532 RECENT. 



is overlaid by the retentive Lower Eocene Clays. (See Fig. 70,. 

 p. 414.) 



Intermittent springs are sometimes fed by cavities in the strata 

 wliich are filled during rainy seasons, and gradually drained off by 

 the springs : there is an intermittent spring at Tideswell in Derby- 

 shire. They may be regarded as temporary outlets after unusually 

 heavy rains. 



The 'Bournes' or ' Nailbournes' are intermittent springs of the 

 Chalk districts, being channels used during excessive rain-fall, 

 when the line or plane of saturation of the Chalk rises to a higher 

 level than usual. The Lambourn and the Winterbourn in Berkshire 

 are examples of these overflows, and so are the Croydon Bourne, 

 the Assendon, near Henley-on-Thames, and the Hertfordshire 

 Bourne, which falls into the Bulbourne at Bourne End.^ (See p. 

 421.) 



Swallow Holes* — Sometimes on reaching a limestone formation, such as the 

 Carboniferous Limestone, after passing over the outcrop of the Lower Limestone 

 Shales, a stream will follow the clayey strata, and disappear beneath the Limestone 

 in ' .S wallet ' or ' Swallow ' Holes. Examples of these swallow holes occur at 

 Downhead Mill, at Priddy, and at Charterhouse, near Blackdown, on the Mendip- 

 Hills. The exit of the streams may be traced with much probability to the follow- 

 ing points : that of the Downhead stream to the limestone chasm about a mile east 

 of the village of Downhead ; that of the Priddy rivulet to Wookey Hole, and that 

 of the Charterhouse brook to the foot of Cheddar cliffs.^ A similar spring no 

 doubt produces St. Andrew's Well, at Wells, in Somerset. 



Subterranean rivers are frequent over the Carboniferous Limestone district of 

 the north of England. Among these is the Manifold, which passes beneath the 

 limestone hills about three miles south-west of Ecton mine in Staffordshire, and 

 after traversing a cavern through the base of the limestone hills four miles in 

 length, re-appears near Ham. The River Aire, in Craven, Yorkshire, issues from 

 Maiham Tarn, a circular lake about a mile in diameter, on the summit of a lofty 

 moor : proceeding thence, it descends through a subterraneous passage, and again 

 issues at the foot of Maiham Cove, a perpendicular limestone rock 288 feet high. 

 During heavy rains the subterraneous passage is not sufficient to carry off all the 

 water, the remainder of which makes its way over the surface, till it reaches the 

 top of the rock and precipitates itself thence in a magnificent cascade.^ Keld is a 

 term applied to the large springs so common in limestone districts, where the 

 water collected in the pot-holes and crevices of the rock runs out a full stream 

 from a cave below.* 



In the Carboniferous Limestone on Appletreewick Moor in the neighbourhood of 

 Skipton (Craven disti'ict), there are hollows, like inverted cones, sometimes fifteen 

 yards across, and of equal depth. An account of these has been published by 

 Mr. L. C. Miall.5 Thund Pot and Helm (Helln or Allum) Pot are amongst the 

 largest. They are sometimes called ' Butter-tubs,' as between Swaledale and 

 Wensleydale. The Swallow-holes or 'water-sinks' are of two distinct kinds, 

 those due directly to chemical erosion (butter-tubs) and those to subsidence of an 

 undermined crust (pots). Many of the latter occur around Ingleborough, at 

 Weathercote, and near Chapel-le-Dale ; these include Jingle Pot, Hurtle Pot, etc.^ 



' Whitaker, Mem. Geol. Surv. iv. 391 ; J. Evans, Trans. Hertfordshire Nat. 

 Hist. Soc. ii. p. Ivii ; T. R. Jones, G. Mag. 1885, p. 148. 



^ Buckland and Conybeai-e, T. G. S. (2), i. 210. 



3 Conybeare and Phillips, Geol. Eng. and Wales, p. 397 ; see also P. GeoL 

 Assoc, vii. 436 ; Dawkins, Cave-hunting, 1874, p. 47. 



* Hughes, G. Mag. 1S67, p. 349. 



5 G. Mag. 1870, p. 513. 



" Dawkins, G. Mag. 18S0, p. 514; Phillips, Rivers, etc., of Yorkshire. 



