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." 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 Y.C.H. Hertford, in the account of the Westleton Shingle 
and Middle Glacial sands and gravel, on pp. 18-23. 
I 25 4 
