PRESTWICH SAND- AND GRAVEL-PIPES. /D 



level at a depth, below the water-level in the sands, dependent upon 

 the general elevation of the country above the sea*. 



Thus, supposing the mass of Chalk and the overlying Tertiaries to 

 have been placed at the period we are now considering at about the 

 same altitude that they at present occupy, but that the valleys which 

 now traverse the country were not then formed, — supposing, in fact, 

 that the country were restored to that state in which it would seem 

 to have been before some later denudation swept away the beds which 

 must have been continuous in the way represented by the dotted lines 

 in Section 2. PL VI., we should then have a state of things represented 

 by Diagram B, where the Lower Tertiary sands would form an upper 

 water-bearing stratum rising some few hundred feet above the sea ; 

 whilst, supposing the Chalk had some lower outcrop further inland, 

 the water-level in it would stand, at a certain distance from the sea, 

 at one or two or more hundred feet below that level ; but as the beds 

 dipped and the country lowered towards the sea, then these two 

 water-lines would gradually converge towards and ultimately merge 

 in the same common plane (see Diagram B, PL VI., where x, y shows 

 the line of water-level in the chalk C, and m, n the water-line in the 

 overlying sands S) . Or if we restore, as in Diagram A, the conditions 

 prevailing during the gravel period, we might then have the case of 

 an inland plain or valley at some height above the sea, covered by a 

 bed of permeable gravel reposing upon the thick bed of the Chalk, 

 and flanked on either side by higher ground. Under these conditions 

 the calcareous strata would again have a line of water-level lower than 

 that which would exist in the overlying mass of gravel (see PL VI., 

 Diagr. A, x, y being the line of water-level in strata C, and m, n the 

 level in the overlying beds 6r)f . 



Referring now to the supposititious cases representedinthe Diagrams 

 A and B, the consequence would be that the water in the sands iS or 

 in the gravel G throughout the higher districts, having little or no 

 natural lateral vent, would tend to escape downwards to the lower 

 water-level in the Chalk C, and a constant flow would be established 

 through any small fissures that might originally exist or else by ge- 

 neral permeation through the body of the Chalk. I do not, however, 

 consider the sand-pipes as necessarily or essentially dependent upon 

 cracks or fissures in the chalk. These, probably, would rather 



* When the table-land of permeable strata reposing upon chalk is much inter- 

 sected by valleys, as for example in the case of the sands of Blackheath, Plumstead 

 Heath, and Bexley Heath, which are bounded by the valleys of the Ravensbourne 

 and of the Cray, the ready lateral flow afforded to the water by these valleys 

 and their branches prevents any large accumulation or much vertical pressure 

 of water in these sands. Still it is more than probable that in such a case some 

 water will pass from the sands into the chalk, and that therefore some amount 

 of wear is continued. 



t We have an extreme case of such conditions in the " Yailahs" of Lycia, no- 

 ticed by Prof. E. Forbes and Lieut. Spratt. These are more or less basin-shaped 

 valleys of various extent at an elevation of from 2000 to 6000 feet, and with no 

 outlet for the streams which water them. During the wet season the water 

 collects and often forms a lake at one end of the valley : this apparently passes 

 away by infiltration. The rivers pour into caverns and are lost among the preci- 

 pitous cliffs which form the sides of the valleys. — See their " Travels in Lycia." 



