580 DR. II. HICKS ON SOME RECENTLY-EXPOSED SECTIONS 



In some cases there was an almost complete absence of gravel and 

 a preponderance of fine sand, whilst in others the whole thickness 

 was found to consist of a sandy gravel. Everywhere, however, 

 except where the furrows containing the overlying Boulder-clay 

 reach downwards to the London Clay, some amount of sand or gravel 

 is present under the clay, usually in a stratified condition, and. 

 showing well-marked lines of current-bedding. When there were 

 but few pits opened, and no continuous sections had been seen, I 

 was led, on one or two occasions, to form a mistaken idea as to the 

 thickness of the Glacial deposits in some parts of the district, 

 by calculating, from some excavations which happened to have 

 been made in one of the channels containing Boulder-clay (without 

 reaching gravel at the base), that an average thickness of the sands 

 probably occurred below. Prom the excavations which have been 

 made since then for the sewers, and other more continuous sections, 

 I have obtained sufficient evidence to show that, where well- 

 sinkings had exposed an unusual thickness (^. e. 15 or more feet) of 

 the Boulder-clay, they had been sunk in the depressed masses, and 

 there was but little, if any, gravel separating it there from the 

 London Clay. As demonstrating the practical utility of some 

 acquaintance with the physical conditions under which these 

 deposits were accumulated, I may mention that in this district, 

 where sand and gravel command a very high price for building 

 purposes and laying out of paths, the search for gravel has often 

 been discarded, because it was not known that over most of the 

 j)lateau there was a considerable thickness of clay overlying the 

 sand, and where test-holes had been made the}- had frequently 

 either not been carried deep enough, or had pierced one of these 

 channels filled up by the Upper Boulder-clay. From finding a 

 certain amount of washed gravel on the surface of the clay, it had 

 also frequently been supposed that it occurred only in pockets in 

 the clay, whilst a further boring of two or three feet would 

 often have revealed the presence below of 7 or 8 feet of good 

 sand and gravel, equal, if not superior, to that which for building 

 purposes would have to be carted from the Thames Yalley or 

 brought from Hertfordshire or Bedfordshire. On several occa- 

 sions also I noticed that where the lower gravels had been re- 

 moved all further search had been suddenly abandoned, in con- 

 sequence of meeting with a depressed mass of Upper Clay, whilst 

 had the mass been crossed the gravels would have been again reached 

 within a few yards. The largest gravel-pit at present in the 

 neighbourhood, the one from which fig. 1 is taken, was opened at 

 my suggestion by tlie men who had been digging gravel for my own 

 use in my grounds, and it was with difficulty that I could persuade 

 them that the gravel would be reached below the 7 or 8 feet of 

 the stifi' brown clay which is found in that pit. 



As showing how the previously-formed hills of London Clay have 

 been, as it were, mantled by the Glacial deposits, the sections at 

 Hen don Grove are particularly instructive. At the highest point 

 on the edge of the plateau, about 280 feet above Ordnance datum, 



