486 J. O. Goodchild — On the Origin of Coums. 



b. On the age of the Oemhilien Coal-field. 



c. Geological description of the Oemhilien Coal-field. 



d. On the Geology of the Island of Nias, with several maps and sections. 



Fort van der Capellan, West Coast of Sumatra, 



March 10th, 1875. 



II. — On the Origin of Coums. 

 By J. G. Goodchild, F.G.S., of ELM. Geological Survey. 



IN my paper on Glacial Erosion lately laid before the readers of the 

 Geol. Mag. (pp. 323 and 356), I have endeavoured to prove 

 that the origin of nearly all the more prominent surface character- 

 istics of the rock scenery in the Yorkshire Dale District admits of a 

 simple and complete explanation by the theory of the modification 

 of pre-existing subaerially eroded surfaces by Glacial Erosion. At 

 the same time it was shown that the character of many of the phe- 

 nomena is entirely opposed to any theory of their origin by means 

 of Subaerial Denudation alone. In the present communication it is 

 proposed to inquire how far this Glacial Erosion theory may be 

 applied to explain the origin of the deep, semicircular recesses that 

 are commonly found in all well- glaciated mountainous districts, and 

 are variously known by the names of Coums, Corries, or Cirques. 



The more prominent terraces and scars whose origin was discussed 

 in the paper just referred to seem to occur only where the ice moved 

 in the direction of the valley's length : where the ice flowed to a 

 greater or less degree across the valley, or, in other words, where 

 the ice moved across, instead of along, the outcrops of the beds, 

 these characteristic scars and terraces are either slightly developed, 

 or else are wanting altogether. But whatever the minor inequalities 

 of the slopes of the valleys may be like, it is commonly found that 

 there is a striking resemblance in form between the contours of the 

 surface at any given elevation and the contours for a considerable 

 distance both above and below. "Where the side of the valley is 

 convex in contour, the gradations in form are complete between the 

 curves of largest radius near the bottom of the valley, and the more 

 decidedly rounded contours of lesser radius that are found in greatest 

 perfection at the higher parts of the feature. So, too, with the 

 slopes that present concave contours. In these the least regular 

 curves are nearly always found near the base, and the contours 

 gradually become more decidedly concave and of larger radius as the 

 upper limits of the feature are approached; and, as a rule, it is also 

 at the upper limit that the most regular curves occur. 



Some of the rock surfaces whose form I have before endeavoured 

 to show must be due to the unequal resistance to mechanical erosion 

 offered by beds of various degrees of hardness graduate, by insensible 

 degrees of form, from surfaces with contours that are nearly straight, 

 through others that are more or less concave, into semicircular recesses 

 that remind one rather of gigantic pot-holes than of anything else. A 

 very beautiful example of this kind occurs at the head of Snaizholme 

 Beck, about a mile to the south of Hawes, in Wensleydale. Others 

 of similar but less perfect form may be found in the neighbourhood. 



