300 
ME. PRESTWICH ON THE GEOLOGY OE THE DEPOSITS 
notice is at Drucat, near Abbeville, and which I take to belong to the high-level gravel 
period. This outlying mass of sand and gravel is in a depression of the chalk, which 
probably accounts for its preservation; at the bottom of the depression there was 
exposed on the occasion of my first visit a deep circular shaft in the underlying chalk 22 
feet in diameter at the top and 18 feet at the depth of 30 feet, to which extent the sand 
and gravel had been cleared out. The prolongation of this great natural excavation in 
the chalk probably reaches a depth of at least 100 feet. A number of these sand pipes 
underlie this quaternary outlier, but I saw no others of the dimensions of this one. 
These pipes are not filled up indiscriminately, as if they had been formed first and 
subsequently filled up, but they show, as usual, a succession of concentric and conti- 
nuous vertical layers, following the encircling surface of chalk and enclosing a core 
distinct from the outer coats. Further, where the beds of sand and gravel are undis- 
turbed and in their horizontal position, it is found that the core of the pipe always 
subtends from the uppermost bed or seam of gravel or sand, or not unfrequently from 
the superincumbent Loess, which proves that the superincumbent beds were deposited 
before the excavation of the pipes, and that they were lowered into them by the gradual 
removal of the chalk. These excavations have been referred to various causes, of which 
I consider the action of carbonic acid held in water as the only one possible*. It is 
evident that to have an excavation of this sort we must have the slow and constant 
passage of water. If the line of water-level in the chalk had remained permanently 
near the level of the high-level gravels, this prolonged downward action could not have 
occurred. The water-line, although at first necessarily on that level, must, as the exca- 
vation of the valley proceeded, have gradually been lowered 50 to 100 feet or more; so 
that the surface-water collected in these beds of sand and gravel, left standing above 
the base of the gradually deepening valleys, would, in draining off, have to pass down, 
along the lines of least resistance, through a successively increasing depth of chalk, 
before it met with the line of permanent water-level into which it would merge. The 
gradual and constant operation of this percolation of water through definite lines in the 
chalk, from the first emergence of the high-level gravels above the old river-bed and 
continued in the same channels down to the time of the lower valley-gravel, resulted in 
eroding these perpendicular shafts or funnels, into which, as the excavation proceeded, 
the overlying gravel and sand coordinately subsided, while the Loess of the periodical 
floods continuously tended to level the resulting inequalities of surface. The process 
must necessarily have been extremely slow. That these pipes are connected with a 
former state of things and not with the present, is shown by there being now no indi- 
cation of their presence on the surface of the ground f . 
* See a paper by Sir Charles Lyell in Phil. Mag. 3rd ser. vol. xv. p. 257, and another by the author in 
Quart. Joum. Geol. Soc. vol. xi. p. 64. 
f This in some cases may arise from the cultivation of the surface. In a few favourable localities a slight 
action of this sort would still, however, appear to be going on, if we may judge from an occasional sinking or 
giving way of the ground. 
