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 1 06 feet.i 



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 develop- 

 ment 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 com- 

 menced 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 



1 For particulars relating to the Triassic rocks see G. H. Morton, ne Geology of the Country around Liverpool, 

 ed. 2 (1891), with Appendix (1897). 



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