LOW TEERACE. 59 



End ' (op. cit., p. 222, footnote). The maximum thickness at 

 Ponder 's End is 22 ft., including a few feet of the brickearth 

 overlying the gravel. 



East of the Lea the division between the Middle and the Low 

 Terraces exists, but is frequently * buried ' and difficult to follow 

 exactly ; on the south of the river it hardly exists. 



On the south side of the Thames the low gravels are well 

 developed ; west of the Wey we find a large area, partly within 

 the map, between Egham and Thorpe, and an eyot of gravel on 

 which Chertsey is built. The great spread near the mouths of 

 the Mole, between Walton and Ditton, may be connected with the 

 change in the course of that river (p. 64). Ham Common again 

 is formed of Low Terrace gravel, the Thames having now migrated 

 to the west from its old cliff between Kingston and Richmond. 

 Similarly, from Richmond to Greenwich we find areas of low- 

 level gravel included between the rising ground of the London 

 Clay and the successive northerly loops of the present course of 

 the river. Probably this deposit is as a whole newer than the 

 buried channel, since that feature can be traced through it, 

 and the beds with late implements and that with Pseudunio 

 auricular ius (Spengler) occur. 



From Greenwich to Erith the Holocene alluvium abuts 

 against the Chalk and Tertiary rocks. In the bay between 

 Erith and Greenhithe at the mouths of the Cray and Darent 

 some of the gravel is, no doubt, of the same age as one or both 

 parts of the ' Low Terrace ' ; but it is continuous with deposits 

 which must clearly belong to the Middle Terrace. Mr. R. H. 

 Chandler has shown that this gravel can be traced from beneath 

 the Crayford brickearth to about 100 ft. below Ordnance datum. 1 



The Coombe Deposits and Trail. Besides the bedded gravels 

 and brickearths which have been laid down at various periods 

 by the rivers, there are deposits which, although occurring in 

 the valleys, show little sign of arrangement by running water. 

 In many of the smaller valleys, more particularly in Kent, we 

 find the bottom filled with an accumulation of mixed material 

 to which the name ' Coombe Deposits ' may be given. 2 These 

 masses have been formed by the descent of material from the 

 sides of the valleys and take their character from that of the 

 deposits available at any spot : thus, in the valley starting near 

 Bean and opening into the Thames at Greenhithe (Sheet 4) the 

 Coombe Deposits include gravel from the High Terrace deposits, 

 a sandy loam formed of Tertiary material and best described 

 as brickearth -it has, indeed, been used on a small scale for 

 brickmaking and chalk rubble. The last resembles the deposit 

 known as * Coombe Rock,' the origin of which has been discussed 

 by Clement Reid, 3 and the whole assemblage has probably been 



1 Proc. Oeol. Avsoc., vol. xxv, 1914, pp. 61-71, and fig. 3. 



2 ' Summary of Progress for 1920 ' (Mem. Geol. Surv.), 1921, p. 9. 



3 Quart. Journ. Geol. Sot., vol. xliii, 1887, pp. 364-373. 



x 17412 C 



