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 conditions 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 Basin, 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 continued during the Middle Glacial periods still proceeded; consequently the strong northern current was, as we have seen, gradually replaced by a more open sea. During the succeeding era the bottom of this sea became covered with a thick deposit of ice-transported clay. This clay, made up of chalk and the débris 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 covering 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 upward 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;
