52 LONDON DISTRICT. 



(below). On the existence of such a cold stage all lines of argu- 

 ment converge and all authorities agree, but the brief outline 

 of the palaeontology given above is sufficient to draw attention 

 to a curious anomaly. It has been assumed in the previous 

 chapter that the Chalky Boulder Clay is older than the oldest 

 (Boyn Terrace) river gravels ; but the fossils of that terrace 

 indicate a genial climate and seem to connect it with the Pliocene 

 epoch : for instance, the giant beaver (Trogontherium cuvieri) 

 found near Swanscombe is well known from the pre -Glacial 

 ' Cromer Forest Bed.' 1 At the same locality a mollusc (Theo- 

 doxus crenulatus Klein, better known as Neritina grateloupiana) 

 is found in abundance and is otherwise known only from the 

 Upper Miocene of Germany. It is urged by some of those to 

 whom we are much indebted for detailed work on the Pleistocene 

 mollusc a that the fossils indicate no break such as might be 

 caused by an Ice Age between the Cromerian and the Boyn 

 Terrace stages, but do show such a break in late Pleistocene 

 times, after the beds with arctic plants just mentioned were 

 formed. Accordingly the Chalky Boulder Clay must be newer, 

 not older, than the river terraces. Since, however, only two 

 species of mollusca, found in the Cromerian and in the river 

 gravels, died out as a result of the late Pleistocene cold period, 

 and 43 are still living in Britain, 2 this argument from their 

 survival-values is not strong; and the possibility that some of 

 the Swanscombe mammals and other forms are derived from 

 the destruction of older deposits, and not contemporaneous, 

 must not be overlooked. Were the Chalky Boulder Clay newer 

 than the river terraces the section at Hornchurch, where 

 T. V. Holmes described the river -gravels as resting on a shelf 

 cut in that boulder clay, must have been wrongly interpreted ; 

 it was, however, accepted, while still visible, by many expe- 

 rienced geologists. 3 The advocates of a late Pleistocene date 

 for the Chalky Boulder Clay have yet to produce a plausible 

 explanation of the presence in all the river terraces of Bunter 

 pebbles, igneous rocks s etc., usually considered to have been 

 brought into the area by the ice. On the whole, we consider 

 that the traditional view of the sequence of events must still be 

 maintained. 



Among the more interesting fossils found in the river gravels 

 and brickearth and not yet mentioned are the bison and urus, 

 wolf, beaver, great Irish elk (moose), hippopotamus (very frequent 

 in the Flood-plain deposits), two species of rhinoceros, saiga 

 antelope, brown and grizzly bears, a monkey and an interesting 

 series of lemmings and voles which has been specially studied bv 

 Mr. M. A. C. Hinton. 4 "_ 



1 Newton, E. T., GeoL Mag., 1902, pp. 385-388. 



2 Kennard, A. S., and B. B. Woodward, ' Post Pliocene Non -marine 

 Mollusca of Ireland,' Proc. GeoL Assoc., vol. xxviii, 1917, pp. 109-190 and 

 table. 



3 Holmes, T. V., Quart. Journ. GeoL Soc., vol. xlviii, 1892, pp. 365-372; 

 vol. 1, 1894, pp. 443-452 and discussions. 



4 Proc. GeoL Assoc., vol. xvii, 1902, pp 414,415; vol. xx, 1908, pp. 39-58; 

 vol. xxi, 1910, pp. 489-507. 



