Page:VCH Hertfordshire 1.djvu/64

 A HISTORY OF HERTFORDSHIRE covered nearly 1,000 feet beneath the surface in south-east Herts has the Devonian rock tilted 25 from the horizontal, and the Silurian about 40. Whilst great changes in the relative distribution of land and sea were taking place elsewhere in Britain during Carboniferous, Permian, Triassic, and earlier Jurassic times, we have no evidence that this tract sank entirely beneath the sea until we come to the Upper Cretaceous period ; but the submergence of its north-western flanks began in Upper Jurassic times ; in Lower Cretaceous the sea had reached almost to its highest point ; and it was entirely submerged during the whole of the Upper Cretaceous period, except perhaps towards the close of the deposition of the Chalk. Whether it was a land-area whilst the higher beds of the Upper Chalk were forming in the south of England, and still later whilst the Maestricht Beds of Germany were being deposited, we have no evidence to decide, but we do know that a vast amount of chalk has been carried away. Most probably it is the higher beds of the English Chalk only which have been removed by denudation, Hertfordshire, with the rest of the British region, being above the sea when the Maestricht Chalk was deposited. In early Eocene times Hertfordshire was again beneath the sea, but not at first deeply submerged. Scotland, Ireland, Wales, and the north and extreme west of England formed parts of a land-area from which sediment was brought down by rivers flowing towards the west, and for a time the climate was tropical. As the land again rose the climate became temperate, and in the Miocene period there was much volcanic activity in the north-west of Britain which probably caused some disturbance of the strata in our area. In the Pliocene period the county was apparently subjected to a vast amount of sub-aerial denudation, and then the sea encroached upon it and its diversified surface was levelled to a considerable extent into a plain of marine denudation. With the next rise of the land the climate became of arctic severity and a great ice- sheet came from the north as far as the range of London Clay hills, still further levelling the land by its erosive action. Submergence followed, and the climate became milder, but only to again become arctic with partial emergence. As the land continued to rise however, and the surface began to assume its present aspect, the climate gradually became temperate, and the most important event of all in the history of the county came to pass the advent of man by migration from the south. It was at about this time also that England was finally severed from the continent of Europe by the formation of the Straits of Dover. The rainfall then was heavy, much of our present land was under water, our rivers rose higher up their valleys and were often in flood, streams ran down our present dry valleys or combes, and most of the county was densely wooded. Man then, probably naked and living much in trees by the margins of lakes or swamps, had to contend with the wild beasts for existence with no other weapons than such as might be made by chipping one flint with another; or perhaps to seek safety first in climbing trees, and then, gaining some insight into constructive art, by 26