Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/205

1869.] SEARLES WOOD BOULDER-CLAY. 105 extruded by that sheet and carried away by floating ice from the ice-foot during summer and distributed over the south of England, there to intermingle with material brought from other parts of the ice-foot; and it is just at the part where the section so encounters this foot, on the south-west side of the Lincolnshire Wold, that the chalky clay is in the condition of almost pure chalk and is quarried for lime. The middle representation then, after leaving the northern portion of the Lincolnshire Wold enveloped in ice, intersects the ice-foot again where the chalky, or basement, clay of Eastern Holderness (a) indicates that the Wold debris was also extruded. It then exhibits the Yorkshire Wold, over which the purple clay without chalk extends, as well as the great vale beneath the Wold where the same chalkless clay occurs, as entirely occupied by the ice, and the sea blocked out by it.

The lowest representation of this triple section indicates what I conceive to have been the position when the purple clay without chalk, and with Shap boulders, were deposited. This representation supposes a further depression of the land to have taken place, so as to bring it about 1500 feet below its present level, and that in consequence so much of the ice-sheet as had before enveloped the Wold, and filled the great vale below it, had been floated up and wasted away by the sea, so that only the higher elevations of the north and north-west of England, and the extreme summits of the eastern moorlands, remained enveloped in ice. This depression having also covered the lowest parts of the dividing ridge with sea and placed the Wolds many hundred feet under water, floating ice, carrying Shap boulders, was enabled to pass over the lowest parts of the dividing ridge; and thus, coincidently with the termination of all supply of chalk by the retreat of the ice-sheet from the Wold, consequent upon its deep submergence, we have the formation of a clay destitute of that material and containing Shap-boulders, which was thrown down direct on the floor of old formations over the great vale that skirts the Wold-escarpment, and generally over the north of England previously occupied by the ice.

This deposit of 1500 feet submergence is represented as extending over the purple clay with some chalk, because we can prove its extension there by outliers still remaining: but how much further it may have spread in a southerly direction I would not venture to conjecture. It may have thinned out greatly in that direction — probably it did so — so that the commencement of the denudation wrought by the sea as the land rose removed it, and with it went the Shap boulders; for it is a remarkable fact that, wherever any great area of denudation has occurred in the Boulder-clay, the various boulders of that formation have disappeared along with the clay itself.

The immense volume of the chalk debris which has been shed out to form the chalky clay appears to me to necessitate the admission that there was a long period in which the land and its enveloping ice-sheet remained stationary, at about the limits indicated in the map and in the middle representation of the triple section. I