EAST ANGLIA DURING THE GLACIAL PERIOD. 195 



which are much rolled and waterworn. In the exceptional cases 

 where shells do occur, they are found to be of such species as would 

 perhaps indicate a climate warmer than that which prevailed before 

 and after the Middle Glacial period. This fact is probably due not 

 so much to a real difference of climate as to the temperature of the 

 current, which would be governed by its direction and the condi- 

 tions affecting it before reaching these islands. 



If these views be correct, there is between the Lower and Middle 

 Glacial no definite line of demarcation. 



Upper Glacial. — The Great Chalky Boulder-clay in East Anglia 

 extends in mass, or in patches that once formed part of the main 

 mass, over the northern half of the London Easin, the Pliocene area, 

 and the dip slope of the Chalk, rises over the escarpment of the same 

 formation, and plunges down into the Cambridge valley. It now 

 caps the highest hills; and it occupies the deepest valleys, except 

 where it has been removed by recent denudation. 



The submergence which began with the Lower Glacial and con- 

 tinued during the Middle Glacial periods still proceeded; conse- 

 quently the strong northern current was, as we have seen, gradually 

 replaced by a more open sea. During the succeeding era the bot- 

 tom of this sea became covered with a thick deposit of ice-trans- 

 ported clay. This clay, made up of chalk and the debris of other 

 rocks also found to the northward of the area, having been brought 

 down and dropped in masses, presents within itself no traces of 

 stratification. Still it is seen in most instances resting evenly on 

 the Middle Glacial beds ; it is, indeed, as a bed, stratified with them, 

 although its own structure is not the result of stratification. There 

 are no signs of grinding or thrusting of their surface, such as 

 must have been apparent had the clay been formed beneath a cover- 

 ing of ice sliding over the land. This latter mode of formation is 

 accepted by many as a true explanation of the phenomena presented 

 by the Boulder-clay ; but for this reason especially I differ from that 

 conclusion. Moreover, if the clay be a direct result of ice moving 

 over the land, an emergence after the deposition of the Middle 

 Glacial gravels must have intervened ; but of this there seems to be 

 no evidence beyond the disputed point of the method of the clay's 

 formation. 



In this district we have no indications of the greatest depth of 

 the glacial submergence ; it was probably many hundreds of feet. 

 That it continued for a lengthened period is certain, judging from 

 the great thickness in many places of the Boulder-clay. Whatever 

 oscillations of level may have occurred elsewhere during the Glacial 

 period, there appear to be in East Anglia no marks of any but one, 

 and that a gradual and long-continued movement of depression 

 succeeded by another, equally gradual, of reelevation. 



Postglacial. — When at length the land again assumed an up- 

 ward movement, and as it rose from beneath the sea, every part in 

 turn, as a receding shore-line, would be subject to the action of the 

 waves, and the surface of the Boulder-clay thereby eroded and to 

 some extent reassorted. A clayey gravel would naturally result ; 



