GEOLOGY
Marls reached a bed of rock-salt at a depth of 258 feet, the rock-salt with a layer of shale being nearly 300 feet thick. Rock-salt is of widespread occurrence in the Keuper Marls, more especially in Cheshire, where, in the Marston Mine, are two beds, one 85 feet thick and the other 106 feet.[1]
PLEISTOCENE
GLACIAL PERIOD
After the deposition of the Trias there is no evidence of rocks of later age in Lancashire until we reach the Glacial Drift, a thick layer of boulder-laden clay and sands which occupies the bottoms of the valleys in the Coal Measure country and occasionally spreads up their sides, even to a height of over a thousand feet. On the low Triassic plain the boulder clay masks the solid geology almost everywhere.
It must not be supposed, however, that rocks later than the Trias and older than the Glacial Drift never were laid down in the Lancashire area, because the presence of a small patch of Lias in Cumberland, at Orton, west of Carlisle, and the presence of extensive deposits of Liassic and Cretaceous age in the north of Ireland, indicate that these formations had a much greater development than now, and might very probably have extended over the county, and have been denuded before the Glacial Period commenced.
The Glacial Period occurred when the greater part of the British Isles and Northern Europe became covered in by snowfields and mighty glaciers, the climatic conditions being such that the snows of winter were not wholly dissipated in summer, and the accumulation of snow thus formed increased until the mountains and mountain valleys were filled, and a downward movement commenced which went on until the lower levels were encroached upon and covered, and the ice sheets ultimately reached the sea, and even travelled over parts of its area. The conditions were in all probability like those which now exist in the Alps, but were more widespread and general. Where rocks or mountain-tops projected through the snow and ice, masses were broken off by the expansive force of water in its freezing, melting and re-freezing, the blocks from time to time falling upon the glacier fields and becoming entombed in them by the opening of crevasses. The lower layer of the snowfields became compacted into ice by the superincumbent weight, the passage of water, and partial melting. Every high mountain peak became a centre of dispersion, and from the centres of high altitude, such as the mountainous region of the Lake District, North Wales, and similar areas, there began a steady outward flow of glaciers to lower levels. As the glaciers moved along, their great weight and the stones locked up within caused them to exercise an erosive action upon the ground over which they moved. The surface soil was worn away until the hard rock was reached, and the latter then became deeply scratched and polished by the slowly sliding mass of stones and ice. As far as the glaciers travelled, so far, of course, were stones carried away from their parent source, and strewn along the course of the glacier stream. The grinding-down of the surface rocks and the ice-borne stones gave rise to clays, which were deposited over the whole country traversed. How much rock material was thus carried away from the high ground, and deposited upon far-away and lower levels, we shall never be able to accurately determine, but there is no doubt that it was enormous. By some authorities it is believed that many, if not all, the basins of the lakes in the Lake District were ground out during this period, the old river valleys everywhere widened, and the hill crests much reduced in height. In some cases river valleys were filled up by earthy material and ice, and the general ice movement passed across them and not along their length. By a close study of the boulders of rock now found in the glacial clays, and an equally careful mapping of the ice scratches upon the rocks below, it has been possible to trace the general course of these 'erratics,' as they are called, back to their source, and to construct maps showing the lines of flow and centres of dispersion. In this way, for example, it can be shown that the glacial clays of Lancashire are derived from the Lake District and the south of Scotland, examples of Criffel granite being strewn in the Boulder Clay along the Cumberland coast, and as far south as Liverpool and the Wirral peninsula. Rocks derived from the Lake District are numberless in the clays of Lancashire, most of them being derived from the mountainous district on the west of Westmorland, but others from the Shap Fell area. They consist mainly of flattened and polished specimens of felspathic rocks, rhyolite, Shap granite and slate, intermingled with local rocks which were also caught up and carried forward. In many places the clays contain boulders of large size, weighing tons, and in several Lancashire towns these have been set up in parks and public places. A fine example is to be seen in the quadrangle of Victoria University at Manchester. The Boulder Clay in the Furness district is known as Pinel, and contains fragments from the Coniston Grits and Shales in
- ↑ For particulars relating to the Triassic rocks see G. H. Morton, The Geology of the Country around Liverpool, ed. 2 (1891), with Appendix (1897).
