Page:VCH Hertfordshire 1.djvu/61

 GEOLOGY only evidence in our county of such a mild interval is the presence of Ostrea edulis where the land must have been submerged at least 300 feet, for Stevenage now stands higher than that above sea-level. The land rose and the cold increased ; the snow-line gradually extended southward from the northern islands of Scotland to the south of Ireland and South Wales ; and glaciers descended from the snow- fields and ploughed up the land at least as far south as the Chalk of the eastern counties. The debris was deposited in the depths of the valleys and on the slopes of the hills, and even up to the top of the escarpment of the Chalk, as on Reed Hill near Royston, but none is to be seen on the higher part of the escarpment towards the west, which would then be an island in the Glacial sea. This is but one of many views which are held as to the conditions under which the ' great chalky boulder- clay ' was deposited, and it seems to be the most likely, but it has been well said : ' Where, as is too often the case with Glacial deposits, there is room for much diversity of opinion, geologists fully avail themselves of it. Hence it is best to picture the Glacial period in a general way, and to admit that glaciers and ice-sheets, icebergs and coast-ice, have all had their share in the production of the phenomena, although we cannot always localize their action. 1 The Upper Glacial boulder-clay (Middle Glacial of S. V. Wood) is generally known as the ' great chalky boulder-clay,' owing to the numerous boulders of chalk which it contains. It is usually a rather dark bluish-grey calcareous clay, containing chalk in all forms ground up with it, as small pellets or pebbles, and in all gradations of size up to very large masses, most of the larger chalk boulders being so hard as to have preserved, with the protection afforded by the clay in which they are imbedded, the scratches and grooves made by contact with harder rocks whilst they were being carried along imbedded in ice, this being the meaning of the somewhat misleading term 'ice-grooved rocks.' Imbedded in the boulder-clay are also many chalk-flints ; boulders from various formations, chiefly of rocks of Jurassic age, but also of much older and more distant strata, such as Carboniferous Limestone, deeply ice-grooved ; pebbles of quartz and small boulders of granite derived from formations still more distant both in time and space ; and fossils derived chiefly from the Lias and Oxford Clay. No fossils contempora- neous with its formation have been found in it. Boulder-clay is spread over the greater part of north-east Hert- fordshire as a continuous bed except where it has been cut through by the rivers ; it covers most of the higher ground in the centre of the county where the rivers have cut more deeply into it than on the east ; and the most south-westerly patch is at Bricket Wood between St. Albans and Watford. Sections of it may be seen there and at Little Berkhamsted, Bayford, Hertford Heath, Buntingford and several other places. 1 H. B. Woodward, The Geology of England and Wales, and ed. p. 486 (1887). 23