PRESTWICH — SAND- AND GRAVEL-PIPES. 81 



Map, on which the sections are planned. The numbers over the 

 sections refer to tlie elevation in feet above the mean sea-level. 



The pipes which are laid down from personal observation are 

 marked with a small cross. Those portions of the lines of section 

 which did not admit of examination show merely the probable posi- 

 tion of the pipes. 



The faults are omitted in the sections. 



Section No. 1. — In this section the base was taken at the river-level 

 in the valley of the Colne between Uxbridge and Rickmansworth, 

 estimated at about 150 feet above the sea-level (the level of the Grand 

 /unction Canal at Uxbridge being 113 feet), nearly three miles in a 

 direct line S.S.W. from Rickmansworth. On the banks of the 

 Grand Junction Canal, and a few hundred yards northward of the 

 Harefield Copper-mill, is a large chalk-pit on the side of the hill, 

 showing a section of from about 80 to 100 feet deep. The chalk 

 is capped at the highest points by a thick bed of ochreous flint gravel, 

 with pebbles of the older rocks. This gravel here occupies a nearly 

 level plane on the hills, and is distinct from the gravel which occu- 

 pies the adjacent valleys ; between the surface covered by the 

 former, and the gravel in the valley, the face of the chalk is de- 

 nuded and comparatively bare. The chalk in this pit is full of 

 gravel-pipes, many of them of a very large size, and descending to a 

 depth of 40 to 60 feet. On the slope between the main mass of the 

 gravel and the river-level, isolated portions of gravel-pipes are com- 

 mon. As, however, the hill is here very steep, these latter are 

 confined to a small breadth, and the phsenomenon is not so well 

 shown as at many other places. In a pit near Troy Farm, on the 

 west side of the valley, two small ends of gravel-pipes are seen about 

 40 feet above the canal level. Our line of section, however, passes 

 up the lane by Corner Hall, a third of a mile west of which the 

 cutting on the lane-side (10 to 20 feet beneath the level of 

 the main mass of gravel, here denuded) shows three or four 

 small terminations of gravel-pipes in the bare chalk. The summit 

 of the hill by Warren Farm and Ninnings is covered by the main 

 mass of the gravel, without any sufficiently deep sections ; but in 

 descending the lane by Hill Farm down to Chalfont St. Peter, the 

 chalk comes to the surface, and, at an elevation of about 50 feet 

 above the valley, exhibits sections of part of several large and deep 

 pipes, into which a mass of angular flint debris and of tertiary pebbles 

 and sand has been let dovm. Crossing the valley, the gravel sets in 

 again on the opposite hill, but no sections are exposed until we reach 

 a small wood through which the lane from Later' s Green to Stamp- 

 well passes. In descending the hill at this point there is a chalk-pit 

 on the right-hand side of the road. The chalk is quite bare, but 

 contains one good detached gravel-pipe 12 feet deep, and about 10 

 to 15 feet beneath the level which the hill gravel, if prolonged, 

 would occupy. On Stampwell and Pitland's Wood Hill the ground 

 rises rather higher, in consequence of a thin outlier of the mottled 

 clays and sands, round which the gravel sweeps, barely spreading 

 over it in places. Continuing our line over these hills and through 



VOL. XI. — PART I. G 



