The Superficial Geology of the Country adjoining the Coasts of South-West Lancashire

Abstract

THE memoir of the Geological Survey by Mr. De Rance recently published is an interesting contribution to our knowledge of the superficial deposits of the area between the Mersey and the Ribble, which carries the classification of the Cheshire Plain as far to the north as Morecambe Bay. The whole of this district is covered with glacial drift and recent sands, gravels, and peatbogs, except here and there where the solid rocks come to the surface in the hills. The drift forms an inclined plane dipping from the hills towards the sea, and probably deposited during subsidence upon an old rocky plain of marine denudation, bounded to the east by a line passing from Eccleston to Euxton and Ribchester, and thence through Broughton, Garstang, and Cockerham to the present sea-margin. Were the superficial deposits stripped off this area, the rock-surface would be seen to be not very far from the present sea-level, although the surface of the ground is often 170 feet above it. This plain also dips gently seaward, and has been worn into hollows by the denuding forces before the glacial period. Very generally it has been cut up into hills and valleys by pre-glacial streams, as, for example, the buried valley of the Mersey described by Mr. Mellard Reade, now filled with 200 feet of sands, gravels, and clays. These buried valleys may be traced inland, rising nearer to the present surface of the ground as we approach the high ground, until at last their tributaries come to the active surface in the higher hills, and are traversed by the same streams as those now finding their way to the surface, and through the accumulation of drift filling their ancient lower courses. It seems tolerably certain that the hill and valley system of Lancashire and Cheshire was produced by sub-aërial agents before the glacial period, and that the ice merely acted on the solid rock by rounding off and smoothing the raw edges left by the streams and rivers. Indeed, as a rule, it may be said that the relative importance of the agency of rain and rivers, and of ice in scenery making is precisely that of chisel and sand paper, the one carves, while the other rounds off, smooths, and polishes. But whatever view may be held of the cause of this uneven surface below the mantle of the drift, it is a most important fact to be noted, that the surface configuration bears little or no relation to the rock-surface below, as engineers have frequently found out by experience in making reservoirs. In one case, for example, the ‘puddle trench” had to be carried down 160 feet, so as to render a ravine filled with drift water-tight, and this ravine, with the big boulders at the bottom left in the bed of the stream, by which it was hollowed, was intercepted twice in the course of the works.