90 



Some of this latter gravel appears in the festoons and 

 underplight, but it must not be regarded as having formed 

 them, only as remaining on the surface from the thawing 

 of previous dirty masses of ice, and so becoming included 

 in the next crumpling of the surface. In the neighbour- 

 hood of rivers this product of glaciation accumulated in 

 ■ great sheets of gravel, shewing something of aqueous 

 arrangement even before it reached the bed of the river, 

 which was the result perhaps of ponding back or debacles. 

 This appearance in our district is not perceived at an 

 elevation above 20 feet O.D. near the Thames.* 



When light clays and sands had been deposited in 

 channels in gravel, the gravels sometimes got pushed into the 

 more yielding soil {PI. 'i-.Jig. 6) and in the case of gravel 

 containing sand and clay in layers, which has been piped 

 through by the dissolution of the chalk beneath, the over- 

 riding of the heavy masses has pressed the layers of 

 differing hardness out of their normal shape in such a 

 manner as to crush in the sides of the pipe, frequently 

 contorting the edges of the layers tipwards, and often 

 pushing portions of the clay and gravel some inches into 

 the core of the pipe, thus contracting the pipe, keying the 

 core and hardening it so that after the ice has passed away 

 and the solution of the chalk had continued to let down the 

 gravel, a cavity was formed below a certain point, the roof 

 of which was the result of the above-mentioned compression. 

 These cavities are common, most are small but some are 

 large, on Dartford Brent I saw one in which twenty men 

 could have found standing room, and I heard of another 

 into which a cart was backed. I would suggest that it 



*It is a question whether the shghtly washed sheets of gravel which 

 overlie the latest Trail on the foreshore of the Thames are not of a 

 modern date, inasmuch as the Trail and Underplight beneath them can 

 certainly be traced under the Tidal mud deposits for twelve feet at least 

 below T.H.W. mark; and I have seen the two latter in particular, under 

 the Tidal -mud at Eiith, ijwanscombe, Northfleet, Higham, and other 

 places on the river. 



