwart the general axis of the Island and from the presence of wide tracts of lower ground on both sides, would be likely to emerge at the margin of the waning ice-sheet in the path of streams flowing southward over the ice ; and, as already stated, it is dificult to understand how otherwise valleys of this character could have been excavated.
On the gentle southern slopes of the ridge the head-waters of the Groudle River are collected. The most westerly branch, known as the Ballacottier River, rises directly under Sheau Meayl and_ flows southward in a broad hollow thinly covered with clayey local drift. In the lower part of its course, 700 yards above its confluence with the main stream, it reveals red stony clay, unlike the grey till in its upper reaches; the included stones, however, seem to be alllocal. At Low Ballacottier (misengraved Ballacollie on the old one-inch map) at 400 ft. above O.D. there is an ill-defined patch of yellow loamy sand ; and in a little gully 100 yards farther south rather pebbly boulder clay is exposed, in which besides the usual local stones a few small foreign pebbles were noticed, including one, an inch in diameter, of the Ailsa Craig rock. At about the same elevation a mile farther west, in the drainage basin of the Baldwin River, the extra-insular material is still better represented, as will subsequently be described (p. 455). It indicates the invasion of the area by an ice-lobe laden with foreign material, which probably passed up the broad shallow depression westward of Clay Head. The electric tramway follows this depression and on the S. slope of Glen Glawne (Garwick), near Ballig, reveals slaty gravel and sand, with v sprinkling of foreign pebbles ; similar material, mixed with slaty till, is alyo exposed between Ballameanagh Beg and Baldroma Beg (Sh. 11) ; and again on the north side of Groudle Glen, where it rests on a glaciated surface of slate. The undulating ground east of the tramway is drift covered up to Ballacreggan, Ballakilley and Ballavarane; and is trenched by a broadish drainage hollow in which some alluvium has accumulated. Scattered over this track, along with numerous Dhoon Granite boulders, are many of extra-insular origin, including Galloway granite, coarse Silurian grit, porphyrite, diorite, etc. These are especially, noticeable in the fences around Baldroma Beg, where one of the blocks, 94 feet in diameter, in a fence 370 yards 8.W. of the house, resembles the altered tuff of the Lake-district Borrowdale series.
To the westward of the tram-line, north of Groudle, extra-insular ingredients are rare or absent, the ground being overspread by a sheet of hard slaty till, which is exposed in a bank 30 to 40 feet high on the north side of the stream 150 yards east of the Laxey highroad-crossing at White Bridge. North-west of the road, between Ballakilmartin and Begoade, we find in the fields and fences scattered blocks of the peculiar igneous rock to which reference has been made in another chapter (Chap. IV., p. 160) ; these increase in abundance towards Ennamona and Cawnrhenny where, as previously stated, it is probable that an outcrop of the rock may occur, thinly covered by rubbly drift. These boulders were at first thought to be of extra-insular origin.
On the southern side of Groudle Glen, a little eastward of the highroad, a shallow depression leads into an alluvial hollow which curves round at Onchan (Conchan of new Ordnance map) and drains into the sea at Port Jack (Sh. 14). The alluvium consists principally of clayey wash, but is said to contain peat with wood east of the dam at Onchan. The depres sion probably marks a Late-glacial flood-channel. 1t is bounded on both sides by thin slaty drift, through which the rock protrudes in the bank north of Onchan Church.
We will now return to the western side of the Island, and work eastward across the drainage basins which discharge into the central valley.
The rivulets which descend to the western coast at Glon Shellan, Glion Cam and Glion Broign (Sh. 6) have each their source in a little basin of slaty drift. In the second of these, the flatter part of the basin has a floor of alluvial wash thinly covered with peat. Small areas of this character are [453] common on the uplands, and have probably at one time been occupied by shallow tarns, though none now exists in this condition. The subsoil in these tracts is usually a cold wet clay full of bits of vein-quartz, the relics of the decomposition of local slaty wash ; and Manx farmers reckon such, ,, Ce land, which they recognise by the abundance of the "white stone" (quartz) as the worst in the Island (p. 569).
