MIDDLE TERRACE. 57 



On the south side of the Thames we find Middle Terrace 

 deposits of the Wey and Mole, consisting mainly of sand from 

 the Bagshot Beds; these may be traced to Surbiton and also 

 occupy much ground (more than is shown on the map Sheet 3) 

 in the valley extending from Kingston to the Wandle at Merton ; 

 the meaning of this distribution is discussed below (p. 64). 

 Narrow bands of Middle Terrace gravel of the Thames are found 

 at Mortlake and Putney, but further east the river has cut away 

 all the deposits, and low level gravels or alluvium abut against 

 a ' cliff' of Tertiary Beds and Chalk as far as Erith. The 

 southern tributaries, however, have important gravels of the 

 Middle Terrace. Those of the Wandle form the surface around 

 Mitcham and as far south as Beddington; they have yielded 

 fossils and implements. It appears probable that the gravels 

 in the valleys of the Ravensbourne, Cray, Darent and other 

 tributaries belong in the main to the Middle Terrace period ; in 

 several cases the streams now rise far below the point to which 

 the gravels can be traced up valleys that are now dry. 



Between Erith and Crayford is a stretch of brickearth in 

 which many pits have been opened and of which many descriptions 

 have been written j 1 it appears to belong to the end of the Middle 

 Terrace period, that is to say, the Mousterian stage of palaeolithic 

 culture (p. 69), but the gravels of this area seem to be continuous 

 from the 50-ft. level to the bottom of the buried channel, and 

 consequently no line can be drawn between the Middle and Low 

 Terraces; the confusion may be due to changes in the course 

 of the main river (p. 64). 



The Low Terraces. The deposits at lower levels than the 

 Middle Terrace are not as simple as some writers have taken 

 them to be; they descend below the present level of the river 

 and east of London far below sea-level; consequently open 

 sections are seldom seen. The evidence available suggests that 

 there may be two periods of deposition, the gravels of which are 

 not clearly to be differentiated by their level. From Brentford 

 eastward it is sometimes possible to trace an upper and a lower 

 Flood-plain gravel, and in the same area we find evidence from 

 borings and excavations of a deep channel of the river descending 

 in the east of our district as much as 100 ft. below Ordnance 

 Datum. Probably the two phenomena are connected, and the 

 sequence of events appears to have been as follows : after the 

 deposition of the Middle Terrace gravel and brickearth a period 

 of corrasion ensued, and the river lowered its base level some 

 20 ft. ; deposition of the upper Flood-plain gravels and brick- 

 earth ensued, a cold climate being indicated at the base of the 

 gravel. Later a fresh elevation of the land enabled the river to 

 cut the ' buried channel ' ; this channel is narrow and gorge-like, 

 and cannot be traced above Brentford; its gradient is steep, 

 compared with that of the Upper and Middle Terraces, about 



1 e.g., ' Geology of London ' (Mem. Geol. Surv.}, vol. i, 1889, pp. 432-439; 

 Chandler, R. H., Proc. Geol. Assoc., vol. xxv, 1914, pp. 61-71 . vol. xxvii, 1916, 

 pp. 72-76 and references there given. 



