HTML Transcription © F.Coakley, 2011
[From Isle of Man, Cumming 1848]
View from the Brough-Varying composition of the pleistocene marls.-Return to Castletown.-Notice of recent raised beaches at various points along the coast.-Remarkable undulations of the limestone beds at the Stack of Scarlet caused by the protrusion of basaltic rocks.-Glacial striations, groovings and indentations. -Mud glaciers not solving the phænomena.-Recent action of littoral ice at Cape Blomidon in the Bay of Fundy affording a clue to the true solution.-Probable gradual sinking of this area at the beginning of the glacial period.
The view from the Brough is sufficiently pleasing to repay the toil of the ascent, its height being not more than 160 feet above the level of the sea. The most toilsome way is really the most picturesque. About 400 yards above the caves the Silverburn makes a sudden angle, and its course from running nearly magnetic north is directed more to the coast. The face of the valley is very steep here, but after mounting 100 feet we come to the top of the schist and meet the old red conglomerate very finely developed as a mass of boulders and pebbles of quartz, quartz-rock and grauwacke, in at first a deep ochreous setting, which, as we rise still higher, becomes at length a gray carbonaceous matrix. A station on the top of this old red conglomerate, looking into the valley at the angle, presents an interesting scene both seaward and landward. In the latter direction we may catch glimpses of the course of the Silverburn for several miles up towards the mountains, and the structure of the valley is easily ascertained.
Proceeding onward towards the summit of the Brough we soon cross the basset-edge of the carboniferous limestone, and may observe its dip towards the centre of the basin.
The composition of the Brough itself may next engage our attention. It belongs to the boulder formation, which seems to attain to a considerable thickness upon it, if we may judge of it by a comparison of the height of the hill with the depth under its summit, at which, according to the dip of the beds, we should meet with the limestone. The extreme red colour of the soil on the hill would indicate that it is formed in great part of the denuded portions of the escarpment of the old red conglomerate which appears to the eastward above Coshnahawin,Head. This is another evidence of the extremely local character of the lower portion at least of the boulder-clay-formation. As we proceed westward across this limestone basin we shall observe how it changes in composition, and tallies in chemical character, as well as in lithological appearance and colour, with that of the subjacent rock prevailing a very little to the eastward of any spot on which we may fix for its examination1.
We may descend from the Brough to Ballahick, and thence get upon the high road between Ballasalla and Castletown, or we may take the road to Ronaldsway, and begin there to examine the evidences of the last raised beach, which we may then trace very distinctly all round the coast where there is no lofty cliff presented to the seaward. Just by the Mill at Ronaldsway we may observe perhaps in the bank an accumulation of a bed of sea-shells of recent species. We have before noticed the beach at Hango Hill, and in proceeding thence towards Castletown we may perceive it very continuously at the back of the houses which front towards the sea all the way to the Bowling Green.
Let us set out again from Castletown towards the Stack of Scarlet. We have the same beach, with plenty of shells all the way round from Knockrushen by Sea-view and beyond Scarlet House. The same thing occurs at Poolvash, Strandhall, Mount Gawne, and Port St. Mary.
In passing from Castletown to the Stack of Scarlet, the series of trap-dykes which are seen between high and low water, the undulations on the surface of the limestone and its frequently altered character, will certainly attract attention. The great Knockrushen dyke, in width twenty-one feet (sending out three other smaller ones), I have supposed to be the continuation of the more southerly of the two great dykes which intersect Langness, and which we meet with again at Poolvash divided into three branches.
It is however evident, as we approach the Stack, that another disturbance than that of the trap-dykes has affected this portion of the limestone basin. The direction of the undulating ridges is changed, and the undulations become more frequent and marked. The long swell becomes the crested wave just ready to break upon the shore.
Close by the limekilns the contortions become very violent, and the great wonder seems to be, that the limestone beds have not snapped under the extreme tension. There is merely a jagged crack running down the crown of the undulation, though its curvature is as rapid as the rim of an ordinary sized carriage-wheel. Either the superincumbent pressure must have been excessive, or the beds were in an extremely new and plastic condition at the time of the contortion.
At these limekilns it is worth while to linger a little, both for the fine view here afforded of Castletown bay and the adjacent country, and also to observe the groovings and scratchings on the surface of the limestone where the quarriers have removed the boulder-clay.
There are three kinds of markings to be noticed; the deeper polished groovings, the striations or finer scratches upon the groovings, and the indentations. The direction of the first is very nearly magnetic east and west, a point north of east and south of west; the second, though generally having the same direction as the groovings, sometimes cross them at acute angles; the third have the appearance of being produced by some hard, sharp-pointed object brought suddenly in contact, grooving the surface for an inch or two, and then removed.
