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 glaciation seems to have been a few feet higher than that of the present day, for glacially transported erratics are found strewn over the flat coastal plain of Sussex. One erratic block, probably derived from the Channel Islands, was discovered under the loess as far east as Sangatte cliff, close to Calais. The icy English Channel must therefore have met the icy North Sea some time during the Glacial Epoch. Some time after the cold had passed away there came in the period with which this book deals—when the lowest submerged forest flourished, on land now 50 or 60 feet below the sea. This elevation of the land, as already shown, converted a great part of the North Sea into a wide alluvial plain. At the same time it raised above the sea-level and obliterated the newly-formed strait, leaving it in all probability as a shallow valley sloping both ways and filled up with alluvium. The Strait of Dover was again a watershed, or perhaps its position was occupied by a small stream, which may have flowed in either direction. Thus the work done during the Glacial Epoch was almost cancelled and had to be done again; but now there was merely a low narrow divide of chalk and a strip of marsh between the two basins, and the chalk ridge was steadily being attacked by the waves of the sea from the west.