SWALLOW HOLES. 533 



Swallow-holes, known as 'man-holes,' are met with in the New Red Sandstone 

 of Ripon ; ' they occur also in the Furness region. 



In the Chalk country Swallow-holes are not uncommon, as near Canterbury, 

 Farnham, Newbury, Hungerford, Corfe Castle (see Fig. 54, p. 347), etc., also in 

 Hertfordshire.- They are from six to forty feet broad, and twenty to thirty feet 

 deep, and funnel-shaped. Prof. Prestwich remarks that old Tertiary sand- pipes 

 may have determined the formation of some of these Swallow-holes. Referring 

 particularly to the Swallow-holes in Berkshire, Mr. Whitaker observes that they 

 are formed by streams which, rising in the higher ground, flow down the escarp- 

 ment of the Tertiary beds, imtil they reach the more pervious and jointed Chalk, 

 'Or until they come within a short distance of that rock, when tliey work their way 

 into it through the few feet of the softer overlying beds. In the course of time, 

 through the chemical action of the carbonic acid in the water, and the mechanical 

 action of the water itself, funnel-shaped basins are worn out in the Chalk and the 

 beds above, the operation being made more easy by any pre-existing fissures. 

 These hollows are generally thickly overgrown with vegetation. The streams may 

 •often be seen running through them, though sometimes they merely flow into a 

 small pool, the level of the water in which remains the same, notwithstanding the 

 constant flow. In Berkshire these Swallow-holes always occur at or near the 

 junction of the Reading Beds and the Chalk, and they are therefore of much use 

 in drawing the boundary-line betvv'een those formations, especially where there 

 are no sections. The Mole, as is well known, sinks for a while in the Chalk 

 south of Leatherhead. 



Suggestions have been made to increase the supply of water in certain strata at 

 a depth beneath the surface, by means of artificial Swallow-holes or ' Dumb-wells.' 



In studying the water-bearing strata, it is interesting to trace their connection 

 ■with the distribution of villages, for no doubt these were fixed according to the 

 means of obtaining a supply of water, and one easily accessible, either from 

 river or well. And the practice of well-sinking dates back to early historic times 

 in Western Europe. 



Prof. Prestwich has shown how the earliest settlements in and around London 

 were dependent upon the geological structure, and for this reason. The London 

 Clay, which occupies so wide an area, is covered to a large extent by the valley 

 gravels, and here and there in the north of London by an outlying hill of Bagshot 

 Sand. Wells sunk through these sandy and gravelly deposits were always 

 supplied with water, which was kept up by the impervious London Clay. Hence 

 the bare London Clay was unoccupied until the New River- and other water- 

 works did away with the necessity for wells ; and the clay districts of Holloway, 

 Camden Town, Regent's Park, St. John's Wood, Westbourne Park, and 

 Notting Hill, received town populations much later than Stepney, Hackney, 

 Islington, Paddington, Kensington, Chelsea, and Camberwell, which are situated 

 on gravel. In the same way on the outskirts of London a succession of villages 

 grew up for miles on the great beds of gravel, ranging on the east to Barking, 

 Ilford, and Eomford ; on the north, following the valley of the Lea, to Edmonton 

 and Hoddesdon ; and on the west, up the Thames valley, to Hammersmith, 

 Ealing, Hounslow, and beyond.* Around Harrow, which stands on the Bagshot 

 Sand, a large area of bare London Clay extends, which is still remarkably free 

 from the encroachment of houses, particularly between Harrow and Ickenham on 

 the west, and Edgvvare on the north-east. The few springs that occur in the 

 London Clay are more or less charged with salts of iron. 



Similarly, formations composed chiefly of limestones, sands, and sandstones, 

 such as the Cornbrash, Corallian Beds, Upper Greensand, etc., are often marked 

 in their outcrop by villages, while the Oxford Clay, Kimeridge Clay, and Gault 

 ■in comparison form very thinly populated districts. Where large cities and towns 

 •are situated on Clay formations, there is usually much valley-gravel, as at London, 

 Oxford, etc. 



' A. C. G. Cameron, G. Mag. 1879, p. 575. 



^ Prestwich, Q. J. x. 222 ; Whitaker, Mem. Geol. Surv. iv. p. lOO. 



* Address to Geol. Soc. 1872. 