On my field-maps I applied the term "Colby Wash" to ground of this description, from the district in which I first encountered it. Its origin may be assigned to the washing of bare drift-slopes by heavy rains immediately after the recession of the ice-sheet, and before the growth of vegetation (p. 413). The absence of tarns among the hills at the present day is somewhat unusual for a glaciated district, but may be explained by the short course and steep fall of the streams, by which they have been enabled to break through the drift-barriers and to re-excavate their filled-in preglacial channels with rapidity.
In the sprinkling of drift-rubble which covers the slopes of slate in this neighbourhood, a few foreign pebbles may be observed, among which a fragment from Ailsa Craig was noticed at about 500 feet above sea-level east of Knocksharry. To the east of this hill there lies another upland drift basin with a flat of "Colby Wash," which drains to the valley of the Neb below Laurel Bank. Glacial striz occur at Staarvey, Lambfell Mooar, Beary Pairk and other places in the vicinity of the Neb, as recorded in the List (p. 477). The next depression eastward, through which runs the main road from St. John's to Kirkmichael, has been mentioned before in connection with peculiarities in its drainage (p. 8). _ Its northern portion, draining to Glen Mooar, is occupied by slaty till, while the water-parting between the north- and south-flowing streams lies in a strip of alluvial wash underlain by rubbly drift, like the basins above described.
The main head-waters of the Neb are in drift-filled hollows on the southern slopes of Sartfell. In the east branch or Rhenas River (Sh. 7), there are good sections showing 40 to 50 feet of slaty till capped with. slaty rubble, at about 1,000 feet above sea-level, just below the col east of Cronkdoo, as well as at several points lower down the stream. The feeder north of Cronkdoo starts high up on Sartfell in a bare coombe-like hollow with cliffs of slate, but has banks of drift a little lower down the slope. South of Little London the river runs for the most part on bare rock, bordered here and there by strips of alluvium, down to the mouth cf Glen Helen. The steep western slope of the valley is also free from drift, but on the opposite side the bank has a capping of local till and rubble which extends eastward over most of the enclosed ground and moorland up to the spurs of Colden and Lhargee Ruy, except where eroded through by the side-streams. A small quarry near the edge of the moorland, 1,000 yards E.S.E. of Ballashimmin, reveals 5 feet of till resting on a glaciated surface of slate, with striae in two directions, both somewhat unusual, viz., W. 30° N. to EH. 30° 8., and E. 20° N. to W. 20° S., the ice having evidently suffered local deviation in this confined basin. The drift, covered with 2 or 3 feet of hill-peat on the watershed, sweeps over the ridge into the basin of the Glass river between Slieaw Maggle and Colden, and again between Colden and Lhargee Ruy, at elevations of 1,200 to 1,300 feet.
Below Glen Helen the deep valley is free from drift on the western side, but has hard slaty till and rubble banked thickly against its. eastern slope. A brickyard on a rather extensive scale was started in this bank 600 yards above the bend at Ballig, but the material seems to have proved unsuitable; the section here shows 20 feet or more of bluish slaty till. To the north and south of this place the valley-spurs are capped with flood-gravel, 12 to 15 feet thick, probably the relics of a high terrace of Late-glacial age connected with the great terraces which line the central valley around the mouth of the glen (p. 410). No trace of foreign material was found im any part of the basin above the termination of the glen at Ballig: below this point the river occupies an alluvial fiat, but erodes the northern bank just before issuing into the central valley, revealing 8 feet of stratified stony rubble resting on 15 feet of slaty gravel belonging to the higher flood-terrace. In the bed of the river below this section red pebbly clay containing masses of crushed slate, foreign stones [454] and many fragmentary marine shells is revealed, while slate in place is seen a few yards farther eastward. The shelly clay is like that in the river bed near Peel (p. 457).
