PRESTWICH SAND- AND GRAVEL-PIPES. 83 



two well-marked detached pipes, both filled with the Tertiary sands ; 

 one appears 15 feet deep and 20 broad, but this arises in conse- 

 quence of the section only exposing the upper and outer portion of 

 the pipe ; the other is 1 2 feet deep by 1 and 2 in diameter. Pass- 

 ing then on to the lane which leads from Stroud to the "Three 

 Crouches," at a distance of about ^ to ^ of a mile from the high 

 road, the sides of the lane, which are 4 to 8 feet deep, exhibit a 

 series of segments of sand-pipes, some of which must be of great 

 depth and size. I counted eight, varying in diameter from 4 to 50, 

 80, and even 100 feet, within a distance of less than a quarter of a 

 mile. A few feet higher up the lane the Thanet Sands set in. Two 

 small chalk-pits, a short way beyond the Three Crouches, show bare 

 chalk and no pipes, but on the road-side, between the two, I found 

 traces of one in a cutting 2 feet deep. A little further on is a con- 

 siderable outlier of the Lower London Tertiaries, capped at Shorne 

 Wood Hill by a small patch of the London Clay . Traversing this 

 Cobham ridge, we meet with the chalk agam as we approach Single- 

 well. 



The country between these hills and the next important tertiary 

 outlier at the Swanscomb hills forms an extended chalk plain with 

 here and there small cappings of Thanet Sands, the elevation of 

 which is on an average from 150 to 200 feet above the Thames. 

 There are many shallow chalk-cuttings and several chalk-pits in 

 this district, but not many sand-pipes are exposed. There was one 

 in a pit opened a few years since near Ifield Church ; traces of one 

 are also visible on the road-side just W. of Singlewell. I could find 

 no other until near Dundle Farm, where the shallow lane-side 

 cutting showed indistinct traces of several. Next in a pit, half a 

 mile N. of Betsham, remains of a sand-pipe are seen*. Then,, 

 •crossing the Swanscomb hills, in a pit by the lane skirting the W. side 

 of Hockenden Wood, and at a depth of 5 to 10 feet beneath the 

 Tertiary base, is a well-marked sand-pipe, measuring 12 feet by 6. 



Beyond this spot there are few good sections and fewer sand- 

 pipes. Some years since I remember seeing one or two in some old 

 pits, now grown over, on the hill E. of Dartford. Between this 

 town and Cray ford the chalk is bare or else capped by drift. 



The same general facts are observable along this hne of section as 

 along the other. The sand-pipes are confined to a vertical zone of 

 chalk not extending usually to a depth of more than 60 to 80 feet 

 beneath the base of the Tertiaries. Where there are cuttings in 

 this zone, pipes are constantly exposed, but in the numerous cuttings 

 situated lower in the valleys than this zone, as a general rule no 

 traces of sand-pipes are to be found. Both in Sections 1 and 2 the 

 chance of meeting with pipes decreases, even in this pipe-zone, the 

 deeper we get from the parent beds. 



Diagram A represents a theoretical restoration of the surface at 



* A clearer instance however ocoiirs, at a short distance from our line of section, 

 in a pit just N. of Swanscomb, where the chalk is traversed by a sand-pipe 15 feet 

 deep, 130 feet above the Thames level, and apparently at about the level of the 

 i)ase of the Tertiaries. 



