GEOLOGY above the sea ; no German Ocean and no English Channel then existed ; and animals were free to roam and plants to spread across the land which connected our country both on the south and on the east with the con- tinent of Europe. A period of gradual depression followed, the cold at the same time increasing, and during Pliocene times at first temperate and then boreal or arctic Mollusca teemed in the shallow seas and estuaries of the eastern counties, while remains of Mammalia, in grad- ually increasing quantities, were brought down by rivers from adjacent land. In all probability there then roamed over our county animals of the same species as those whose remains we thus find in the Crag (a Suffolk term for a shelly gravel or sand), but there is no evidence that our rivers flowed in that direction and contributed their sediments to any of the existing Crag strata. It is more probable that the rivers of Hertfordshire then flowed to the north, and removed thence vast quan- tities of Cretaceous and Tertiary strata, cutting off our Chalk from that of Yorkshire and Lincolnshire, and commencing to form the Chalk escarpment across the north-western margin of the county, its present features being due to subsequent erosion by the springs which form the sources of the Great Ouse. What was the cause of this gradual refrigeration of our climate, culminating in the Glacial epoch, has given rise to a vast amount of controversy. This is not the place to discuss it, but it may be mentioned that, without bringing in cosmical changes, a very great alteration in climate might be produced by a different distribution of land and sea. A deflection of the Gulf Stream which might be brought about by changes in the distribution of land so far off as the continent of America, might at any time give to our islands almost an arctic climate. It is necessary to look a little beyond our boundaries, for the student of Hertfordshire geology alone might be justified in assuming that there was a great gap between the Tertiary and Quaternary epochs, the greater part of the Eocene and the whole of the Oligocene, Miocene, and Plio- cene deposits being unrepresented in our county ; but in the eastern counties there is an almost unbroken sequence between the two, the Forest-Bed series, which is the newest of the Pliocene deposits, passing upwards almost imperceptibly into the oldest of the Pleistocene strata. The justification for the change of name from Tertiary to Quaternary lies more in the alteration in the nature of the deposits than in any decided physical or palasontological break, for we have no longer to deal with regularly stratified beds which can easily be correlated over wide areas. In the Pleistocene period marine gravels were being formed at one place while rivers were accumulating gravel of somewhat similar com- position in another not far distant ; mud was being deposited in the estuary of a river while on the sea-shore near it the wind was piling up heaps of sand, as in the present day. This great diversity of operations carried on at the same time makes the study of the Pleistocene period one of great difficulty, and our chief authorities vary greatly in their views. Much of the following attempt to show how this period is i 17 c