Leaving for the present the consideration of the central valley itselt, we pass eastward on its northern slopes into the little drainage basin of the Greeba River. This is covered with a thin but persistent sheet of slaty drift, up to the slopes of Beary Mountain on the north and Greeba Mountain on the east. The latter hill shows glacial striae a little to the east of the summit, at 1,268 feet above O.D.; and others along with the characteristic rounded surfaces, indicating movement from the west and north-west, on the top of the crags overlooking the central valley. Greeba Glen is for the most part clear of drift ; but in the bed of the stream, 300 yds. above the mill, a 3-ft. boulder of grey (Galloway ?) granite was noticed, and a little lower down another of pebbly grit which did not seem to be insular. These have no doubt been transported eastward along the central valley by the ice-lobe previously described (p. 351). The singular effect of the old delta of flood gravel at the mouth of Greeba River in diverting the present drainage has already been noted.
Bordering the central valley east of Greeba Mountain, there is again a broad drift-covered incline, out of which rise islands of bare slate in Cronk Breac and Cronk ny Mucaillyn (Sh. 10), while in several other places the sheet of slaty till and rubble is so thin that the rock peeps through. In deed in all tracts of this kind in the Island, while on first impression it may appear that the solid rocks are buried deeply, one finds on closer investigation that the smoothing down of the surface has been effected with ereat economy of material, and that only exceptionally, in the deeper hollows, have the glacial deposits accumulated thickly. Hence 'live-rock' crops out, in little patches too small to map, in all sorts of unexpected places. The drift-mantle is therefore far more ragged than appears on the published map. Sometimes, as in this tract, the positions of a few of the Louk exposures are indicated on the map by dip-arrows or other rock symbols.
On the slope north of Eyreton Castle an unusual number of large boulders of actinolitic greenstone (p. 311) have been ploughed out, and now line all the fences up to Braid ; and are equally plentiful on the opposite slope leading into the West Baldwin (Glass River) valley, around Ballalough and again near Ballaquine.
The principal known exposures of dykes of this character are near the heads of the Glass and Baldwin valleys, 4 miles to the northward (pp. 157 and 159), but there may of course be nearer outcrops concealed by drift. In any case, the blocks form a well-marked train crossing the ridge in a N.N.E. to S.S.W. direction, in agreement with the trend of the glacial striae of the neighbourhood. On the same slopes 4 mile farther east, near Nab and Ballakelly, while the greenstones diminish, boulders of pale micro granite of the Foxdale elvan-type (p. 158) become plentiful ; and though somewhat intermingled, a parallel train of the latter may be traced down to the central valley near Ballabeg and Ballahutchin. These have no doubt been derived from the dyke which is exposed at Ballachrink in the West Baldwin valley, though not necessarily from this particular part of its cutcrop. Still farther east the boulders are principally of quartz-veined orit, like that seen in place north of Mount Rule and on the moorlands east of the Baldwin River. The drift with which all these boulders are associated is hard slaty till ; and this material lines the northern side of the valley down to Union Mills.
