A HISTORY OF DERBYSHIRE 



may mark the site of a volcanic vent occurs in the western part of the 

 inlier. 



At Snelston Common south-west of Ashbourne is a small dome- 

 shaped inlier of Mountain Limestone surrounded by Keuper Marl. 

 Lead and copper ore were formerly worked here. Some portions of the 

 limestone are dolomitized. 



At Diminsdale, Calke and Ticknall are three small inliers consisting 

 of the upper beds of Mountain Limestone, which are succeeded con- 

 formably by the Limestone Shales. They are brought to the surface 

 through the combined agency of a series of gentle rolls and of a great 

 fault which brings them against the Ashby Coal Measures. 



CAVERNS 



The upper beds of Mountain Limestone are in many places honey- 

 combed with caverns. Some of the caverns of Derbyshire are old lead 

 mines, but others are natural and connected with the underground drain- 

 age of the district. 



The existence of unexplored caverns may be inferred by the 

 numerous swallow holes found in the limestone. These swallows vary 

 in depth and diameter and have generally the shape of a wide cone with 

 a blunted point at the bottom. The sloping sides may be covered with 

 grass with or without any signs of rock. The swallows are formed by 

 the action of water containing carbonic acid finding its way into joints 

 and enlarging them by dissolving away the rock and ultimately emerging 

 at a lower level and at some distance from the place of entry. As the 

 volume of water increases the subterranean course is enlarged by the 

 solvent action of the water, and caverns are formed. The caverns are 

 often enlarged by the falling in of the roof and the removal by water of 

 the resulting debris. 



Poole's Hole or Cavern near Buxton illustrates the intimate relation 

 between caverns and underground drainage. The river Wye, a tributary 

 of the Derwent, rises on Axe Edge, and after flowing over the Yoredale 

 Shales disappears down a swallow in the limestone called Plunge Hole. 

 It reappears in Poole's Cavern, and after flowing for some distance in the 

 part shown to visitors suddenly disappears again and finally emerges from 

 a small hole called Wye Head a few fields away from the mouth of the 

 cavern. 



The Speedwell Cavern near Castleton is an instance of those caverns 

 which are partly artificial and partly natural. The entrance is at the 

 foot of the Winnats. A level was driven into the limestone in order to 

 reach some of the rakes which run through the hill. After passing 

 several serins or calcite veins the level reached the New Rake at a hori- 

 zontal distance of 750 yards from the entrance. The level entered a 

 large narrow cavern which extends to a great height and was hollowed 

 out in the New Rake by underground waters. A solid platform has 

 now been built on the sloping floor of the cavern which is inclined at 

 an angle of about 45 to the horizon. The level now contains water 



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