PRESTWICH — SAND- AND GRAVEL-PIPES. /3 



This phsenomenon is not confined to these eocene sands, but is 

 equally common in connexion with one of the most recent of our 

 drift-gravels, and in beds of various other ages. In the instance 

 of the drift-period, this is shown in the line of section (No. 1, 

 PL VI.) extending from the neighbourhood of Rickmansworth 

 to the hills above Wycombe, where the gravel caps a succession 

 of Chalk hills and exhibits a considerable number of very illus- 

 trative gravel-pipes, including the fine case mentioned by Dr. 

 Buckland between Beaconsfield and Wycombe. Both this section 

 and the other (No. 2), referring to the sand-pipes, are actual 

 sections, in which all the pipes found on the lines of section are in- 

 troduced and distinguished from those supposed to exist where the 

 chalk is not exposed. The relative position of the pipes and their 

 distance from the main masses are maintained, the height of the 

 ground at the various points Where they occur having been approxi- 

 mately determined by the aneroid barometer. 



Some of the most remarkable instances of these detached gravel- 

 pipes, and instances affording a test to a much-debated period of 

 denudation, occur in the slopes of the North Downs. A thick drift 

 of ferruginous clay, sand, and gravel extends generally to the very 

 edge of this escarpment, and is there abruptly truncated, whilst the 

 slopes of the hills present a bare and clean chalk surface ; but dotted 

 a considerable way down these slopes may often be found portions of 

 detached and isolated gravel-pipes, — the termination of pipes descend- 

 ing from the main mass of the gravel when prolonged above them. 

 Such tail-pieces may readily be observed above Charing, Wrotham, 

 and Westerham. The annexed sketch gives a section of the hill above 

 the latter town (see fig. 3) . This fact is important, as it affords a 



Fig. 3. — Section of the hill-side above Westerham, showing the pipes 

 on the top of the hill filled with drift, and the terminations 

 only of similar pipes some way down the slope of the hill. 



Drawn on scale. The height from the dotted line to the base line is about 300 feet. 



* Pipes of which the ends still exist on the slope of the hill. Descent by the London road, 

 ffl Chalk. 



b Ferruginous clay, sand, and gravel filling pipes on the surface of the chalk, 

 c Outlined continuation of former surface of the chalk with similar pipes. 



strong proof that the final excavation of the valley of Holmesdale, 

 including, I believe, that of the Weald, was not effected until after this 

 comparatively recent drift period. This, however, is a point I had 

 not intended to have entered upon at present, and the further con- 

 sideration of which I must reserve until I have occasion to treat of 

 the drift as a separate question. 