in the lower part of the valley drained by Awin Darragh (Sh. 10) which joins the River Glass at Baldwin, we find an unusual quantity of local till, scarcely any rock being exposed below Cregwine. Above this place there is only a narrow strip of drift in the glen until we reach its head at Braid ny Scarrag, at about 1,100 feet above O.D., where it is again banked deeply and sweeps over into the Neb basin as already described. Similar conditions occur at the head of the Injebreck River on the northern side of Colden Mountain, but this stream flows over bare rock for some distance above its junction with the Glass. [455]
The upward margin of the drift around the head of the main branch of the Glass, between Injebreck Hill and Beinn-y-Phott, is ill-defined, fading off on the higher ground, as usual, into a sprinkling of slaty rubble containing a few sciatched stones. In the valley there is much slaty . till; above Injebreck the stream has cut through this to its old floor, but below that place the glacial material occupies parts of the valley bottom, and is exposed in high banks on the eastern side, both on the main stream and in transverse gullies. One of the. best of these sections is the river-cliff 300 yards S. of Ulican (Sh. 10), which exhibits from 40 to 50 feet of till interbedded with rough slaty gravel. This drift extends westward up to the foot of the Awhallan crags, on which there are excellent glacial strize (see List, p. 477); and eastward, though not quite continuously, across the dividing ridge into the East Baldwin depression. Below Injebreck traces of fluviatile gravel-terraces are visible here and there in the bottom of the valley, the higher of which probably correspond to the more conspicuous platform below the confluence of the ee River which attracted the attention of Macculloch 1 and Cumming 2 (p 405)
The conditions in the East Baldwin valley resemble in all respects those of West Baldwin, except that the drift is not so thick in the bottom of the glen ; wherefore the stream runs for longer on a rocky floor, and its alluvial flats and terraces are confined to lower reaches than in the sister-valley.
Its eastern feeders, the Creg-y-cowwn fiver and a nameless stream 700 yards farther south, rise in basins deeply filled with glacial material, and afford interesting illustrations of the post-glacial erosion of these deposits. On the moorland-ridge of Slieau Ree to the east of the valley, glacial striae were observed in several places (see List, p. 477). The eastern slopes of this ridge are thinly overspread with drift, apparently all of local origin, In which small sections are revealed in places in the gullies that occur at the head of a little stream (Sulby River) tributary to the Glass.
The rising ground east of this stream is also more or less drift-covered ; and on the southern part of this tract, between Abbeylands and Glenville, at 300 feet above sea-level, a section occurs which deserves especial notice, since it shows an admixture of extra-insular material with the local detritus. This is revealed in a shallow clay-pit 500 yards west of Glenville, from which material was dug for use in the construction of the Douglas water works reservoir on the Groudle River ; the section visible in 1896 showed 2 feet of slaty gravelly clay of 'Colby wash' character (p. 453), resting on 6 feet or more of dull red clay with stones of which the majority and the larger were local, but with a fair sprinkling of foreigners.
Among the local boulders was one of greywacke, 6 feet in diameter ; the extra-insular stones included fragments of grey (Galloway ?) granite ; red sandstone and grit (rather plentiful) ; Carboniferous Limestone, etc. Specimens of the Liassic fossil Gryphiea incurva were said to have been obtained in this pit by the workmen, but their authenticity seemed very doubtful.
The little glen to the S.E. of this exposure revealed no clear section ; but a brickyard by the side of the highroad at Highton, 300 yards S. of the glen, showed 10 feet of mixed slaty clay ; in this a few red sandstone pebbles were noticed, and possibly other foreigners might have been obtained by longer search. This drift was rather gravelly at the top, and thinned off southward upon a bank of slate. A slate quarry 400 yards east of Willaston (Sh. 18), 1/4 mile S.E. of the section last described, is capped by gravelly drift, 2 to 3 feet thick, containing numerous foreign pebbles; and the same admix ture probably prevails in most of the material which covers the slope between Cronk-ny-Mona (Sh. 10), where the bare rock is seen, and the valley of the Glass. Where thin, such drift weathers into a reddish or yellowish stony loam, differing from the soil of the unmixed slaty boulder clay. It is frequently difficult to decide whether it is least misleading to map such material as gravel, or as boulder-clay,—a difficulty often still more acute in the case of the mixed drift south of the central valley.
