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 GEOLOGY the greater part of the country north of the Thames was covered by a vast accumulation of snow and ice, there is much reason to assign the formation of some of the older gravel-deposits of Surrey to this Glacial Period. The ice-sheet during its maximum development spread south- ward as far as the northern margin of the Thames Valley, but there is no evidence of a permanent ice-field to the southward of that valley. Thus the conditions in Surrey at this time would be peculiarly favourable to rapid erosion ; for although the ice-sheet itself planes away the land in its gradual outward flow, it also protects the surface from the severer action of streams and of alternate freezing and thawing. We may picture the Surrey of the Glacial Period as a bleak tract lying just beyond the margin of the ice-fields ; its surface frozen and rendered impervious by the autumn frosts ; then deeply covered by the winter snows ; to be drenched and torn during the thaw in the late spring by the sudden re- lease of the waters. And as the land probably ^tood higher above sea- level than at present, the torrential denudation during the short summer may have been extremely great. The turbid rivers, laden with mud from the disintegrated clays and with stones from the harder strata, deepened their channels rapidly, and cast down wide sheets of detritus wherever their course was checked. Hence, throughout all the Glacial time there was everywhere in the county a rapid wasting of the hills and slopes, and a transference of the material to lower and lower levels. In the vicinity of the main valleys, like that of the Thames, it is more or less difficult to distinguish between the lower portion of the high-level accumulations and the higher of the deposits clearly connected with the existing valley, although in some other districts the distinction seems well marked. It is probable that at the close of the Glacial Period there was no such radical change in the conditions in this area as in places which had been actually overridden by the ice-sheet, but only a gradual and progressive amelioration by which the past was merged insensibly into the present. The streams and rivers still maintained their courses and continued to deepen their channels, but with diminishing activity as their flow diminished ; until they reached their present shrunken state, in which the main streams are able to transport only the finer detritus, while many of the smaller head-valleys, especially those traversing the more porous strata, are no longer able to maintain a permanent stream. The older of the undoubted valley-deposits are especially interesting from the fact that they yield the earliest indications of ancient man, in the form of coarsely-chipped implements of flint, associated with the remains of extinct animals. 1 Though these ' Palaeolithic ' implements have not been obtained so plentifully in Surrey as in some of the neighbouring counties they have already been recorded from many localities, and fresh discoveries are constantly being made as the search by qualified observers 1 For numerous records of Palaeolithic and Neolithic implements in Surrey consult Sir J. Evans' Ancient Stone Implements, 2nd ed. (1897), where references to previous literature will also be found. 25