Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/916

 Fleetwood, the grains of sand are much larger, probably owing to the proximity of shingle beaches derived from the Boulder-clay, which, by the action of the waves, becomes sand.

Denudation of the Gorge of the Ribble. — The Kibble has been mentioned as having cut through the great- drift plain, between Chorley and Lancaster, down to the rock beneath. It has, in fact, excavated its bed, near Preston, to the lowest possible level, namely to low- water mark, the spring tides flowing two or three miles up the river beyond the town. The river has excavated for itself a gorge, with an average width of a mile and a half, bounded by steep walls, or bluffs, here and there worn into cliffs, exhibiting fine sections of the glacial drifts, of an average elevation of from 80 to 200 feet.

The whole of the excavation of this gorge, 20 miles long and 150 feet deep, must therefore have taken place since the era of the Upper Boulder-clay, or in postglacial times, and since the country had acquired its present level ; for if the country had stood lower, the sea would have shaved the country across, instead of excavating a comparatively narrow valley ; and if the country had stood higher, the river would have excavated its bed deeper, or below low-water mark, which is not the case, as, whenever the rock is visible, it is seen extending under the drift as a flat surface, in every direction.

The Ribble wanders in a series of S-like curves through an alluvial plain ; and wherever the bend of the S cuts the boundary- walls of the plain, there the process of river-cliff-making ensues : the cliff is worn back and back, until the river, by cutting across a bend, finds a new channel ; then the talus formed at the base of the cliff, being no longer removed by the river, accumulates, until a gradual slope, covered with grass, is formed, and the cliff becomes a bluff.

As the bends of the river are nearly a mile apart, and they alone exercise the primary horizontal denuding power, it will be seen that, for the formation of the bluffs, once cliffs, the bends of the river must have been successively upon every point of the bluffs forming the bounds of the plain. And as the vertices of these bends move with extreme slowness, it follows that, when the river twice denudes the same point, it must flow at a lower level the second time than it did the first.

This will account for the fact that terraces of Lower Boulder-clay have been left at the base of the bluffs, at higher levels than the alluvial plain — the top of the terrace marking the level of the river when it last denuded that point.

The river is now depositing silt, or alluvium, during every flood produced by a freshet from the land or a high tide from the sea, except at the points where the denudation of the old banks is at work. Similarly we find, here and there, superimposed terraces of alluvial gravel, at heights of from 20 to 100 feet above the present level, formed in the concave curves of the S's, when it stood higher than at present, and cut the terraces in the Lower Boulder-clay before referred to. The Ribble, in other parts of its course, flows across