J. G. Goodchild — Glacial Erosion. 323 



The above numbers call for two remarks. Firstly, it will be seen 

 that though the density of zircons from some localities is increased 

 by ignition, this is not the case with the Expailly or Mudgee speci- 

 mens, which remain unaltered by heat. Secondly, some zircons are 

 of very low density (No. 63 above). This density remained the 

 same after heating. The stone was a true zircon however, giving 

 on analysis the per-centages of that species. 



VII. — Glacial Erosion. i 



By J. G. Goodchild, F.G.S. ; 



Of H. M. Geological Survey of England and "Wales. 



THE Lower Carboniferous rocks of the Yorkshire Dale District — 

 Wensleydale, Swaledale, Dentdale, Garsdale, and the adjoining 

 parts — consist of a series of alternations of limestones, sandstones, 

 and shales, not usually much inclined from horizontality. The 

 harder beds of these commonly form terraced outcrops, which are 

 often several hundred yards in width from the scar or the steep 

 escarpment at their outer edge to their inner margin where the next 

 bed above comes on. Owing to the nearly horizontal position of the 

 rocks throughout the greater part of the district, many of these ter- 

 races and scars can be followed for miles almost without interruption. 

 Hence they form perhaps the most prominent characteristics of the 

 Dale District scenery, and they offer a striking contrast with the 

 generally regular outline of the dome-shaped hills, and the short 

 and irregular scars that characterize the adjoining area of Silurian 

 rocks. 



The principal object of the present communication is to endeavour 

 to show how these Carboniferous terraces and scars were formed. 



"Whilst engaged with Professor Hughes upon the Geological Survey 

 of the Dale District I often noticed that the swallow-holes marking 

 the presence of the limestones there occur along only the inner 

 margin of each terraced outcrop, while nearly all the rest of the rock 

 exposed is entirely free from such indications of Subaerial Denudation. 

 Then again, the inner and the outer margin of each rocky shelf are 

 rudely parallel to each other and to the outlines of the terraces both 

 above and below ; and where the valley is not very wide, the outline 

 of each scar, whether convex, nearly straight, or concave, is matched 

 by the corresponding form in the scar formed by the same bed on the 

 opposite side of the dale. Not less striking is the frequent absence 

 of any debris from the higher beds on the same hill-side, even from 

 those close above the limestone of the terrace. 



Hitherto it appears to have been assumed that the long-continued 

 action of subaerial agencies is sufficient to produce the phenomena 

 here referred to ; but, however plausible at first sight this theory 

 may seem, when it is applied to explain some of the facts that an 

 attentive examination brings to light, it fails completely. 



1 The substance of the following communication was laid before the Geological 

 Society 24th June, 1874, by permission of the Director-General of the Geological 

 Surveys. It is reproduced in its present form in order that the accompanying theories 

 may evoke some criticism, which they could not receive when the original article, 

 together with 26 others, was read in brief abstract at the last meeting of the Session. 



