Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/270

198 have seen, if they had had access to the valley, and had not been excluded therefrom as suggested, some undoubted traces of the series of gravels and sands would remain and testify to the fact. There are certain gravels which, I admit, bear a striking resemblance to those of undoubted Middle Glacial origin; but my reasons will presently be given for believing them to be of much more recent date. In some cases the Boulder-clay has a gravelly base; but it is that and nothing more, representing the gradual change of physical conditions which occurred when the Chalk-ridge was nearly submerged. At this time, although the southerly set of the water still continued, its currents possessed much less power of transporting and rearranging gravels, and their deposition ceased. But the change was gradual; and we may readily conceive that during the transition -period the surface of the Chalk, being slowly encroached upon, would be partly covered over with a wash from the rock itself, and perhaps also from the gravels. This wash is not a clean gravel; and it gradually passes up into the Boulder-clay, which then began to be deposited.

We have seen that the Cambridge Valley was excavated in preglacial times, and that in all probability no Lower or Middle glacial beds were formed within its area. But when the Chalk was wholly submerged, the sweeping currents, hitherto confined to the east side of the range, were replaced by an open sea extending over every part of East Anglia that does not now attain to an elevation of 500 feet. In this sea the icebergs laden with rocky débris from the north were slowly melted; and their freights, descending in mass, formed on the bottom the unstratified Boulder-clay. There had been icebergs borne along in the Middle-glacial currents also; but they were swept off more quickly to the southward, and probably a great majority of them may have melted over an area still beneath the waters of the Atlantic.

The Upper Boulder-clay is found all along the top of the Chalk range; it caps the minor elevations on the flank, and it occupies some of the lower ground in the valley. At one time it doubtless spread as a sheet over the whole area, from the highest point of the 'scarp down to, if not below, the present sea-level. It has since been so much denuded that its main mass on the hills is disconnected from the remainder, which now exists merely as outliers on the smaller hills and ridges.

Noting that the Boulder-clay is found on the back and top of the escarpment, as well as on the low ground beyond the foot of it, we might assume that there is no "horizontality" in its mode of occurrence, and that its present boundary-lines would ignore all the contours, and features even, of the country. To some extent, and in small areas, this may be so; but looking at the Boulder-clay as a whole, there is a striking regularity in its occurrence. When viewed on a true scale, in the exceptional case of the Chalk escarpment it is seen to plunge down about 500 feet; but this being in a distance of not less than 10 miles, represents a fall of 1 in 100 only, or an