342 ITNEDALE ESCARPMENTS; 



they vary, in tliis district, from mere dimples up to cups twenty 

 or twenty-five feet deep. They are not generally permanent 

 for any considerable time. The fissures that occasion them are 

 at once so numerous, and of such varying width, that the opening 

 of now holes seems to divert the centres of the little drainage 

 basins from place to place, without allowing time for the elonga- 

 tion of the oiifice in waterfall style. When discarded they seem 

 to be liable to obliteration either by the filling up of the cup, or 

 the removal of the cover wliich it perforates. 



Tlie formation of swallow-holes is well illustrated in certain 

 streams. When the course of a stream flowing upon clay drift 

 happens to coincide with the outcrop of a limestone, a point is 

 by and bye reached in the deepening of the stream-bed at which 

 the clay is swallowed into a fissure and the stream engulfed. The 

 lower reaches of the course then become nearly dry : but above 

 the hole, deepening progresses as before ; new chasms open suc- 

 cessively above the first, and bit by bit tlie stream is shortened. 

 Sooner or later, of course, it diverges from the line of the lime- 

 stone, and the hole last formed is, comparatively speaking, per- 

 manent. The old channel remains for a time a curiously irregular 

 succession of burrows and sadtUe-backs, sometimes like the rout- 

 ings of a huge animal. 



It is difficult to see how some of those more prominent swallow- 

 holes can ever be superseded unless by the engulphing of the 

 stream by another limestone. Yet so inconspicuous, generally 

 speaking, are the effects of the water upon the limestone, and so 

 inconsiderable the modification of the original circular form, that 

 they can scarcely be otherwise than of recent formation. En- 

 largement to the extent of several cubic yards is exceptional, 

 though it must be said that the full effects are rather apt to be 

 masked by clay. But as we have seen, the solution of limestone 

 is apt to be exaggerated. I may add, that the best examples 

 in this district of streams burrowing in tliis fasliion lie to the west 

 of the Church at Thockrington. Ordinary swallow-holes are so 

 numerous as to make it superfluous to name localities.* 



* Artificial drainage, in those claya, IncreaHei the importance, and will soon alter the 

 npiKsnrnnce of many (wallow-holea. I have seen half a doxon drain* led into a Hlngle oue, 



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