596 DENUDATION AND SCENERY. 



The coast-line is indented, as might be expected, at the mouths of rivers where 

 the action of the waves is increased by contraction, producing the estuaries of the 

 Thames and the Bristol Channel, the estuaries of the Dee and Mersey, and the 

 Solway Firth. In the Bristol Channel much destruction of the coast is taking 

 place near Watchet, and this is facilitated by the workings for alabaster, and by 

 the removal of beach pebbles of limestone for lime-burning. Between Aust Cliff 

 and Pollock Bay the Triassic marls and sandstones have been most easily denuded ; 

 while the harder Carboniferous Limestone and Devonian rocks stand out more 

 boldly ; again on the north side of the channel, Swansea, Oxwich, and 

 Caermaithen Bays are excavated in the Coal-measures. 



TJie indentations further north in the coasts of Wales and Lancashire (Cardigan 

 and Morcambe Bays, etc.) are due to the varying texture of the rocks. ^ 



Notwithstanding the great erosion of our coast-line, it is satis- 

 factory to find that in many places additions are being made both 

 naturally and artificially, so that, according to Mr. Topley, it is 

 probable that the total land-area of England and Wales is as great 

 now as it was 500 years ago.- 



The waste of the land by subaerial agents, although greater, is 

 not so conspicuous as that caused by the sea. Much denudation is 

 subterranean, for the action of rain and springs is chemical as well 

 as mechanical. The denudation by rivers can be estimated by the 

 amount of solid matter which they annually discharge into the 

 sea, a subject to which attention was directed in 1852 by Alfred 

 Tylor.^ Such estimates are often surprising. Thus I\Ir. E. Witchell 

 has calculated that 200 tons of solid matter are annually removed 

 in apparently clear water from every square mile of the Cotteswold 

 country, while a large quantity of solid matter is also removed 

 in suspension.* The formation of caverns and hollows in limestone 

 rocks, and the many mineral springs, testify to the subterranean 

 denudation of the strata. INIany land-springs too carry away mud 

 and sand from the strata through which they sink before issuing at 

 the surface, and in this way they may gradually lower the general 

 level of a tract of ground without influencing in any marked way 

 the surface configuration. The influence of streams in forming 

 deep underground channels may be frequently witnessed in cliff- 

 sections, where porous strata rest on clayey beds. In other cases 

 where the strata below have been dissolved away, falls of earth or 

 subsidences of the ground take place from time to time. 



Local subsidences of the land are produced both naturally and artificially. In 

 the former case by dissolution of the strata, and in the latter case by excavations 

 for Coal, Chalk, etc. 



In the salt-districts of Cheshire the dissolution of the rock-salt by springs, near 

 Northwich and other places, has caused alarming subsidences, and these have 

 sometimes given rise to meres, such as those of Great Budworth, Pick, and 

 Rothorne.^ (See p. 241.) 



^ See J. E. Thomas, Prize Essay upon the Encroachment of the Sea between the 

 River Mersey and the Bristol Channel, 1867. 



- Address Geol. Assoc. Nov. 18S6, Nature, Nov. Ii, p. 38; see also Reports 

 on Erosion of Sea-coasts, Brit Assoc. 18S5, 1S86. 



3 Q. J. ix. 47, G. Mag. 1875, p. 443. 



* Proc. Cottesw. Club, 1867, p. 225. 



^ G. W. Ormerod, Q. J. iv. 269; G. Mag. 1874, p. 259 ; T. Ward, Nature, 

 Oct. 14, 1880, p. 560. 