If we examine the action of the breakers upon the surface of the limestone, wherever it is exposed at the present time, we shall find the result very different to that seen on the rocks under the boulder clay. We have the proof plain before us, about one hundred yards south-westward of these limekilns, nearer the Stack of Scarlet, on a shelf of rock which is intersected by a trap dyke. The surface of the limestone, which is just exposed to the sweep of the waves at the highest spring tides, or when a storm rages from the south, is drilled with a series of holes of every size and depth. How are they formed ? Look at that pebble or heap of pebbles which lies at the bottom of one of these clear briny pools. These are the tools with which the work is done; the natural augers which have pierced the solid stone. The effect is thus produced. The action of the atmosphere on a small crack or flaw in the limestone (and being in such close contact with trap-rock, and contorted so fantastically, no wonder that it is in some places much cracked!) produces a small hole. A little pebble driven on by the breaker lodges in it: the next high tide sets the pebble in motion, and the instrument begins the drilling operation. As the hole increases, other and bigger pebbles or hard boulders find a lodgement there, and assist in widening and deepening the hole till it is too deep for the refluent surge to be capable of moving the collection at the bottom, and then of course the action ceases: Now here is plainly a very different result from that found on the surface of the rock under the boulder clay. Indeed, I am not aware of any instance in this neighbourhood where the sea now produces anything like the groovings, scratchings and indentations which we are now considering.
It has been suggested that the effect has been brought about by the sliding forwards of the entire mass of the boulder series upon the inclined surface of the limestone beds; in fact, the boulder clay has been spoken of as a kind of mud glacier, which, rolling onwards, has abraded the subjacent rocks, and left the traces of its course in those groovings and striations which in so many places meet the eye. Such an explanation might possibly stand had we to do simply with groovings, or with the striations only parallel to them; yet even in this case the objection would have to be met, that these do not always coincide with the dip of the rock; and further, that since it is now pretty generally allowed that the motion of glaciers is due to gravitation, and as this would be specially the case in the so-called mud-glacier, it would require a rather nice engineering adjustment of the inclines and application of forces for the motion to be propagated through several miles up one hill and down another to an extent which greatly taxes one's credulity. But the grand difficulty which still remains on this hypothesis is to solve the problem of the cross-scratches and the indentations. I have never heard of such a solution, and I certainly cannot offer one.
And why should we go out of our way to frame hypotheses to account for these marks upon the rocks, made at the period of the boulder deposit, when we have similar phænomena to adduce of a recent date, where the cause and effect are distinctly set before our eyes in the closest possible connection
Mr. Lyell has supplied us with the necessary data for determining the problem in his recent travels in North America2. Strolling one day along the beach, at the foot of Cape Blomidon, in the Bay of Fundy, he was startled by observing, upon a ledge of soft sandstone, some recent farrows, the exact counterpart of the grooves of ancient date attributed to glacial action. Some of these furrows were half an inch broad, and very nearly parallel; others were rather divergent, and crossed each other. The direction of the parallel furrows coincided with that of the shore at this point. His guide was asked if ever he had seen much ice at this spot. He replied, that in the preceding winter of 1841 he had seen ice, in spite of the tide, which ran at the rate of ten miles an hour, extending in one uninterrupted mass from that side of the bay to the opposite coast of Parisborough, and that the icy rocks, heaped on each other, and frozen together, or packed at the foot of Cape Blomidon, were often fifteen feet thick, and were pushed along when the tide rose over the sandstone ledges. He also stated that blocks of a black amygdaloid, containing numerous geodes coated with quartz crystals, fell from the summit of the cliff, were frozen into the ice, and moved along with it. Need we say, that Mr. Lyell,-like any other man whose mind has been trained in the inductive principles of the Baconian philosophy, hesitated not an instant as to the. agent which had produced the groovings and furrows upon the ledge of soft sandstone in the Bay of Fundy ?
And need we hesitate to ascribe the groovings, striations and indentations on the limestone at Scarlet and elsewhere, wherever the boulder-clay is removed, to the same agency?
We must again recur to the circumstance so often before stated, that at the period of the boulder formation the Isle of Man was a cluster of islands, and that powerful currents in all probability swept through the channels between them; that tide-ways would be formed parallel to the coast-line, and that the climate was, to judge by the fossils included in the drift, of a more Arctic character than it is at the present time. Is it very difficult to connect the phænomena of the grooved, striated and indented rocks with the action of shore-ice, ice-floes and icebergs ?
