360 ME. C. REID ON THE PLEISTOCENE 



similar to that found on the Sussex coast. We begin with Chalky 

 Boulder Clay : a natural equivalent of the lower-lying marine glacial 

 deposit 50 miles farther south. Next come fossiiiferous loams, full 

 of Corbicida, Succinea oblonga, and Uydrobia marginata, as in 

 Sussex ; and yielding also bones of Rhinoceros and Elephant, and 

 leaves of plants, among which I may mention the ivy, a speciCiS 

 which cannot bear great cold. Above the fossiiiferous loams usually 

 occur widespread sheets of rough flint-gravel and stony brick-earth, 

 with Palaeolithic implements and occasional bones, exactly as in 

 Sussex, except that the material, being derived largely from older 

 morainic deposits, and not directly from Cretaceous rocks, yields 

 little chalk. 



It has been usual to speak of the gravels in the Thames Yalley as 

 river- gravels, or terrace-graveis, marking successive stages in the 

 excavation of the valley. A study of the Sussex levels, however, 

 makes me incline to the opinion that the widespread sheets of 

 gravel in the Thames Yalley were deposited in the same way as the 

 similar sheets in Sussex. In short, they were not river-gravels, but 

 frozen- soil gravels, laid down on plains sloping gently towards the 

 Eiver Thames. They may have been deposited at all heights con- 

 temporaneously, as long as the slope did not exceed about 100 feet 

 in the mile. In favour of this view of the origin of the greater part 

 of the Thames Yalley gravel and brick-earth, may be mentioned the 

 rarity of all fossils except land mammals, and the absence of aquatic 

 species ; as is the case also in the Coombe Rock. There is moreover 

 the occurrence of old land-surfaces on which flint-chips were 

 scattered, and on which the flakes occur in close proximity to the 

 cores from which they were struck. The bearing of this last fact 

 seems to be important, for these floors may be covered with coarse 

 detritus, which needed considerable force to move it, and under 

 existing conditions would be accompanied by considerable erosion. 

 If, however, the climate were colder, the flakes would freeze into 

 the soil, and a sudden fall of rain might wash gravel or loam over 

 them, without disturbing the old surface, which at the time was 

 frozen into a solid rock. 



This view of the origin of the Thames Yalley gravels is so contrary 

 to the ordinarily accepted opinion that for several years I have 

 hesitated to bring it forward : but I may point out that it will 

 greatly simplify the geology of the district. Instead of our having 

 to deal with a complicated series of gravel-terraces formed when the 

 Thames stood at different levels, or formed during a Pluvial Period, 

 there is probably one sheet, belonging to a single period. Beneath 

 this sheet of gravel and unfossiliferous loam are found, at different 

 levels, relics of lacustrine strata belonging to an earlier mild period 

 when Corbicula fluminalis flourished, exactly as in Sussex. The 

 excavation of the Thames Yalley dates back to still earlier times, 

 when the river flowed parallel with the southern margin of the ice- 

 sheet, and received the sub-glacial drainage of this sheet as well as 

 that of the present catchment-basin. No trace of truly pre-glacial 

 deposits has yet been found in the Thames Yalley. 



