GEOLOGY existing south-eastern river-system originated, and that the northern es- carpment of the Chalk commenced to be formed by the erosion of a river with its tributaries flowing towards the north. There is no reason to suppose that any great physiographic alteration has taken place since that time in the lower valley of the Thames, besides its deepening and its widening on the north where it has encroached upon the catchment- basin of the northerly-flowing river ; but many changes must have taken place in the course of the latter river during the long period in which it has been developing into the present sinuous Ouse. But the features then impressed upon this district were afterwards obliterated by the great ice-sheet which advanced from the north and ploughed up the surface, leaving its record in the chalky boulder clay which covers the greater part of Bedfordshire. This is not nearly the earliest of the Pleistocene deposits, earlier beds of a very varied nature, showing great changes to have taken place both in climate and in the distribution of land and water, having been deposited elsewhere. It is thus seen that a very long period elapsed of which there is no record in the county, even in Pleistocene times, and to this must be added the whole of the interval since almost the commencement of the Eocene period. It would be useless to give a sketch of the geological history of Britain during all this time, for how Bedfordshire fared would have to be left to the imagination. 1 What was the condition of the district when the chalky boulder clay was deposited, whether the land was sub- merged or not, is a disputed point; but from the presence here and there of marine shells in beds of gravel and sand intercalated with the boulder clay, it seems most probable that in the southern portion of its course the ice-sheet travelled over submerged land, abrading the submarine hills and filling up the submarine valleys by material dropped into the sea from icebergs or from the under-surface of the ice to which it was frozen and from which it would become detached on the partial thawing of the ice as warmer latitudes were reached. The northern land from which this ice-sheet came, whether Scandinavia or Scotland or no farther than Shap Fell, was no doubt much above the level of the sea, the submerged area probably extending over the whole of the midland and eastern counties, including Bedfordshire. The ' great chalky boulder clay,' as it has been called, covers an area in these counties of nearly 5,000 square miles, including nearly the whole of Bedfordshire except the valley of the Ouse, which has been cut through it, and part of the county south of Bedford where it occurs in patches. Portions of the hills of the Lower Greensand and of the Chalk are free from it, and in some places it is difficult to say whether it is present or not, for over the Oxford Clay and the Gault there has been such a mixture of the older and newer clays that the presence of the boulder clay can only be inferred by the drifted rocks and fossils which occur in it and work up to the surface of the land. This admix- 1 Part of this period is treated of in the V.C.H. Hertford, in the account of the Westleton Shingle and Middle Glacial sands and gravel, on pp. 18-23. « 25 4
