[Vannin Lior (Yn Lioar Manninagh) Vol 1 No 1 pp16/18]
By W. BOYD-DAWKINS, F.R.S.,
Professor of Geology and Palaeontology, in Owens College.
iii. The Source of the Pebbles,
iv. General conclusions.
[Geological Survey Map of the Isle of Mann (six inches to the mile) Sheets xvi. and xix.]
i. Introductory.The Conglomerates of the South of the Isle of Mann were described and mapped in 1847 by the late Rev. J. G. Cumming, who first laid before the world a scientific account of the Geology of the Island. They form one of the many interesting features in the Geology which induced me to select that portion of the Island as a starting point for the six-inch Geological Survey, which I hope to finish in the course of a few years, and two sheets of whichxvi. and xix.are now laid before the Society, The Conglomerates which are the subject of this preliminary note are those which occur beneath the Carboniferous Limestone and above the Phyllites and Clayslates, and must not be confused with the comparatively modern Conglomerates and breccias which rest upon the water-worn surface of the Limestone in the low cliff at Balladoole, South of the road from Port St. Mary to Castletown, and which occur also to the East of Ronaldsway House, in the Bay of Derbyhaven.
ii. Description.The true character and relation of the Conglomerates in question may best be studied in the peninsula of Langness, where they are seen to rest on the water-worn edges of the Clayslates there locally coloured red, and to have been protected from the denuding forces by which they have been swept away from a large area in the South of the Island, by their having been faulted down into the Clayslates. In the extreme South of Langness they occupy the eastern half of Dreswick Harbour, and, wedged in between the Phyllites of Langness Point and those forming the eastern part of the peninsula, extend northwards to form the foreshore and the picturesque cliffs, and caves, and arches which are so well-known between Port Bravag and the mine shaft. From this point they still continue their northward course, forming the eastern part of the coast of Castletown Bay, as far as Poyll Breinn (sheet xvi). Passing thence under the, blown sand they reappear on the shore on the south of Derbyhaven. Throughout this area the boundary between them and the Phyllites and Clayslates is a faulted boundary, with the doubtful exception of a small portion to the south of Port Bravag (sheet xix). They dip gently under the Carboniferous Limestone on the shore, on the eastern side of the Bay of Castletown, and form a faulted inlier in the Limestone to the west of Poyll Breinn.
They also form a faulted inlier on the shore at Derbyhaven, and may be traced at low-water to the north-east from the Breakwater to Ronaldsway, opposite which they disappear beneath low-water mark. From this point they are concealed by the over-. hanging limestones as far as Casna Hawin where they are again faulted in, forming the top of the cliffs at the south of Santon Burn. Here they pass under the sands and gravels of the boulder clay series, and do not reappear at the surface until they are revealed in the valleys of the Awin Ruy and Silverburn streams. In the first of these they are exposed in the bed of the stream from the foot of the ravine to a point opposite the Crossag farm, where they dip under the Limestone at an angle of g degrees, and are about 159 feet thick. In the second they are thrown in by a north and south fault, passing through the garden of Rushen Abbey, and are seen on the roadside on the west of the garden wall. From this point they pass to the north-east to Ballahot, where they crop to the surface in the farmyard. From this point they are. concealed by the Boulder Clay series of deposits as far as the Silverburn farm-house on the road to Ramsey, south of Athol Bridge, and are finally abruptly cut off by the great Port St. Mary Fault, at the Athol Bridge over the Silverburn. This is by far the most important fault in the south of the Island, since it throws down the carboniferous series of rocks along a line passing from the end of the Pier of Port St. Mary and the shore, by the south. of Kentraugh to Athol Bridge, running out westwards at Perwick Bay, and concealing the conglomerates along its whole course.
The physical character of the Conglomerates may be summed up in few words. They are mainly composed of quartzite pebbles stained red, and embedded in a sandy paste more or less red and calcareous. At Langness there are pebbles of red clay slate. Their thickness in the bed of the stream at Awin Ruy is about 159 feet. They are in all cases conformable to the Carboniferous Limestone above, and rest on the water-worn edges of the Phyllites and Clayslates below. They belong to the series of Conglomerates which, in North Wales, in the valley of Clwyd, and in Scotland are found at the base of the Carboniferous series, of which they form the lowest member. They are simply the shingle accumulated by the waves of the Carboniferous sea as they dashed against a shore in the Isle of Mann composed of Phyllites and Clayslates.
iii. The Source of the Pebbles.Until I had carefully examined the Phyllite and Clayslate Rocks in the south of the Island, I had considerable difficulty in tracing the quartzite pebbles to their true sources. The first intimation of the presence of quartzites in the older (? Lower Silurian) series I owe to Mr C. le Neve Foster, H.M. Inspector of Mines, who gave me specimens of grey quartzite from the Laxey Mine. I have since met with thin beds of the same rock in the Phyllites of the south of the Island, and in the Awin Ruy and Silverburn Ravines a considerable thickness of grey and red quartzite is exposed. In the former they. dip at a high angle (22 to 45 degrees) to the south, and have an approximate thickness of 632 feet. It is, therefore, to these local rocks that we must look for the source of the quartzite pebbles rather than to shingle drifted from some other district, such as Scotland or Wales. The existence of these thick quartzites in the Isle of Mann opens out a new source for the possible derivation of the quartzite pebbles in North Wales and England not only in the basement beds of the Carboniferous Rocks, but also in the shingle beaches of the New Red Sandstone.
iv. General Conclusions.The general conclusions which may be drawn from the study of the Conglomerates of the South of the Isle of Mann may be summed up thus :
1. They represent shingle beaches of the Carboniferous sea, slowly acccumulated by the dash of the waves on a line of cliffs composed of Phyllites and quartzites.
2. While the accumulation of shingle was going on the area of the Isle of Mann was being depressed, so that the shingle beaches were covered by the Limestone which was formed in deep and clear water.
3. The submergence went on until nearly the whole of the south of the Island was submerged, indeed probably all the Island, except some of the loftier peaks such as Cronk-na-Ireylhaa and Snaefell.
4. Next profound disturbances took place after the deposit of the Carboniferous rocks, by which the whole Carboniferous strata were faulted and inlaid, so to speak, in the older Phyllite series.
5. And, lastly, by the ordinary process of denudation subaerial, glacial, and marinethe whole surface of the Island has been cut down to its present rock contours and afterwards covered with the boulder drift and other surface accumulations.