446 Maw — On Subaerial and Marine Denudation. 



Now nearly all materials have what is called an angle of repose, and 

 will fracture or slip off as soon as their inclination attains to within 

 a given number of degrees of the perpendicular. Some very solid 

 rocks will we know stand with almost vertical sides, and in such 

 rocks running water w^l scoop out a vertical trough as indicated on 

 the right hand side of Fig. 6, and of this structure examples occur on 

 the Khone a few miles below Greneva, on the Ehine in its passage 

 through the Via Mala, and on a smaller scale in the Aber valley ; but 

 in the great majority of cases softer strata and rocks abounding in a 

 variety of lines of jointing, cleavage, and bedding will find an angle 

 of repose like any loose material, though at a steeper pitch ; tjie sides 

 at tJie very bottom of the valley, especially ichere tJiere is quick running 

 water, will first exceed in tlieir slope the angle of repose of the roch, and 

 the result will be the breaking down and sliding off of the sides up 

 to a hard angle, as at ^'' on the left hand side of Pig. 6. At first this 

 may be to no great extent, but as the stream deepens its course down- 

 wards the confines of the slipping sides will proportionably recede 

 upwards, and a large V-shaped valley be produced, having apparently 

 no harmony or community of origin with the more rounded outline of 

 the valley in which it is placed. The mountains of Wales afford an 

 almost endless number of illustrations of the variety of effect produced 

 by the different degrees of concentration of the waterflow, and the 

 vai'ious combinations of the several kinds of outline brought about 

 by subaerial denudation. In some localities the V-shaped gully is 

 but a subordinate feature in the general rounded outline of the 

 valley. In other places the angle of repose will run up to the crest of 

 the watershed, producing a steep gorge with straight sloping sides, 

 and here and there the vertical cliff will, from variation in the struc- 

 ture of the rock, boldly stand out from the straight slope of repose. 



The Aber Valley. — In the earlier part of the article I have endea- 

 voured to point out the difficulties that the position of this steep 

 valley present to its having been excavated by a marine current 

 directly assailing the coast, and there are certain points in its struc- 

 ture that also militate against the possibility of its marine origin, the 

 most obvious being that it is a complete cid de sac affording no tho- 

 roughfare for the passage of a current. At about thi'ee-quarters of a 

 mile from its mouth the valley splits up, and on the face of the head- 

 land separating the two divergent branches which above all other 

 parts would have received the full force of the sea, there is no ap- 

 pearance of any escapement, indeed the sides of the valley throughout, 

 except at its very end, exhibit that graduated contour which appears 

 to be characteristic of subaerial denudation. At the extremity of the 

 left hand fork of the right hand branch below the waterfall a delta 

 like mass of drift occurs, partly filling up the valley ; it is evident 

 that its deposition could not have been contemporaneous with the 

 excavation of the valley, as denudation and deposition cannot go on 

 simultaneously on the same spot. The general disposition of the 

 mass appears as though it was deposited when the valley was partly 

 submerged, the debris having been brought down by the stream 

 above the waterfall, and through this delta-like heap the stream has 



