Kugh Miller — Escarpments and Terraces. ' 31 



I believe it to be possible, however, by selecting only such, 

 gorges as are post-glacial, to arrive at results of some value: 

 1st, as showing the actual amount of the scarp removed since 

 the Ice Period ; and 2nd, as a means of comparison between 

 sandstone and limestone under weathering. I give the follow- 

 ing measurements more to illustrate my meaning than as con- 

 clusive on the subject, which no single instance can be. The cases 

 are taken from the neighbourhood of the village of Birtley, North 

 Tynedale. A is a post-glacial gorge in sandstone and shale, oc- 

 cupied by a stream vigorous at least in winter, and doubtless rapidly 

 lengthening. B is a short limestone gorge terminating in a rocky 

 ledge now dry, but in the " Eain Period " succeeding the Glacial 

 Epoch, and even during the extension of the forests swept away 

 since the Roman Era, doubtless containing its waterfall. This 

 evidence, so far as it goes, shows, 1st, that since the Glacial Period 

 A 



I Vertical Section, 

 b I Sandstone 8-10' ....;... 



21 Shale 20' 317' 



Vertical Section, 



Limestone 18-20' 



912' r-b' 



the waterfall of A has cut back 190 feet, while from each side 18 feet 

 has been disintegrated ; and indicates, 2nd, that the recession of a 

 limestone scar from atmospheric disintegration has been to that of 

 a scar of sandstone and shale as 19 is to 18. 



Though I adduce these instances as in some degree bearing out 

 my views, I regret that North Tynedale does not afford the materials 

 for satisfactorily working out the question, which must be decided 

 in a district supplying several instances of both kinds. Mr. Good- 

 child's method may then — -with the amendations above proposed — 

 lead to definite and interesting conclusions. 



One fact is sufficiently clear, namely, that atmospheric disintegra- 

 tion is an agent so gradual that little can have been accomplished 

 by it in post-glacial times. Accordingly it is only to be expected 

 that the quantity of debris strewn about the escarpments should 

 never be verj^ great. In this respect there is indeed, a marked 

 difference between the sandstone and the limestone outcrops of West 

 Northumberland. The former are sometimes considerably ruined ; 

 the latter are, on the contrary, so free from dislodged fragments that 

 it is rare to find many together. This fact explains the paucity of 

 debris remarked by Mr. Goodohild for the Yorkshire terraces. The 

 isolated pieces toppled from the limestone scars are even more ex- 

 posed to reducing action than while in situ, while the opposite is 

 probably the case with sandstone debris. It is to be remarked, too, 

 that sandstone escarpments, even when standing over a pile of fallen 

 blocks, give no signs of becoming buried iu their own ruin. On the 

 contrary, the more rapid the dislodgment and the larger the masses, the 



