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.
When subsidence again set in the strip of alluvium was soon submerged and the two seas again met; but in all probability for a long time the Strait was only a narrow one, over which animals could easily swim. Then tidal scour, deeper submergence, and the action of the waves did the rest, so that ever since that time the Strait of Dover has been getting steadily wider and wider, and also deeper. Its bottom is to a large extent composed of bare chalk with patches of gravel; and the movement of this gravel during storms, combined with the action of boring molluscs must slowly eat away the chalk far below ordinary wave-action.
The above explanation is needed, for it will not do to take existing soundings, and say that all the sea-bottom below a certain level, corresponding with a particular submerged forest, was then sea and all above was then land. This is an easy way of reconstructing the physical geography; but it may be a very misleading one. A little consideration will show that whilst in large areas sand-banks have accumulated to a great thickness, in other areas, of which we know the Strait of Dover is one and the Dogger Bank a second, there has been much submarine erosion, which is still going on. In neither case is it safe entirely to reconstruct the ancient contours from the present-day soundings.
Even such a gigantic feature as the continental platform, which ceases suddenly at a depth of 100 fathoms, is in all probability in the main a feature formed by the deposition of sediment during long ages. Its outer edge marks, not the limit of some ancient continent, but the limiting depth at which gentle wave-action has been felt, and beyond which the sediment cannot be carried.
After this necessary digression we must return to our study of the actual evidence for such changes of sea-level in the English Channel. It has been pointed out already that for this purpose the present depth below sea-level of the rocky floor of the Strait cannot in itself be accepted as sufficient evidence. Nor can the depth at which rock was met with under the Goodwin Sands; though here a cylinder was sunk 75 feet before it reached the chalk. Unfortunately no record of the strata passed through seems to have been preserved, though it is perhaps implied that nothing but sea-sand was penetrated.
Romney Marsh is a wide alluvial flat occupying a silted-up bay, the floor of which in places lies at least 70 feet below sea-level. There are here unfortunately no extensive excavations for docks, and all we can say is that the few borings which have penetrated the alluvial strata prove the existence of a slightly undulating rock-surface below. In short Romney Marsh appears to be a submerged flat-bottomed open valley, like that which we have already seen underlies the marsh deposits of the Fenland.