60 LONDON DISTRICT. 



formed in the same way ; the period of formation appears to be 

 that of the down-cutting between the Middle and the Low 

 Terraces and of the cold period at the base of the latter. 



A good example may be seen at East Wickham, where the 

 valley has been described as an ' adult coombe.' It is filled to 

 a maximum known depth of 30 ft. with brickearths, gravels and 

 chalk rubble (Coombe Rock) which have yielded mammalian 

 fossils including the musk-ox (Ovibos moschatus}. 1 At Wallhouse 

 Farm, between Erith and the mouth of the Darent, a chalk 

 rubble has been noted beneath low -lying gravel; 2 where such a 

 rubble is overlain by Pleistocene deposits its upper surface is 

 frequently re-cemented by lime, a phenomenon which sometimes 

 serves to distinguish Eocene deposits in situ, beneath which this 

 ' calcrete ' is never found, from similar material subsequently 

 moved and re-arranged. The deposits in the valley leading to 

 Greenhithe have already been mentioned ; they can be seen 

 to a depth of 20 ft. and may be much thicker; neither fossils 

 nor implements have been found here, but at Northfleet, a few 

 miles to the east, remains of mammoth and implements of 

 Mousterian type are abundant on the surface of the solid chalk 

 beneath Coombe Rock. 3 Coombe Deposits may also be seen in 

 the Chalk area of Purfleet and West Thurrock. 



It is probable that much of the gravel found in the dry valleys 

 of the Chalk area is of the nature of a Coombe deposit, as it 

 shows little or no sign of stratification by flowing water; the 

 process of formation may be slowly continuing at the present 

 day. In the upper parts of the Darent Valley, as noted by 

 Prestwich, 4 there are accumulations of white chalk rubble which 

 merge downwards into undisturbed Chalk. The rubble contains 

 broken flints and occasional Eocene flint-pebbles, and appears 

 to be of older date than a local red rubble which has been derived 

 by downwash from the Clay -with -flints. In the gravels of the 

 Ravensbourne valley near Hayes Common and West Wickham 

 remains of reindeer, mammoth, horse, and rhinoceros have been 

 found, and at Green Street Green in the upper part of the Cray 

 Valley the musk-ox occurs as well. 



With the Coombe Deposits we may describe what is known as 

 'Trail,' a term introduced by the Rev. 0. Fisher in 1866 to 

 include tumbled and frequently contorted material which forms 

 the surface layer over wide areas ; it appears to have been formed 

 by movement down a slope of semi-fluid masses comparable with 

 ' the flowing soils of northern lands which arise from the snow 

 and frozen earth partially thawing in summer and sliding down- 

 hill with their burden of rubbish.' Here and there such flows 

 have dragged up the underlying strata into waves or festoons 

 which may be overf olded in the direction of movement ; in these 



1 ' Geology of London ' (Mem. Oeol. Surv.), vol. i, 1889, pp. 431, 432; Proc. 

 Geol. Assoc., vol. xix, 1906, pp. 341-347. 



2 Proc. Geol. Assoc., vol. xxv, 1914, p. 66. 



3 Smith, R. A., and H. Dewey, Archceoloyia, vol. Jxiv, 1913, p. 195. 



4 Quart. Journ. Oeol. Soc., vol. xlvii, 1891, pp. 126-163. 



