Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/200

100 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 8, debris that makes up so large a part of the chalky Boulder-clay, it will, I think, be apparent that had there been either a sea or a dry valley unoccupied by ice to receive it, the chalk debris, so far from being entirely absent in the clay would have been extremely abundant. None but those who have spent years in the examination of it over the greater part of the east of England can form an idea of the enormous volume of the chalk contained in the great Boulder-clay of the south-east. The proportion of this material may be estimated at from 10 to 90 per cent in different localities, the proportion being usually greatest in the counties of Norfolk, Suffolk, Essex, Hertford, and Lincoln — in the latter county, for a great distance along the western flank of the Wold, the clay being so nearly chalk itself as to be quarried for lime; and the quantity is still considerable in other counties, such as Huntingdon, Cambridge, Rutland, Leicester, Northampton, Warwick, Bedford, and Buckingham. Most of this chalk debris consists of lumps of rocky chalk of various sizes, unlike the soft material of which the upper Cretaceous formation of the south is principally composed, and so hard as to require a hammer to break it*. In these characters it is identical with the chalk of the Yorkshire Wold, which is all of this hard kind; and in it I have found, in sections where this chalky clay overlies the middle Glacial sands, rolled lumps of the red chalk which forms the base of the northern chalk, but is absent from the southern. The highest position at which the red chalk crops out in England is more than 300 feet below the higher elevations to which the chalky clay attains, and which were therefore under the sea when the red-chalk lumps, coming from a much lower level, were imbedded in its deposit.

If we consider the soluble nature of chalk, it must, as it seems to me, be evident that none of this debris can have been detached from the parent mass either by water-action or by any other atmospheric agency than moving ice.

The action of the sea, of rivers, or of the atmosphere upon chalk would take the form of dissolution, the degraded chalk being taken up in minute quantities by the water, and held in suspension by it, and in that form carried away; so that it seems obvious that this great volume of rolled chalk can have been produced in no other way than by the agency of moving ice; and for that agency to have operated to an extent adequate to produce the quantity contained in the great chalky clay before its denudation (a quantity that I estimate as exceeding a layer 200 feet thick over the entire Wold) nothing less than the complete envelopment of a large part of the Wold by ice for a long period would suffice. Nor, as it seems to me, can we explain the detachment of lumps of the red chalk from the outcrop of the parent stratum, far below the level reached by the sea that de-

into which the contorted drift passes, and of which great masses are also imbedded in the coast, or silty development of that deposit, is a soft greasy accumulation, formed out of the soft chalk of the cretaceous districts south of the Wold.
 * Quite different is the chalk in the Lower Glacial of Norfolk. The marl