COOMBE DEPOSITS AND TRAIL. 61 



cases, where parts of the contorted material are in continuity 

 with that which is undisturbed, the term ' underplight ' has been 

 used by Spurrell. 1 Though the formation of Trail may to a 

 small extent have taken place throughout a long period and 

 even continue at the present day, there are two lines of evidence 

 which suggest that it belongs mainly to a single definite epoch. 

 The phenomenon may frequently be observed in beds of all 

 ages up to the Low Terrace gravels, into which the Trail appears 

 to merge ; and mammals and other fossils indicative of late 

 Pleistocene age have frequently been found, as, for instance, 

 hippopotamus at Wembley Park 2 and mammoth at New Maiden 

 in Trail consisting mainly of London Clay; again the character 

 of the deposit strongly suggests a cold climate. The combined 

 evidence points to a period immediately after the Middle Terrace ; 3 

 in other words, the Trail is equivalent to the Coombe Deposits 

 but is material which has not actually reached a position of rest 

 at the bottom of a valley. 



A good example of Trail was described by Hudleston on 

 St. George's Hill, where laminated lilac clay of the Bracklesham 

 series may be seen to be nipped up and contorted (the under- 

 plight), the hollows being occupied by disturbed masses of gravel, 

 with green sands and loams derived from higher parts of the 

 Bracklesham Beds. He remarked ' it is not difficult to imagine 

 that, when the climate was colder, the steep northern slope of 

 St. George's Hill was occupied by a kind of neve, or a sliding mass 

 of indurated snow. This, adhering to the greensand beds, caused 

 large masses of them to slide downwards involving also a portion 

 of the original plateau; hence the occasional jambs of flint 

 gravel.' 4 



To the same group of phenomena belongs the descent of the 

 Upper Terrace gravels, at their margin, to rest on the surface of 

 the Middle Terrace, between West Dray ton and Ealing. Here 

 and there Mousterian working-sites have been discovered on the 

 surface of the old Middle Terrace preserved by the overlying 

 Trail from the terrace above (p. 68), affording another indication 

 of the date of the movement. Throughout the London Clay 

 area Trail is constantly found, and frequently small deposits of 

 gravel, which may yield fossils or implements, are completely 

 buried. Mr. G. E. Dibley tells us that he has detected gravel 

 terraces at many points along the western bank of the Pool 

 River, but owing to the layer of clay that has slipped from the 

 higher ground they can be seen only when excavations are made. 

 There are notable instances of Trail in the brickearth and gravel 

 that extend eastwards from Ilford to Purfleet and Grays. In 

 the tract near the latter town the superficial disturbances, very 

 suggestive of glacial action, have been attributed by Mr. T. V. 



1 ' History of the Rivers and Denudation of W. Kent,' Greenwich, 1886. 



2 Quart. Journ. GeoL Soc., vol. xlviii, 1892, p. 468. 



3 Dewey, H., Proc. Prehist, Soc. E. Anglia, vol. ii, 1915, pp. 114, 115. 



4 Quart. Journ. GeoL Soc., vol. xlii, 1886, pp. 169, 170. 



C 2 



