A HISTORY OF HERTFORDSHIRE Hertfordshire the views of Sir Joseph Prestwich have been adopted, but it should be mentioned that they are not universally accepted. The correlation of these gravels with the Westleton Shjngle of Westleton has been disputed, and Mr. Clement Reid 1 now believes that the Stanmore gravel ' presents all the characteristics of an Eocene deposit.' He also remarks that ' It now seems doubtful whether outside the glaciated area any plateau gravels (i.e. gravels more than about 150 feet above the Thames) are to be found that are not either of Eocene age or derived wholly from Eocene deposits at a higher level.' A gravel derived from Eocene deposits may be of Westleton or any age subsequent to Eocene, but the higher the level the older is the gravel likely to be, and Prest- wich considered the 'southern drift' of Stanmore Heath to be older than the Westleton Shingle. After the deposition of this high-level shingle, which, with the Tertiary strata beneath it, then extended at least nearly to the edge of the present escarpment of the Chalk, the land gradually rose and the cold gradually increased until arctic conditions prevailed. A great ice- sheet spread over the Scandinavian peninsula and crept southwards over northern Britain, covering the whole of Scotland and nearly the whole of Ireland and the north of England, and extending over the Midland and Eastern counties including nearly the whole of Hertfordshire. Here it planed off the Chalk, cut away the Tertiaries, and carried off most of the Westleton Shingle, devastating the county as far south as the hills of Brockley, Elstree, and Stanmore. At the period of its greatest advance the ice-sheet terminated just north of these hills ; snow-fields rested on the highest points in North Wales, Ireland, and northern Britain, glaciers descending from them and adding their quota to the field of ice ; and here and there where evidence of ice-action is wanting there appears to have been an exposed surface of land. Eng- land was then joined to the continent of Europe, perhaps only between Kent and Normandy except by means of the ice-sheet on the north, the English Channel even then dividing the south of England from Brittany, while the Atlantic Ocean existed on the west. The climate becoming milder, the ice-sheet receded, and as the ice melted, liberating a vast volume of water, the resulting rivers took its place as a denuding agent, excavating the valleys of the Colne and Lea. It was also probably soon after the retreat of the ice-sheet that, with a very heavy rainfall, the greatest deepening of the valleys of the Thames and Ouse took place, and the present general features of the escarpment of the Chalk were impressed upon it, but in quite a different manner from that in which the Tertiary escarpment was formed. The Chalk escarpment has been and is still being cut back by the springs which issue from the Totternhoe Stone at its base, mostly at right angles with the strike of the Chalk ; the cutting back of the Tertiary escarpment is effected by the rivers which flow along its foot 1 Summary of Progress of the Geological Survey for 1899, p. 140 (1900). 20
