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 A HISTORY OF HERTFORDSHIRE It is evident that boulder-clay once filled up most of our valleys as well as covered all but our highest hills. In the east of the county the sources of our rivers are upon it ; the rivers then cut through it in places into the Chalk ; and lower down their valleys the boulder-clay has been completely cut through by them, exposing the underlying Glacial gravel and sand. After the deposition of the boulder-clay the land again rose, but not for some time to its present level, the old shore-lines which pass through the southern counties, where they have left their mark in beach-shingle and sea-cliffs, being evidences of successive elevations of the land, at one time up to 140 feet below its present level, and subsequently to within a few feet of it. 1 The shingle-beaches at various levels indicate pauses in the upheaval, and by the time the sea had receded (or rather the land had risen) so far as to form a shingle-beach at least 100 feet higher than that now forming on our coasts, the arctic climate had given place to one milder than that which now prevails. This period of upheaval marks the time when marine gravels finally ceased to be formed above the present sea-level, giving place to estuarine, alluvial, and lacustrine deposits. An instructive example of the latter is the ancient Hitchin lake-bed, for the most complete knowledge of which we are indebted to Mr. Clement Reid, 8 although it has also been investigated by Sir John Evans, Mr. William Hill, Mr. William Ransom, and other Hertfordshire geologists and archsologists ; for the formation of this lake-bed and the overlying deposits embraces the period during which the study of geology gives place to that of archasology, bringing Man upon the scene. The alluvial or lacustrine deposits known as the Hitchin lake-bed lie in a channel or trough running nearly north and south, which appears to have been excavated, or re-excavated, after the deposition of the boulder-clay, the geological position of the lake-bed being between the great chalky boulder-clay, representing the close of the Glacial period in this neighbourhood, and the brick-earth in which Palasolithic flint implements are of frequent occurrence. The deposit is very variable in character, consisting of sandy, marly, and loamy beds, white, yellow, brown, and black, sometimes, from the abundance of decomposed plant- remains, even forming a lignite. It contains the teeth and bones of several mammals and fishes, remains of a few insects, the shells of many molluscs, the leaves and seeds of numerous flowering plants, several mosses, and a few charas. Most of the species still exist with us, but all the mammals have long been extinct in Britain in a wild state, and two, the mammoth and woolly rhinoceros, are altogether extinct. Their remains have all been found in a whitish marly silt which occurs locally above the deposits from which all the other fossils have been obtained. On this silt rests the Paleolithic brick-earth, which until recently yielded 1 Clement Reid, Victoria History of Hampshire, vol. i. p. 23. ' The Paleolithic Deposits at Hitchin and their relation to the Glacial Epoch,' Prac. Royal Soc., vol. Ixi. p. 40 (1897) ; Trans. Herts Nat. Hist. Sac., vol. x. p. 14 (1898). 24