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VI] now experience? For the proper understanding of many different problems it is essential to settle this point.

It is scarcely satisfactory to read history backwards, though geologists are often compelled thus to work from the known to the unknown. We will therefore not in this case ask our readers to follow us through the detailed evidence and arguments which have enabled geologists stage by stage to reconstruct the physical geography of this part of Britain as it was in days before written history They must take this preliminary work for granted, and allow the description of the changes to be taken in their correct historical order.

We need not go back far geologically. In late Tertiary (probably Newer Pliocene) times there was a ridge of chalk joining the range of the North Downs to the corresponding hills of France; but the divide between the North Sea and the English Channel was low at this point. Afterwards, during the Glacial Epoch, when an ice-sheet accumulated and blocked the northern outlet of the North Sea, the water was ponded back in the southern part. There was no easy outlet northward for the water of the Rhine and other great rivers, so the level of the North Sea rose slightly till it overflowed this low col and cut an outlet where lies the present Strait of Dover.

The general sea-level during this period of