198 W. H. PENNING ON THE PHYSICAL GEOLOGY OF 



have seen, if they had had access to the valley, and had not been ex- 

 cluded 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 pre- 

 glacial times, and that in all probability no Lower or Middle glacial 

 beds were formed within its area. But when the Chalk was wholly 

 submerged t, 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 debris 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 occur- 

 rence, and that its present boundary-lines would ignore all the con- 

 tours, 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 

 * Ante, p. 194. t Ante, p. 195. 



