CHALK. 42 1 



lower portion of the Chalk, the clayey Chalk Marl being, as a rule, impervious ; 

 at Lydden Spout, Folkestone Hill, there is a copious perennial spring. (See 

 Fig- 69.) The Totternhoe Stone yields many springs. Inland the streams come to 

 the surface at different levels in a valley according to the season and the amount of 

 rainfall ; hence in Wiltshire we find the names Winterbourne Basset, Winterbourne 

 Monckton, etc., applied to villages in the district. (See sequel.) Prof. Prestwich 

 has stated that it is only under certain conditions that he considers the Chalk a 

 source of good water supply. "Those conditions are that there should be a large 

 receiving surface, and that the spring or well should be in a low situation relatively 

 to the mass of exposed chalk. A spring shows itself; but in the case of a well, 

 as the water does not permeate freely through the mass of the chalk, it is further 

 necessary to hit upon a fissure below the level of the water-line. The depth or 

 size of the fissure determines the supply." ' Suggestions have been made to increase 

 the supply of water in wells from the Chalk by means of galleries ;- and this 

 system has been adopted in many places with success. 



Chalk water is hard ; that at Grays contains about 15 grains of carbonate of 

 lime in the imperial gallon. 



Over parts of the dry Chalk Downs of Berkshire, etc., shallow excavations 

 known as Dew Ponds have been made to supply water for cattle. These, 1 am 

 informed by Mr. F. J. Bennett, are filled with water, preferably with snow, in the 

 first instance, and that they rarely dry up, the supply being maintained by 

 mists, as well as by rain, etc. The ponds on the Chalk generally require to be 

 clayed. 



Fuller's Earth has been recorded from the Chalk at Bepton, near Midhurst, in 

 Surrey.^ 



Sulphuretted hydrogen, produced by the decomposition of iron-pyrites by 

 water, is sometimes rather troublesome to well sinkers, and carbonic acid gas 

 (choke-damp) is apt to accumulate in tunnels and borings in Chalk. ^ 



The Chalk Downs are in many places celebrated for the extent of their prospect, 

 while the boldness of the escarpment and the whiteness of the substance have 

 given the idea of forming figures in various parts by cutting away the turf. The 

 White Horse, south of Uffington in Berkshire, occupies about an acre of ground, 

 and may be seen from some points of view at the distance of twelve miles. There 

 is also a land-mark of the same kind at Cherhill, near Calne, cut about the year 

 1780, by Dr. Alsop, a physician of that town; and another White Horse at 

 Bratton, near Westbury, in Wiltshire. On the chalk hill that faces Weymouth 

 there is a representation of George HI. on horseback ; and near Cerne Abbas 

 there is a figure of a giant (180 feet in height) holding a club in one hand and 

 extending the other. Near Prince's Risborough a large cross has been cut in the 

 Chalk. One of the largest artificial mounds in Europe, that of Silbury Hill, near 

 Avebury, was formed of Chalk rubble. Its height is 170 feet and the circumfer- 

 ence at the base 2027 feet. 



The Chalk Downs produce a short herbage, adapted for sheep, of which the 

 South Down and Hampshire Down breeds are noted. In some parts good crops 

 of turnips and wheat are produced ; and much fine barley is grown on Chalk 

 soils, near Devizes, Warminster and Salisbury. The Lower Chalk and Chalk 

 Marl are, as a rule, more fertile than the Upper Chalk. The Chalk Downs are 

 considered to retain their ancient character more than any other tracts of country. 

 Beech trees grow exceedingly well on the Chalk (especially where it is covered with 

 clay-with-flints or brickearth), and in Buckinghamshire the wood is largely used 

 for chair-making ; as a rule, however, the Downs are bare and open. 



The fissured and sometimes rubbly surface of the Chalk is to a large extent 

 caused by rain and frost, but occasionally in the Eastern Counties the Chalk has 

 undergone disturbance by some more powerful agent, so that the mass of the rock 



1 Report on the Water-springs at Grays, 1S60 ; Water-bearing Strata around 

 London, 1851, p. 57. 



2 J. Lucas, Horizontal Wells, 1874. 



3 F. Sargent, T. G. S. (2), i. 168. 



* Dr. J. Mitchell, Proc. G. S. iii. 151. 



