Vol. 65.] 



GLACIAL EROSION IN NORTH WALES. 



299 



small and of intermittent flow, accordant junctions of trunk and 

 branch prevail. All the more essentially accordant must the junc- 

 tions of main and side-streams be when the main stream has not 

 only worn down its course to grade, but has widened its valley- 

 floor by the relatively slow process of lateral erosion ; for, during 

 the considerable interval of time required for this process, all the 

 side-streams, even the smallest, will wear down the mouths of 



Fig. 4. — Diagram of a young 

 main valley, with normal 

 hanging lateral valleys. 



Fig. 5. — Diagram of a mature 

 main valley, ivith accordant 

 lateral valleys. 





^ 



their valleys to as great a depth as that of the main valley. 

 Examples attesting the truth of this principle may be counted by 

 the hundred in the Appalachian plateau, and in any other maturely 

 dissected non-glaciated region. The contrast of these normally 

 accordant valley-junctions, with the discordant or hanging junctions 

 such as prevail in the Snowdon district, is most striking. 



The main valleys around Snowdon are prevailingly even-sided 

 or trough-like; few spurs stretch into the valley-floors; and, by 

 reason of the hanging relation of the lateral valleys, no lateral em- 

 bayments are opened in the side of the main valley-floor. The cross- 

 profile of the valleys is often a fine catenary curve, of which more 

 will be said in a later section. A rock-step of siidden descent may 

 occur in the floor of such a valley, as in the left side of fig. 3 

 (p. 296) ; it is sometimes dependent on, sometimes apparently 

 independent of rock-structure. Although the valley is maturely 

 widened, the valley-stream cascades down the face of the rock-step 

 in a very youthful fashion. Evidently, under no normal conditions 

 of ordinary weather and water-action can the rock-step have been 

 made : for a cascade in the course of a normal stream is a mark of 

 youth, and (except in special cases of hard and soft horizontal 

 strata) is normally associated with a narrow valley-floor, hardly 

 wider than the stream at the cascading point, and with steep 

 valley-walls ; if the valley is wide open even in the hard rocks, the 

 cascade should normally have disappeared. 



In striking contrast with the sudden increase of slope at the rock- 

 steps is the abnormal decrease of valley-floor slope, even to the 

 point of becoming negative or reversed, as where lake-basins occur. 



