102 W. Gunn — Subaerial Denudation v. Glacial Erosion. 



According to Mr. Goodcliild, subaerial denudation in forming 

 swallow-holes by the action of streams should give rise to a series 

 of ravines extending across the whole width of the terrace (p. 359), 

 and as on the high ground ''a considerable portion of the pre-glacially 

 weathered rock was left " (p. 361), we ought there to find the remains 

 of such ravines. Yet it is true enough, as he says, they are very rare. 

 I know of one in Arkindale, about 150 yards long, which, however, 

 does not stretch all across the outcrop of limestone. It is more 

 common to find, beyond the place where a stream now sinks, an 

 irregular line of swallow-holes ; which shows that the stream 

 generally'" finds some place to sink in, higher up in its course, before 

 it can cut its way back. Thus a new swallow-hole is formed and 

 the old one is disused ; and if we are only granted a small portion 

 of the enormous dissolving power of rain on limestone rocks claimed 

 by Mr. Goodchild^ the swallow-holes may disappear by general 

 waste of the surface ere the scar is cut back to them. 



The argument against the fluvial origin of these rock ledges, 

 "that many of them are situated a thousand feet above any stream 

 that could possibly have produced them" (pp. 324, 328, 359), seems 

 to me to tell against their glacial origin ; for the highest of the 

 principal limestone terraces — that formed by the Main Limestone — is 

 generally the largest of any, and its scar the most continuous, while 

 it is situated at a height where the ice could have little abrading 

 power, and where, in Mr. Goodchild's opinion, " a considerable portion 

 of the pre-glacially weathered rock was left " (p. 361) ; therefore 

 this terrace should be much less perfect than the lower ones. 



I do not see why a great part of many of these rock terraces may 

 not have had a fluvial origin. Over the greater part of the dis- 

 trict the rocks are only very slightly inclined from the horizontal ; 

 and when the valleys began to be formed, the slope of the ground 

 and dip of the beds very nearly coincided, so that a river must have 

 run for long distances on the same bed. And pace Mr. Goodchild, 

 streams generally in these Yorkshire Dales do not now cut down 

 rocks to one level, as the sea does (p. 356); and as it would "be 

 difficult to find one of the Dale District terraces that is not more or 

 less inclined," so it would be difficult to find in the same district any 

 considerable length of river cliff that is shorn off to one level. 

 Of course I do not suppose that a river could form such scars and 

 terraces as those near Carperby (p. 327) ; nor do I see how they could 

 have been formed by Glacial Erosion, for it is admitted that " where 

 the ice moved across the outcrops of the beds, the scars and terraces 

 are either slightly developed or else are wanting altogether " (p. 486) ; 

 and I do not think the ice would have gone out of its straight path 

 in order to coast the outcrops here. There are several other ex- 

 amples of lines of scars and terraces in these dales, in steeply dipping 

 rocks, that could not have been formed by the action of streams; nor 

 by erosive action of ice, unless it formed scars when cutting across 

 the edges of the beds ; hence one is forced to look elsewhere for the 

 origin of such. 



In modern ravines, such as those Mr. Goodchild has so well de- 