It appears to me highly probable that at the commencement of the boulder period there was a gradual sinking of this area: successively, therefore, the points of different degrees of elevation were brought within the influence of the sea, and exposed to the rake of the tides charged with masses of ice which had been floated off from the surrounding sbores, and bearing in their under surfaces mud, gravel and fragments of hard rock. If the basset-edge of a rock were opposed to the drifting currents, it is probable that their effect would be to detach pieces from it, or to break up the beds, especially when they consisted of alternations of soft shale with limestone. Thus an accumulation of mud, with blocks of limestone and the boulders torn from the old red conglomerate, would be constantly taking place in the hollows, and the sea-bottom would gradually be filled up. If the rock over which the ice-charged current flowed presented no serious obstacle, if it were for instance one of the limestone domes or bosses which are so numerous here,-then, instead of tearing the beds in pieces, the effect of a mass of loaded ice grounded upon it would be to polish, groove and scratch the surface; and though the general direction of these marks would be that of the great tide-ways, yet so long as the rock was subject to the extremes of high and low water (just as at the present time the Carrig boss is in the centre of Poolvash Bay), we can readily conceive how the ice-charged breakers might produce scratches in any direction. Afterwards, as the submergence of the land proceeded, and these bosses became placed at greater depths below the sea-level, they would be beyond the reach of the merely scratching influence of shore-ice, but still suffer from the digs and thumps of icebergs, and by such blows would the indentations and those furrows, which, from being very deep and rough, gradually die out, be produced.
In the separate detail of these operations we may very possibly have erred; in the opinion of some, there may have been no depression of land, but on the contrary, elevation; or the polished furrows may be attributed to icebergs and the identations to shore-ice; but the general theory which seeks for the solution of these phaenomena in the action of ice, in some shape or other, floating in marine currents, does certainly not tax our credulity to any unreasonable extent.
Having spoken of the manner in which the sea-bottom was being filled up in the glacial period, it is easily understood how these furrowed and scratched bosses also became ultimately covered up with the accumulated glacial deposits, and how these marks have been preserved from erosion at a subsequent time, when this area was again upheaved. The foreign rocks of the boulder period are plainly the produce of erratic bergs detached from more distant shores.
About sixty or seventy yards to the north of the limekilns is the boulder of porphyritic greenstone, to which allusion has before been made as having probably been detached from the mass of similar rock at the northern extremity of Langness, and drifted across Castletown-bay. The scratches and groovings on the surface of the rock at the limekilns point directly to that same spot on Langness. But as it may be argued that the current was as likely to have flowed from the magnetic west as from the east, it is desirable to state that there is no distinct trace in the boulder-clay at the limekilns of the rocks which lie to the westward of that point, viz. the trap-tuff and breccia which extend from the Stack of Scarlet to Poolvash Bay, and over which the drifting current would have passed had it come from the westward. It is at any rate a singular circumstance that we do not meet with these rocks, and it is not readily to be accounted for on the supposition that the currents of that period were solely due to the ebb and flow of the tide. At this very spot, however, we fall in with pebbles of foreign rocks in the boulder clay, which must have come from a great distance, from the shores of Cumberland and the south of Scotland; we have for instance fragments of the grit of the coal measures. Now all this leads to the conviction of one great current setting down from the Solway Frith upon these shores, and overpowering the effects of the local currents caused by the flux and reflux of the tide. The origin of such a current is at present a mere matter of speculation; we dare only point to the evidences of its existence.
On the hypothesis of a gradual depression of the land and sea-bottom, it is easy to see that as long as any particular surface was within the direct influence of the tide (lying, we will say, between high and low water), traces of that influence might be left on it, in scratches varying in direction from that of the prevailing great current, as indicated by the polished furrows and groovings ; yet afterwards, as the submergence proceeded, and the surface of rock was placed at some depth below the level of the sea, all the detrital matter, the accumulation of the boulder clay deposited on it, would be the product of the great current only, and this appears to have flowed from the magnetic east.
1 Through the kindness of my friend, George Kemp, Esq., M.D., of St. Peter's College, Cambridge, I am enabled to give the percentage of lime contained in the clay at different places on the Island. A reference to the geological map and sections will show at once its value, as bearing on the present question of the origin of the boulder formation; and it may prove acceptable to agriculturists, as indicating the best localities for marls containing the largest quantity of lime. See Appendix, Note 1.
2 Lyell's Travels in North America, vol. ii. p. 172.
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