396 E. M. BUEAVASH CHALK AXD FLIXTS OF XORTHERX FEAXCE 



whose longest axes are in general parallel with the Yalley, and which 

 may well be formed by the falling in of the roofs of open subterranean 

 channels or caverns. The sloping valley walls are frequently interrupted 

 by scarps, which also run parallel to the valley and of which there may 

 be two or more, one above the other, on the slope, obviously indicating a 

 slipping down of masses of chalk toward the valley bottom. Wells sunk 

 near or in line with the graben encounter a strong flow of water at shal- 

 low depths, and the water supply of many of the towns of this region is 

 drawn from what appear to be subterranean streams. The water is in 

 general of very good quality, though ^'liard,'* owing to its filtration through 

 the chalk. At Staples, before the war, a pumping station in a valley 

 only a few miles in length had furnished the water supply for Etaples, 

 Le Touquet, and Paris Plage, with a total population of six or eight 

 thousand. On the establishment of large base and hospital camps in 

 the neighborhood, this supply was found sufficient for more than ten 

 times as many people, with some enlargement of the pumping j)lant. 



Developmext of the Valleys 



The question of the development of these valleys has some features of 

 interest. At Etretat such a valley debouches at the coast, flanked by 

 chalk cliffs on either side. At low water the stream can be seen emerging 

 and flowing into the sea through the flint-pebbled beach at a point under 

 the clifl on the south side of the valley mouth. This is not a mere dif- 

 fused seepage, but a rapid flow, definitely limited laterally, with a width 

 of about twenty feet. Farther up the valley it appears to be toward the 

 northern side, under the valley floor, where the pumping station is situ- 

 ated. The stream near Etaples also meanders across its valley, as evi- 

 denced by the depressions already described, which are found on opposite 

 sides of the valley at intervals along its length and may be as stated 

 formed by the falling in of parts of the roof of the subterranean channel. 

 The scarps seem sometimes to be so placed as to leave the impression that 

 they have been caused by the cutting of a subterranean meander, which 

 may extend, as above described, under the bank, and tends to travel in a 

 down-stream direction much as do those of surface streams. 



The surface run-ofl carries the soil overlying the chalk down the sides 

 of the valley and spreads it over the valley floor. This deposit is thickest 

 near the lower end of the valley, and here a small surface stream may be 

 formed which flows entirely over this alluvium and is quite distinct from 

 the main stream beneath. 



In a country where intensive cultivation pollutes the surface streams,, 

 the presence of these subterranean supplies of chalk-filtered water is in- 



