666 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 22, 
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 Ribble 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-hke curves through an aliu- 
vial 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 remoyed 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, ex- 
cept 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 
