1894. ^-^^ EVOLUTION OF THE THAMES. loi 



The most instructive area in the Chilterns is that in the neigh- 

 bourhood of Tring, Prince's Risboro', the Wycombes, and the valleys 

 of the Missbourne and the Chess. It is all included in sheet no. 7 

 of the maps of the Geological Survey, of which the " Drift Edition " 

 admirably shows the complex series of gravels in the district. These 

 are also described in two of the Survey Memoirs (i). The map 

 shows that the Chilterns consist of Chalk, covered by a varied series 

 of drifts, intersected by a number of valleys which cut across the 

 ridge from north-west to south-east. The most extensive of the 

 drifts is a series of gravels and brickearths that are coloured pink on 

 the map and marked as " Glacial." There is no Boulder Clay in this 

 immediate district, but the gravels, though somewhat different in 

 composition, appear to be continuous with those which pass under 

 this deposit further to the east. They are, therefore, earHer than the 

 Boulder Clay. Though marked as " Glacial," no one would be likely 

 to maintain that they are Glacial in the same sense as is the Boulder 

 Clay. They are certainly due to water action in some form, instead of 

 to the direct agency of ice like a moraine. To avoid the ambiguity 

 involved by the use of the term " Glacial," it may be advisable to 

 call these the " Newer Plateau Gravels." They must be dis- 

 tinguished from another set, which may be called the " Older 

 Plateau Gravels," which are the " Pre-Glacial Gravels " of the 

 Geological Survey and the Westleton Shingle of Professor Prestwich. 

 Examples of these occur in this area capping the outliers of Eocene 

 beds at Penn and Lane End, near High Wycombe. The former 

 existence of these two plateaux, each capped by its own set of 

 gravels, v/as first demonstrated by Professor T. McKenny Hughes 

 in 1869 (9). 



A third set of gravels occurs on the floors of the set of valleys 

 running through the Chalk ridge ; as these occur right up to the 

 watershed, it is clear that they must have been deposited at a time 

 when the valleys continued further to the north. 



The fact that the valleys of Bradenham, Hampden, and the 

 Miss cut through the plateau and contain none of the " newer high 

 level plateau gravels " shows that the valleys are later than the 

 gravels. The valleys themselves, though now dry in their upper 

 parts, breach the Chalk escarpment, and it is impossible to examine 

 them without feeling that they are due to erosion by rivers that once 

 rose some distance to the north. The breaches made by the heads 

 of the valleys are most impressive ; thus, that on the road from 

 Wendover to Aylesbury crosses by a pass the summit of which is 

 503 feet high, while the ridge on either side rises to 790 and 800 feet. 

 Similarly, the road from West Wycombe to Prince's Risboro' crosses 

 through a breach at the height of 427 feet between points of the 

 ridge that are 700 feet only three-quarters of a mile to north-east and 

 south-west. The deposits on the floor of the valley are also suggestive 

 and tell no less strongly in favour of the formation of the valleys of 



