18 THE VARIOUS DRIFT DEPOSITS 



3. DRIFTS OF DIFFERENT AGES AND ORIGINS. 



It was soon discovered that the superficial beds of 



Drift, instead of being the result of one Cause and 



belonging to one time, were the result of many 



agencies acting at different times and under different 



conditions. Thus the beds of gravel, loam, and 



sand lodged, especially in the north of France and 



the south of England, on the slopes of many of 



the river valleys, are now known to have been 



deposited by the same rivers in their earlier stages, 



when, before they had excavated their channels to 



the present depths, they flowed at various higher 



levels. That this was their origin is shown by 



the fact that these deposits contain fluviatile shells, 



mostly such as now live in our present rivers, 



together with the rolled and worn bones of land 



animals of species now extinct. The preservation of 



such remains is, however, partial and irregular, partly 



because the beds in which they are entombed are in 



general so permeable that the percolation of the 



surface waters has very commonly removed the 



chalky or calcareous matter of the bed itself together 



with that of the embedded shells and bones ; but 



where these have been protected by beds of loam or 



clay, the organic remains are often well preserved. 



The higher and older the terrace, the more rare are 



the organic remains. There are good examples of the 



old river beds at Grays, Erith, Crayford, Oxford, etc. 1 



1 The sections vary from year to year. 



