326 J. G. Goodchild — Glacial Erosion. 



the shale marts the greatest distance that this can have been cut 

 back in Post-Glacial times. 



The thinner kinds of sandstone, especially where they are much 

 split up by beds of shale, seem to go to pieces very readily ; but 

 upon the more compact, blocky, and little jointed kinds ordinary 

 weathering seems able to produce very little effect. A very good 

 example of this is to be found at Mosedale Foss in Wensleydale — 

 a waterfall caused by the superposition of a hard and blocky sand- 

 stone on a bed of soft and thinly laminated shale. The length of 

 the ravine that the fall is found in is nearly eight hundred feet from 

 its outer end to the fall itself; while the difference in the width 

 of the ravine at the two points measured is only sixty feet. Yet in 

 this instance a rapid stream flows within a few yards of the foot 

 of the scars, which have thus receded only thirty feet on each side 

 since the ravine was formed. There is good reason for thinking that 

 this was in Post-Glacial times. The remarks made above relative 

 to the nearness of glacial striae to the outcrop of higher beds of shale 

 apply equally to the accompanying sandstones in similar positions ; 

 thus we get direct evidence that some of the sandstone scars have 

 not been much altered in form since the close of the Glacial Period. 



For our present object the rock of most importance as regards its 

 behaviour before subaerial agents is limestone. Not much more need 

 be added to what has been already stated about the cutting back of a 

 waterfall in this rock : where, however, it is found in thick beds, 

 and is not very much split up by structural planes, limestone seems, 

 under like conditions, to recede not quite as fast as sandstone. But 

 under the influence of the weather, limestone, as is well known, 

 often disappears with great rapidity. Jukes's comparison of it to a 

 glacier melting before the summer's sun conveys an excellent idea 

 of the way this rock is dissolved and carried away in solution by the 

 waters from the surface. The numerous structural planes that every 

 bed of limestone is more or less divided by are developed and rapidly 

 widened to a considerable depth from the surface by the action of the 

 acidulated waters, which thus easily find their way to a lower level. 

 There seems reason for believing that the absolute rate of dissolution 

 of limestone is far from slow, even when measured by years. In 

 Kirkby Stephen Churchyard there was in 1871 an erect gravestone 

 of ordinary mountain limestone that was put up about fifty years 

 ago. As the stone was carved, at least the greater part of it must 

 once have been smooth and unweathered ; when I saw it in 1871 

 there were encrinite stems and bits of other fossils left in relief to 

 the extent of a tenth of an inch or more, because the softer matrix 

 had been removed by the rain that has fallen on the stone since its 

 erection. One cannot be quite sure even that the highest parts of 

 the fossils accurately represent the original dressed surface; but, 

 assuming that they do so, we have in this instance proof that a smooth 

 and quite unweathered piece of limestone, standing in a position the 

 least favourable for erosion by subaerial agencies, is being dissolved 

 away at the rate of one inch in five hundred years. Where the form 

 of the surface is such that water can remain some time upon the rock, 



