PRESTWICH — SAND- AND GRAVEL-PIPES. 



71 



line " m, n " is perfectly straight. This section also corroborates the 

 inference drawn from fis-. 1 . 



Fig. 2. — Section of the upper part of a Sand-pipe in the Chalk 



at Grays. 



Drawn on the same scale as Fig. 1 . 

 n Thanet Sands. 



b Seam of clay and sand with green-coated flints. 

 c Gravel. d Chalk. 



The general arrangement is the same in the gravel-pipes, only it is 

 rougher and less apparent * ; for as the gravel is usually spread over 

 the chalk in a large unstratified sheet of one rough homogeneous 

 structure throughout, it necessarily follows that, however slow its 

 subsidence at any point into a pipe penetrating the chalk may have 

 been, the absence of straight lines of bedding would prevent the 

 clear exhibition of any lines of flexure in the gravel of the pipes, 

 and would cause it to retain the same apparent want of structure 

 which characterises the mass of the gravel itself. But it sometimes 

 happens that the gravel is roughly stratified, or rather spread out in 

 layers of variable texture ; or at times a bed of sandy gravel full of 

 Tertiary flint-pebbles overlies another bed containing almost solely 

 subangular and unrolled flints. In cases where pipes have been 

 formed under such gravels, the curved or inverted conical ar- 

 rangement of the mass, and the descent of the central core from 

 the higher beds, generally become apparent, as in the sand-pipes. 

 Some good examples of these pipes are common on the Chalk downs, 

 especially in some pits above Westerham and Wrotham. The sides 

 of most of these pipes are there formed of an extension of the layer 

 of perfectly angular flint-rubble 1 to 4 feet wide, occurring at the 

 base of the drift, whilst the core consists of worn gravel, or often of 

 round flint-pebbles and sand derived from the Tertiary beds which 

 formerly overspread that area. 



The size of the sand- and gravel-pipes is very variable, some being 

 only a few feet deep, and others reaching to a depth even of 100 feet 

 or more, with a diameter'of 20 to 40 feet. They are very common all 

 over the Kent and Surrey Chalk district ; also in Berkshire, Wilts, 



* There are some very good instances of gravel-pipes in the Chalk-pits at Green- 

 hithe. The neighbourhood of Watford, Henley, and the Downs a few miles N.E. of 

 Maidstone also offer convenient localities for studying this phenomenon ; there 

 are, in fact, few places in the Chalk district where it may not be observed to a 

 greater or lesser extent. 



