596 A. J. JEKES-BEOWNE ON THE KELATIVE AGES OE 



34. On the Belative Ages of Cektain Bjver-valleys in Lincoln- 

 shiee. By A. J. Jukes -Beowne, Esq., B.A., F.G.S. Com- 

 municated by permission of trie Director-General of the Geolo- 

 gical Survey. (Eead June 20, 1883.) 



InTEODTJCTION. 



In a country which is traversed by a series of escarpments or 

 hill-ranges, the valleys by which its drainage is effected are usually 

 separable into two sets or systems, one parallel to the strike of the 

 ridges, and the other more or less at right angles to the same. The 

 origin of these longitudinal and transverse valleys, and the process 

 by which escarpments have been intersected by river-valleys, were 

 first explained by Mr. Jukes *. He showed also that in the case 

 of a river cutting through a ridge or escarpment, and receiving 

 tributaries from the longitudinal valleys which are parallel to this 

 ridge, the primary or first-formed stream is that opposite to the 

 breach in the escarpment, and that the longitudinal branches, 

 though often of much greater length than this primary stream, 

 are really of secondary or subsequent origin. 



Stated in general terms, his theory amounts to this, that the 

 original direction of all rivers which cut through ridges was de- 

 termined by the general slope of the ancient surface over which 

 they began to run, and that when the slope was transverse to the 

 strike of the beds, the channels cut by the earliest rivers necessarily 

 had a similar transverse direction, while the channels in the longi- 

 tudinal valleys were formed subsequently and concurrently with the 

 development of the ridges or escarpments. 



A further corollary to this may, I think, be considered as 

 generally true, viz., that those portions of a river-valley which 

 intersect the same ridge date from the same epoch of time ; ancl 

 that rivers flowing between the same two parallel ridges came into 

 existence at the same time. 



It does not follow, however, that the channels of all the primary 

 transverse streams were permanently maintained ; the extension of 

 a longitudinal tributary may intercept the drainage of minor 

 transverse streams. Thus in fig. 1, A, B, C, are three transverse 

 streams, each of which originally ran across the ridge D K, as in- 

 dicated by the dotted continuations of B and C ; but the extension 

 of the tributary T has intercepted the waters of B and C. 



Jukes, writing of the river-valleys in the south of Ireland, ex- 

 presses this as follows f: — "The longitudinal valley may even be 

 worn back across the course of many minor transverse streams, and 

 deflect their waters down its course." He also observes that " to 



* Quart. Journ. Geol. Soc. vol. xviii. p. 378 (1862). 

 t Manual of Geology, 3rd ed. p. 456 (1872). 