1 Western isles volt. p: 525
2 Quart. Journ. Geol. Soc. vol. 11. p. 338.
[456]
In the valley of the Glass below its junction with the Baldwin, high banks or cliffs of till and partly-stratified drift extend along the eastern side of the river-flat down to Tromode. A section in the upper part of this bank 300 yards N. of the dam of Tromode Mills showed 30 to 40 feet of stratified coarse slaty gravel and loamy sand, with bouldery loam towards the top. In some of the gravel beds in this section the pebbles were arranged with their longer axes vertical. One hundred yards farther south, firm slaty till is seen at the foot of the bank, with gravelly drift above. A little below this place the river has missed its old channel and excavated a trench in slate, as described in a previous chapter (p. 161'. The section where the present bank cuts across the abandoned eastern loop opposite Ballanard shows slaty flood-gravel resting on warp and silt, with boulder clay probably concealed under the talus slope. Slaty till is seen again under the gravel terrace in the little side-ravine 500 yards 8.E. of Tromode. On the western side, the river-flat south of the confluence of the Baldwin and Glass is fringed by a high-level terrace, which, like its equivalents in the Baldwin valleys, borders a more or less drift-covered slope. This terrace is interrupted by a bluff of slate at Sir George's Bridge,
Fre. 107. Section ar the top of the valley bank, 400 yards south-west of the Purt-ny-shee, near Douglas. 3 Late-glacial flood-gravel ; pebbles, chiefly of slate, fairly well rolled, lying flat ; about 3 feet seen.
2, Grey loamy layer, with most of the pebbles vertical ; 1 to 2 feet. 1 Slaty gravel and wash like No. 1, with the pebbles flat.
near Oatlands (Sh. 10), but sets in again a little below, and continues up to-the ravine at Tromode. It is represented on the opposite side of the valley by the gravel which covers the southern spur at the confluence of the Sulby stream. Opposite Castleward the terrace is composed of thin clayey rubble resting on slate, with little or no clean gravel. It falls south ward from 200 feet above O.D. where first seen, to 100 feet at Tromode, but does not quite keep pace with the present fall of the river, and is there fore highest above the stream in the lower part of its course.
Below Tromode the valley bottom expands suddenly into an alluvial 4at 200 to 300 yards wide, and 40 to 50 feet above sea-level. The opinion is generally held in the neighbourhood that this alluvial flat marks a silted up estrarime inlet, an idea recorded by Cumming,1 repeated in the guide-books, and fostered by the name, Purt-ny-shee, by which the place is known. The flat is however evidently of fluviatile origin, and is too high to have been entered by the sea even during the Raised Beach period. On its western side the continuation of the high terrace above-described
1 "Isle of Man," p. 18.
[457]
may be traced down to its junction with the central valley, where the edge of the terrace is 40 feet above the river-flat. Sections of this terrace in the steep bank S.W. of Purt-ny-shee farmstead exhibit in places 20 feet of slaty gravel, with loamy bands in some of which most of the flatter pebbles are vertical even when underlain and overlain by beds in which such pebbles are horizontal, as shown in Fig. 107.
Traces of the gravel terrace are also seen, resting on a high bank of till, on the eastern side of the valley between Tromode and Purt-ny-shee ; but south of the latter place they disappear, and the stream impinges upon a bank of slate. From the form of the ground and the position of the gravel, it seems not improbable that the drainage of the valley may at one time have found more direct access to the sea in Douglas Bay, in the direction marked by the shallow depression due east of Purt-ny-shee, to which reference has been made in describing the Douglas area (p. 446).
The spur of rising ground between the valley of the Glass and that of the Dhoo below Union Mills is almost wholly drift covered, and sometimes apparently to a considerable depth, but there are few sections. West of the road between Oatlands and Union Mills, slaty boulder-clay is the chief con stituent, and no foreign ingredients whatever were found. East of the road, gravelly and sandy material predominates, with a sprinkling of extra insular pebbles among the slaty debris, and the surface is more uneven and moundy. A small sand-pit in this tract 350 yards W. of Cronkbourne House showed (in 1896) 12 to 15 feet of cross-bedded fine gravel and loamy sand. Former pits of larger size adjoining the main road 400 yards W.S.W. of this place are now disused and obliterated. Towards the southern end of the spur the ground becomes stiffer, and has been mapped as boulder-clay.
