71 



level without much more disturbance than a moderate 

 crumpling of the layers. I suggest this in consequence of 

 the peculiar appearance and position ot some of the gravels 

 in the Wealden district, which are like, and at the same 

 time unlike river gravels, without there being a doubt of 

 their fluviatile origin. In the history of our superficial 

 deposits, I look on those of the higher levels already 

 described as preceding the Episode of the deposition of 

 the Marine Glacial Gravels and the Chalky Boulder clay ; 

 but while some of those that follow may belong to the 

 latter part of that Episode, all seem to be of subsequent 

 date. 



The gravel proper to the river Thames in the lower 

 part of its course (or 20 miles on either side of London) 

 form a broad belt of varying width, generally six or eight 

 miles from North to South. All these gravels lie below 

 the present 200 feet contour line (the highest, those at 

 Wimbledon do not exceed 190 feet, up to which they nearly 

 reach). The general average of the upper maximum 

 range is 1 50 feet. All the gravels of the Thames proper are 

 distinguished by a composition differing from those at a 

 level exceeding 200 feet, in as much as they contain, in 

 varying proportions according to locality, erratics or the 

 pebbles and fossils of Northern origin, which have been 

 washed from the Glacial gravels and clays which still line 

 the Northern brow of its valley, and which may have occu- 

 pied a small part of what is now within its present well 

 marked area of denudation. Erratics are the most abun- 

 dant and unvarying companions of the chalk flints, and 

 tertiary pebbles, which form its Northern spreads of gravel. 

 On the South, however, where the supply of material's from 

 the chalk hills, and the Weald, by tributaries which do not 

 pass through glacial beds, is very abundant, we find 

 the erratics diminish in proportion, and frequently we find 

 the gravels under the hills and near the outlets of streams 

 almost free of erratics, though continuous stratigraphically 

 with gravels full of them. This is the case at the mouth 



