GEOLOGY nearly parallel with the strike of the rocks (Chalk and Tertiaries). The Lea was then probably flowing off the high land of which the remnant still left is now known as part of the Chiltern Hills ; between Hatfield and Ware it has followed the trend of the Tertiary escarpment, cutting it back ; and at Amwell it has cut through this escarpment. It would appear that a vast amount of denudation must have taken place on the north-west of the present range of the Chilterns in this district since the Lea commenced to flow towards the south-east, for its source must then have been very much higher than it is now, other- wise it would have flowed in the opposite direction ; but whether this so-called ' Luton gap ' was caused by river- or ice-action is uncertain. The river which takes the name of Ver above Bricket Wood Common and of Colne below it, flowed from the same hills near Kensworth, but only from the south-east, not the north-west side of the present range, and meeting with the Tertiaries south of St. Albans has almost ever since been eating back the Tertiary escarpment past Watford and Rick- mansworth, increasing its steepness, and at Harefield it has cut through it. The chief tributaries of these rivers also flowed from the Chilterns and helped forward their erosive work, especially at the points of juncture. Although some of the main features of the county may date from Pliocene or even Miocene times, when its surface may have been even more diversified than it is at present, there can be little doubt but that many of those features were much modified over the greater part of the area during Lower Glacial times, and that it is only to the period immediately following the recession of the great ice-sheet that we can with certainty trace back the origin of our present river- valleys in the Chalk, the rivers then flowing at a higher level than they do now, but in the same general direction. The land then sank and the sea gradually encroached upon it, the extent of the submergence being much greater in the north than in the south of Britain. In the Lake district of Cumberland stratified gravels of this period are found 1,600 feet above the level of the sea, and on Moel Tryfaen in North Wales there were then deposited sands and gravels since raised 1,350 feet, containing shells of species of Mol- lusca still living in the seas around Britain. There is a similar assem- blage at Macclesfield 1,200 feet above the sea, and from the height at which the flint-gravel sand, and clay of this the Middle Glacial period are found in the south of England we may infer that Hertfordshire was depressed at least 500 feet below its present level. During this period of greatest submergence in Pleistocene times the British Islands formed a scattered archipelago ; the highest mountains of the north of Scotland were the only islands with their summits above the snow-line, for with the submergence the climate became milder, this period being an in- terglacial one ; and in Hertfordshire portions of the Chiltern Hills appeared as islands probably nowhere exceeding 200 feet in height. These Middle Glacial gravels are the ' Gravels of the Lower Plain ' first described by Professor Hughes. They vary much in composition 21