The general description of the drift and alluvia of the central valley given in preceding chapters (Ch. I., p. 8, Ch. IX., p. 349, and Ch. X., p. 409), will be supplemented by a few additional details, without repeating the facts already recorded.
Between Peel Harbour and the rock-gorge at Glenfaba the Neb is fringed on the east by a low alluvial flat, Close Chiarn of the six-inch map (Sh. 9), which is conterminous with the strip of Raised Beach at Peel, and may be in part estuarine. This is burdered on the east by a high bank of drift of the extra-insular type, best exposed in the road-cutting leading to Glenfaba Bridge, consisting of red sandy boulder-clay, associated both above and below with sand, loam, and gravel. The bed of the river 300 to 400 yards S. of Peel Station reveals patches of contorted red pebbly clay and loam with foreign stones and many fragmentary marine shells, including Leda pernula, Lellina balthica, Trophon clathratus, etc., which appears to rest directly on crushed slate, and must have been deposited against the cliff-like rock-slope which forms the western bank of the river beneath Peel Hill. A large excavation into this bank at the brickyard 150 yards farther south, where some of the drift still adheres to the slope, showed (in 1897) 10 to 20 feet of slipping slaty rubble, mixed with red clay and drift-gravel containing pebbles of flint, Ailsa Craig riebeckite-rock, granite, etc., and some scratched stones. This drift rested on the crushed and decomposed shaly blue slate, which is ground up and made into bricks.
It has previously been suggested (p. 410) that at Glenfaba the buried pre-glacial valley probably lies to the eastward of the present gorge ; and evidence in favour of this view is furnished by the report (see p. 279) that 90 feet or more of "soft clay, gravel and clay mixed, and then boulder clay," were passed through in a boring for coal made in 1873 a little to the eastward of Glenfaba Mills. This boring does not appear to have struck solid rock until below present sea-level, although in the adjacent river-bank the rock is seen at 50 to 60 feet above O.D.
The undulating platform of Extra-insular drift extending inland from Peel to the foot of the slate hills at Lhergydho and Poortown is composed [458] for the most part of low sandy and gravelly mounds, but red stony clay comes to the surface on the rise between Ballaquane and Ballagyr, and slaty clay covers the lower slope of the solid hills. The platform is trenched by several drainage channels, now almost dry, falling southward to the Neb. The occurrence of little basins in this tract has been previously noticed in describing the elk-remains found at Close-y-Garey, but the most conspicuous example, at Ballalough, deserves further mention. This is a pit-like hollow about 200 yards in diameter, surrounded to a height of 40 or 50 feet with steep banks of sand, gravel and a little clay, except on the south-west where one of the drainage channels just referred to leads out from it towards the Neb. The bottom of the basin is still boggy, but contains a dry mound which is not unlikely to be artificial, like the so-called "Fort" at Lagagh Mooar (p. 432). Immediately to the south-east of this hollow rises Cronk Lheannag, a little isolated hill of bare slate (p. 155); and we may suspect that the deposition of the drift may have been so modified in some way by the vicinity of the rocky hillock as to produce the depression. A smaller basin lies 200 yards farther north, with another, still smaller, beyond it, the three forming a broken chain in the direction of the drainage-slope.
The upper gravel terrace on the northern side of the Neb east of Balla wyllin is in places only thinly overspread with slaty gravel, with an eroded surface of glacial clay. This clay has been dug for brickmaking both on the terrace and in the upper bank of drift which bounds it, but the works were not in operation during the survey of the area.
