1854.] SORBY ON YEDMANDALE. 329 



now in very nearly the same state as when first it emerged from 

 those conditions of submersion which denuded the district. It is 

 situated about four miles west of Scarborough. The lower end 

 opens into the vale of Pickering at Ayton, and it trends to the north- 

 north-west for about two miles. In connexion with the great pro- 

 blems of denudation, formation of valleys, and superficial deposits, 

 it presents a number of facts of such importance in forming a correct 

 theory for this district, that I shall describe them with some degree 

 of minuteness. 



At the lower end of the valley the sides are inclined at an angle 

 of about 30° ; and it may be seen that there it has been cut out of 

 the sand and gravel with erratic pebbles found in the vale of Picker- 

 ing, Here the bottom of the valley is flat, and about fifty yards 

 broad ; and there is no indication of a brook having ever run in it. 

 Passing upwards, however, we come to two ravines, trending north, 

 and down them come streams, which, when arriving at the main 

 valley, form an excavated course in its flat bottom, gradually vanish- 

 ing, the water obviously passing away through the coarse detritus of 

 which it is composed. Above these ravines the flat tract is again 

 entire for some distance, until we arrive at another brook, which also 

 vanishes in a similar manner. 



Higher up, however, this brook has cut away and removed the 

 whole of the detritus from the bottom of the valley, and there its 

 sides are inclined at an angle of 30° from the top to the bottom, as 

 shown on the plan (fig. 1), where it is marked Sec. 1, and by fig. 2, 

 Sect. 1, where the. upper dotted line indicates the original surface 

 before the valley was excavated, and the lower, the probable extent 

 of the detritus in it, before it was removed by the action of the 

 brook. Farther up the valley, the brook has not washed away the 

 whole of the detritus, but has formed in it a small ravine and little 

 flat alluvial tracts, as shown on the plan. Sect. 2, fig. 3, exhibits the 

 configuration in this part, where the results of the operation of the 

 modern brook, and that of the agent that excavated the valley, are 

 shown by the lower and upper dotted lines. 



Here the nature of the detritus at the bottom of the valley is well 

 seen. It is composed of boulders and pebbles, of a size varying up 

 to several feet in diameter, which are but slightly rounded or worn ; 

 and, in some parts, have been cemented together by the infiltration 

 of calcareous water. Though many thousands are exposed, and 

 though I examined them very carefully, I did not find one that could 

 not have been derived from the immediate vicinity northwards, 

 being all composed of harder and softer calcareous grit ; and I did 

 not see any of coralline oolite, though it occurs in situ only a short 

 distance to the south. 



The brook just mentioned comes down the western arm of the 

 valley, as shown on the plan, and has removed from it all the detritus 

 that probably extended up it ; and, for the whole of its length, which 

 is about a mile, the section is similar to Sect. 1, the ravine becoming 

 less and less. It is, however, the eastern branch that claims the 

 most particular attention. Its form will be best understood from the 

 plan. Except just at the upper end, the sides are incHned at an 



2 A 2 



