364 W. TPliifaker — Chalk of the London Basin. 



In the more exceptional cases where the escarpment is not a simple 

 riflge, but a double one, the two ranges some waj' apart (as near 

 Luton), there is a greater area subject to this flow against the dip, 

 and still more so is this the case where, instead of the usual bold 

 abrupt ridge, we have a more gentle and therefore a broader slope 

 (as in Norfolk) ; so that over a tract some miles in width the plane 

 of saturation will slope in a direction opposite to the dip. 



Another marked exception is given by a range of sea-cliffs, where 

 this runs along the dip, or at no very great angle with the direction 

 of the dip, as on the coast near Dover, in which case the water finds 

 a ready outlet along the free margin formed by the cliffs, and takes 

 a shorter course more or less across the dip, instead of a longer one 

 following the dip. 



In short, the underground flow depends generally on the surface- 

 contour ; the water can generally flow across the bedding when it 

 can more speedily reach an outlet by such a course. 



4. Eelation of the Structure and Position of the Chalk to 

 Water Supply. 



Turning now to the bearing of the different divisions and of the 

 various structures of the Chalk on the question of water supply, 

 we find, in the first place, that the greater part of the area of out- 

 crop is formed by the Upper Chalk, which, rising up from beneath 

 the Tertiary beds, reaches right on to the top of the great escarpment, 

 or bounding-ridge, except where cut through by valleys, which lay 

 open lower beds. Where the escarpment is all in one, as is commonly 

 the case, and is abrupt, it follows that the outcrop of the lower 

 divisions is narrow, very narrow as compared with the broad tract 

 of Upper Chalk inward from the escarpment. This division, too, is 

 the thickest of the three. 



The Upper Chalk, therefore, must be the great gathering ground 

 for water; and, as the water sinks downward to low levels, to the 

 boundary of the Tertiary beds, so that the plane of saturation comes 

 near to, and sometimes up to, the surface of the ground, it follows 

 that, as a general rule, this division must be the great water-yielding 

 one, and that works for vast supplies, over most of the area in 

 question, must be established near the outcrop from beneath the 

 Tertiary beds, where the plane of saturation, though near the 

 surface, is at a low level, so that underground water tends to flow 

 down to such works and to replace that pumped out from them. 



It seems to me that the place taken by the Upper Chalk as the 

 great water-bearing bed may, perhaps, be owing to its favourable 

 position rather than to its structure. There is nothing, as a rule, 

 to prevent water sinking down through the Upper into the Middle 

 Chalk, and we find that at high levels, toward the escarpment, the 

 former is dry, its base being above the plane of saturation, which 

 sometimes may be very far down in the latter. In its frequent 

 layers of flints the UpjDer Chalk may have some advantage over 

 the Middle Chalk, in regard to the flow of water inward from the 

 higher grounds ; but probably these also sometimes act as a hindrance 



