1854.] 



PRESTWICH SWALLOW HOLES IN CHALK. 



223 



incessant addition, remain^ unchanged and without rise, the water 

 being gradually and quietly absorbed as fast as it is supplied. Only 

 occasionally after heavy rains the water stands for a few hours some 

 feet higher. The sides of these excavations are usually sloped with 

 debris, grass, and bramble, and the bottom covered by a bed of sand 

 and gravel so that the chalk surface cannot often be seen. Some 

 of the swallow holes are situated within the boundary of the Lower 

 Tertiary sands, whilst others are just on the edge of the chalk. 

 Between this spot and the river Stour at Shalmford Street there is 

 a descent probably of 200 to 300 feet, throughout which the surface 

 of the chalk is as bare of wood as it is of water. But on the river- 

 bank near that village a large and perennial spring bursts out. There 

 are, I believe, several other springs in the river, but this is a very 

 striking one, and is apparently dependent upon the brooks lost in 

 the swallow holes a mile distant on the hills above. Not that I 

 think that the streams are continued underground in separate and 

 independent channels from the spot where they disappear to that 

 at which they issue in the river-bank, but that they descend, within 

 a short distance, through one or more channels down through the 

 mass of the chalk, until they reach the line of permanent water-level 

 which passes under the hills in a curve rising slightly from the river 

 Stour and descending again towards Feversham. The additional 

 supply made by the brooks at this spot determines a higher local level 

 in the water-line, and consequently the springs issue in greater force, 

 and higher above the river, along the nearest lowest level of the 

 valley' — the river channel. The bulk of the springs are probably in 

 the bed of the river, or low on its banks, and are therefore not so 

 apparent. 



The accompanying section gives a general view of the position 

 of these swallow holes and of the main spring. 



General theoretical Section of the Hills S.E. of Boughton to the 

 River Stour between Chilham and Canterbury. 



Length about 2 miles. s.e. 



1. London Clay. 2. The Lower Tertiarj- Sands. (These should be continued 



between a and a'.) 3. Chalk. 



a, a'. Swallow-holes in the surface of the Chalk. 



b. Line of water-level in the Chalk *. s. Spring on the river-bank. 

 S, », z. Sand-pipes on the surface of the Chalk, filled with the tertiary sands. 



* Assuming b to be placed in a central position between s and the river-level at Fevershara, 

 to the N.W., the line of water-level from b to ,9 would form a nearly regular descending plane 

 in either direction, and the point s would fall rather lower than in the above diagram ; but the 

 concentrated supply of water at a, ii', raises the water-level between b and s, and affects in 

 proportion the water-lev 1 at s, causing it to stand higher than it would in its normal condition, 

 and determining, therefore, at that spot a larger delivery of water than usual. The water- 

 channels conducting from a, n', to the dotted line b, s, may be nearly perpendicular, or may 

 follow for some distance the line of stratification formed by the beds of flmts. 