On the south side of the valley around Kirk Patrick, above the terraces elsewhere described (p. 410), clayey drift containing an admixture of local and foreign detritus sweeps through the low col leading to Glen Meay ; at Gordon and Rheabymooar this is overlain by terrace-like strips of gravelly material, and in the valley bottom by alluvial wash, indicating former drainage through the gap. The manner in which the lower layers of the ice-sheet laden with foreign material were pushed forward into the embayment of the hills in this neighbourhood has previously been discussed. While the greater portion of this ice escaped through the opening at St. Johns and up the Archallagan slope (p. 351), or passed out at Glen Meay by the above-mentioned col, some part seems to have been pushed up the western slope of Slieau Whuallian, where the gradient is comparatively gentle, as foreign material is associated with slaty drift in the patches preserved around the heads of the little glens high up on the ridge, and passes at Eairy over the watershed into a gulley descending to Glen Mooar. A boulder of grey (Galloway ?) granite, measuring 5 feet by 3 feet by 2 feet, lies in the ravine above Ballacosnahan (Sh. 9), 400 yards 8. of the Peel reservoir ; another, of purple grit, 5 feet in diameter, at the head of the glen due south of Ballaspet, with others lower down; and Mr. Kendall records a 3-ft. block of Galloway granite on the ridge of Slieau Whuallian itself. 1
The boulders are most numerous and largest, however, below Eairy, in the above-mentioned rocky gully descending southward into Glen Mooar, at 500 to 6OO teet or more above sea-level, where among others noticed were one of grey granite, 6 feet by 5 feet by 4 feet, and one of pebbly grit, 6 feet by 4 feet by 3 feet. This gully is one of those which cannot be explained by the present drainage and must date back to Late-glacial times. Glacial striae in several places on Sheau Whuallian show that the summit of the ridge has been crossed by an ice-flow from N.N.W. (see List, p. 477).
The character of the drift in Foxdale has previously been outlined (p. 350 and _p. 370). The best sections are in the lower part of the glen and at the head of its western feeders south of Sleau Whuallian, the upper part of the main valley containing only a thin rubbly covering on the slopes, with bare rock in the stream-bed. A 4-ft. boulder of Galloway granite was observed near the east bank of the river 400 yards below the road-crossing at the mines, and two or three others of smaller size lower down the glen, north of Ballanas, where the slaty drift is intermingled with foreign material (see Fig. 94, p. 350). To the eastward similar boulders are scattered plentifully on the upland around the head of Glion Darragh,[459] but blocks of the local granite alone were seen on the moorlands west of Foxdale.
1 Yn Lioar Manninagh, vol. 1., p. 409.
The slopes above the fans of flood-gravel lining the central valley east of Foxdale have a thin covering of yellowish sandy drift, containing both local and foreign pebbles. This is sparingly exposed in the occupation roads south-east and east of Kennaa (Sh. 9); it has been coloured as ' boulder clay' on the map, but scarcely falls under that designation ; and the rather more sandy material dccurring in the same position farther east, around Rhenny (Sh. 10), is s9 far removed from boulder-clay that it has been coloured as 'sand and gravel.' In cases of this kind, where the litho logical characters hang upon the border-line, it must be understood that the strong contrast in the conventional colouring of the map may represent only a slight difference in the actual composition of the deposits. A few boulders of Galloway granite and other extra-insular rocks accompany this sandy drift and mark the path of the ice-lobe previously discussed (p. 350). The crest of the slope is for the most part bare of drift, but on the plateau beyond we again find a covering of yellow loam with an admixture of local and foreign stones, best seen in the road leading from Ballacurry to the Old Cornelly Mines, and in the drainage-trenches of Archallagan plantation. No drift now remains in Glion Darragh (otherwise known as Ballacurry Glen), but numerous large foreign boulders strew its channel, of which a rounded block of grey (Galloway?) granite 8 feet by 6 feet, at the fork below the mines is perhaps the finest example.
On the northern side of the central valley, a little tract of moundy gravel at Ballacraine, at the mouth of the low col leading into the ravine of the Neb, deserves mention. Like the adjacent gravel-platform of slightly newer date, it probably denotes the passage of floodwaters from the upper glen through this now dry gap during the melting of the ice.
