Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/542

418 the depressions; and the sharply defined brinks of the gullies they have excavated can at once be distinguished from the neighbouring form of ground. But streams in many places have made channels where little or no previous depression existed. The steepness of their sides varies with the compactness of the drift, the absence of springs, &c. But though the streams have been flowing in their channels ever since the Glacial period, rain has not been able to bevel down the sides of these channels to a lower average angle than 30°. Geologists ought therefore to hesitate before attributing to rain the excavation, in hard rocks, of valleys the sides of which rise above the river- channels at an angle of only 4° or 5°.

c. Origin of Drift Escarpments and Valley-plains.—As already remarked, depressions must have been formed in drift deposits before their elevation above the sea*. As waves at a stationary level are now wearing the slopes of these depressions back into cliffs, it is reasonable to suppose that the cliff-lines now bounding valley-plains which have partly or wholly risen above high-tide level have been formed by wave-action. Rivers have wandered over these plains, and during floods deposited sand or loam. In many places they have attacked the cliff-lines and worn them back into horseshoe-shaped curves. But some of the concavities in the drift escarpments bounding low-level valley-plains have escaped river-action, as would appear from the direction of the dip of the ground under them, from their form relatively to the old gullies which abruptly break their continuity, from transverse sections of the bases of the escarpments revealing the kind of agency to which they were last subjected, &c.

The river Ribble, at Redscar Cliff, near Preston, has evidently only lately attacked the concave escarpment, a part of which it is undermining and carrying away, while a great part of the cliff-line on both sides of the valley-plain traversed by this river shows indications of its having been formed by the sea†.

5. Between Carnforth and Ulverstone, along the sea-coast, several sections of drift may be seen exposed. To the west of Grange the hard Lower Boulder-clay appears in full force, and looks either like an old beach under an escarpment, or a fringe of a formerly extensive deposit. At Cark Station gravel and sand make their appearance. In the neighbourhood of Ulverstone one cannot proceed very far in observing drift-sections without seeing the importance of beginning his researches with examining the forms presented by the rock-surfaces of the district.

a. Distinction between Glaciated, Rain-worn, and Sea-worn Rock-surfaces.—Ice, especially land-ice, is a planing agent. It uniformly


 * r1 In each successive drift deposit depressions must have been scooped out while the deposit was under the sea, as these depressions are filled up with over-lying drift. The latest depressions formed in drift must have risen above the sea without being filled up excepting by postglacial warp, &c.


 * r2 Mr. De Rance, of the Geological Survey, accompanied me when I examined the drift escarpments near Preston, and agreed with the conclusions at which I arrived.