[From Lamplugh Geology, 1903]

GENERAL DESCRIPTION OF THE ISLAND.

"Rising from the middle of the Irish Sea, within sight of each of the three Kingdoms, with a history and associations so distinct, yet so intimately linked with those of' the rest of Britain, this interesting Island presents in its geological structure features that connect it alike with England, Scotland and Ireland, while at the same time it retains a marled individuality n regard to some of the rocks that form its framework." These words, with which Sir Arch. Geikie preludes his account of the volcanic rocks of the Isle of Man serve equally well as an introduction to its general geology.¹

The predominant feature in its stratigraphy is the central ridge of slate and greywacke, which seems to have constituted an insulated tract at as early a date as the beginning of the Carboniferous Period.² This prototype of the present Island appears afterwards to have been enfolded and obliterated by the sediments of later times; but with the progress of denudation the old ridge has once more emerged from beneath this mantle.

Its insular character is as well maintained in its physical as in its geological features. The erosive agency of the simple drainage system, descending radially to the sea from the central hill-ridge, together with that of the waves which surround it, is adequate to explain all the contours of its present surface. It must indeed frequently during its history have been reunited to the mainland by a continuous land surface; but at such times it probably still retained in some degree its characteristic individuality, and arose above the surrounding plain as a hilly tract with a self-contained drainage, although its streams may then have been tributary to a larger river-system lying beyond its limits.

The sketch map, Fig. 1 will serve to recall the position of the Island as regards the neighbouring shores. Its northernmost point is 16 miles distant from the nearest headland of the Scottish coast, while its closest approach to the Cumberland shore is 31 miles, to that of Ireland, in Co. Down, 31 miles, and to the Welsh coast near Holyhead 45 miles. The same map shows roughly the contours of the surrounding sea-bottom. To the eastward of the Island the depth is nowhere more than 20 fathoms; to the northward it rarely reaches 30 fathoms, to the southward it usually ranges between 30 and 50 fathoms, while to the westward, midway to the Irish coast, there is a long narrow trough descending to over 70 fathoms.

As at present constituted, the Island, with the detached islet of the Calf off its south-western extremity, contains 227 sq. miles (145,325 acres), of which 170 sq. miles, or three-fourths of the whole, are occupied by the slate and greywacke rocks, probably of Upper Cambrian age, composing the hilly massif. Strata of the lower Carboniferous age occur in a small basin of 7 or 8 sq. miles at a low elevation in the south of the Island; and a narrow strip of red sandstone, probably belonging to the same period, borders the coast for two miles about midway upon the western side. The northern extremity consists of a low-lying tract of about 45 sq. miles, which is an addition made to the Island in Glacial times time by deposition of great masses of Glacial Drift upon the Preglacial sea-floor. Deep borings through this drift have recently revealed a rock-floor of Triassic, Permian and Lower Carboniferous strata at a considerable depth below sea-level.

The Island is irregularly oblong in shape, with its longer axis running N.N.E. to S. S.W., which is the direction of strike of the slate-rocks. In this direction, from the Point of Ayre to Spanish Head the land has a length of 30 miles, while the breadth of its wider central portion varies from 8 to 12 miles. Excepting in the well-cultivated northern plain, there is little flat ground. In the interior the physical features bear much resemblance to the Southern Uplands of Scotland. The hills are steep, but not generally craggy, and are arranged in long grassy or heather coverered ridges running with the longer axis of the Island, with broad intervening valleys. The highest of these ridges commences in the vicinity of the eastern coast near Ramsey, and is practically continuous to the south-western coast north of Port Erin, but is broken across in one place by a deep transverse valley, which intersects the Island between Peel and Douglas. North Barrule, with an altitude of 1,840 feet, forms the north-eastern extremity of this ridge, which culminates 3½ miles farther south-westward in Snaefell, the highest point of the Island, with an elevation of 2,034 feet, while Cronk-ny-Arrey-Lhaa, overhanging the south-western coast, is 1,449 feet in height.

Most of the larger streams of the Island rise in the vicinity of Snaefell and fall outward in different directions to the sea, the Sulby River and Glen Auldyn water draining northward, the Cornah and the Laxey Rivers eastward, the Glass and the Baldwin south-eastward, and the Neb south-westward. The drainage of the smaller tract south of the transverse valley is radial from a separate centre in the south western portion of the hill-chain, whence flow the Glen Rushen waters north and northwestward, the Foxdale River northward, and the Santon, the Silverburn, and the Colby southward. Fuller details of the physiography are given in the small-typed portion of this chapter.

The Ordnance one-inch map of the Island, though numbered m the New Series as occupying sheets 35, 45, 46, 56 and 57, is published at present only in a single sheet (No. 100 of the Old Series).

The following Table of Strata shows the divisions which have been adopted for the one-inch map of the Geological Survey, published in 1898. The more southerly portion of the northern urift-plain may possibly be underlain by rocks intermediate age between the Manx Slate Series (Upper Cambrian? ) whicn bounds it on the south, and the Lower Carboniferous strata which have been proved in the borings at its northern margin.

But in the complete lack of evidence as to the character of its rock-floor in the intervening tract, and in view also of the absence of intermediate strata where the base of the Carboniterous system is seen in outcrop in the south of the Island, it would have been rash to introduce into the map any divisions not yet actually proved to exist within the area. Hence the conjectural lines by which an attempt has been made to express on the map the solid geology the buried rock-floor are based on the assumption that no other formations are present therein than those which have already been proved to exist.

TABLE OF STRATA.

Footnotes

1 "Ancient Volcanoes of Great Britain," vol. ii. p. 22

2 See Rev. J. Cumming's "Isle of Man;" p. 239; Rev J. Clifton Ward Geol Mag., dec. ii., vol. vii. (1890), p. 6.