The drift on both sides of the valley contracts eastward towards Greeba, and disappears altogether in the narrowest part of the pass opposite Oregacable, where the rocky bluffs come down to the alluvium for a space of 40 or 50 yds. To the E. of this, sloping banks of drift once more border Sada the depression, that on the southern side, which continues up to the mouth of Cootllingill (Sh. 10), being again in part composed of yellow sandy loam. A 4-ft. boulder of grey Galloway granite was noticed at the lower margin of this bank 480 yards S.W. of Cregbeg. The head of C'oodlingill reaches back to Archallagan, and contains loamy drift with foreign stones at 600 feet above sea-level. Similar material extends southward into the little basin which gives rise to the stream flowing to the central valley at Ellerslie ; and occasional extra-insular boulders, along with many from the local micro granite dykes, may be seen in the fences around and to the east of Kairyjora and Eairykellag (Sh. 13) at elevations up to 500 feet. At Garth in the lower part of the basin this drift becomes assorted into stratified sand, gravel and gtoneless warp ; and an irregular strip of similar material stretches down the depression N.E. of Slieau Chiarn to the mouth of Ellerslie Glen.
A small sand-pit in this strip 350 yards west of Ballingan showed 6 feet of yellow sand with small local and foreign pebbles scattered as in a boulder-clay. A rock-surface exposed at the new buildings 60 yards E. of the pit bore glacial striae in two directions (p. 479); the one set corresponds with striz on Slieau Chiarn, and represents the local flow on the leeside of the hills ; the other agrees with those on Archallagan and Slieau Whuallian, and indicates the main flow. Below Ellerslie the drift on both sides of the central valley seems all to be of local origin, until we pass Union Mills ; it is, as usual, deeply banked on the northern slopes, and thin or absent on the opposite side. Below. Union Mills the south-western slopes, above the flood-terrace (p. 411) and below the outcrop of the slate, are sparingly covered with indefinite rubbly loam ; this has been coloured as ' boulder clay', except where it forms distinctly gravelly mounds as in_the patch between Kirby and Farmhill and again at Pulrose (Sh. 13). The drift of the opposite side of the valley has already been described (p. 457).
The remnants of a flood-gravel terrace, probably equivalent to those of the Glass and the Dhoo, are seen in the lower part of the valley of their confluent, the Middle River, up to its sharp bend above Kewaigue. Its western [460] slope south of Middle is capped by a moundy belt of rubbly and sandy slate gravel with some foreign pebbles, which follows the trend of the valley up to the above-mentioned bend, but then crosses it and runs up into a depression leading to the coast at Keristal, north of Port Soderick (p. 463), no doubt indicating a glacial drainage-course. To the east of the valley, the patches of drift running up into the depressions at Nunnery Howe and Ballaslig though still rubbly are of a more clayey character. Both varieties are exposed in railway-cuttings between Douglas and Port Soderick. In the upper part of the Middle River basin, north of Richmond hill, there are several small exposures of slaty till, while an ill-defined patch of local gravel occurs south of Ballabunt. The drifts of the eastern extremity of the central valley required no further description than that already given (pp. 456-7).
1
2 P. M. C. Kermode. Yn Lioar Manninagh, vol. iii., pt. viii., p. 395.
3 Trans. Geol. Soc., vol. v. (1821), p. 502, footnote. Short notices of this discovery were also given anonymously in the Edinburgh Philosophical Journal for 1821, vol. v., p. 227, and for 1823, vol. viii., p. 198.
4 Kermode, op. cit., p. 397.
5 Kermode, op. cit., p. 398, and Yn Lioar, vol. i., No. 1, pp. 23-4,
6 Cumming, Isle of Man, p. 214.
7 Edinburgh Journal of Science, vol. iii. (1825), p. 27.
8 Yn Lioar Manninagh, op. cit., p. 398.
9 Quart. Journ. Geol. Soc., vol. ii. (~846), p. 344.
10 "Isle of Man" (1848), p. 216, foot-note.
11 Memoirs of the Geological Survey, vol. i. (1846), p. 394.
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