Vol. 65.] GLACIAL EROSION IN NORTH WALES. 297 



growing talus, by which the floor of the era is encroached 

 upon. Thus the cliffs are not so steep now as they were once. 

 It is true that valley-head ampitheatres of a somewhat cwm-like 

 form are found in non-glaciatecl mountains, but they differ from 

 typical cwms of glaciated mountains in two significant respects : — 

 first, their heads are half-funnel shaped, rather than half-cauldron 

 •shaped : their walls are not abrupt cliffs, but are simply the 

 steepest headwater slopes of normally branching stream-lines, under 

 the action of which the funnel-shaped valley-head has been retro- 

 gressively excavated, as in fig. 2 (p. 294) ; second, each valley-head 

 of normally carved mountains is drained by a stream of decreasing 

 slope, while a cwm is fronted by a rock-step of increasing slope, 

 •on which the outflowing stream openly cascades to a lower level, 

 -as in fig. 3 (p. 296). The cwm is therefore evidently due to some 

 special excavating agency that has worked much faster than the 

 slow weathering by which the slopes of the moel have been so 

 evenly graded, and the excavating agency is not now operative, for 

 waste from the top of the cwm- cliffs is accumulating at their bases. 

 It is as if the normal processes, now again in operation, were 

 hastening to restore and regrade, but at a lower level than before, 

 the broken arch of the mountain-dome. Cwm Du breaks the 

 arch of Mynydd Mawr as abruptly as if it were a huge quarry; its 

 sharp edge is hardly exaggerated in fig. 3. 



Even more abnormal than the cwm-head cliffs is the rock-step by 

 which descent is made from the cwm-floor to the broad valley into 

 which it opens. Not enough has been made of this discordance of 

 level in most descriptions of cwms, for it is a most extraordinary 

 feature, the like of which is not to be seen in mountains of normal 

 erosion. As a consequence of the discordance the branch stream, 

 by which a flat cwm-floor is drained, descends to the wide open 

 valley-floor of the main stream in a series of cascades which are 

 only just beginning to cut a ravine in the main valley-side. This 

 last point is also of importance ; for, if the cwm be regarded as the 

 work of normal erosion under the leadership of the cwm-stream, 

 the cascades ought now to be hidden in the depths of a large gorge : 

 but instead they usually flaunt their whitened waters on the ledges 

 of the main valley-side, or at most they have only just begun to 

 ensconce themselves in a cleft. This is manifestly a most abnormal 

 relation ; for, if the main valley is maturely open, every side-stream 

 ought to enter at an accordant grade by a valley proportionate to its 

 size. The disparity of development batween the main and the side- 

 valleys cannot be ascribed to differences of rock-resistance, as if the 

 development of the main stream and its valley had been hastened 

 because they followed a belt of weak rocks, while that of the side 

 stream and its cwm had been retarded because they occurred in a 

 belt of resistant rocks ; for many cases can be pointed out where 

 the main valley and the side cwm occur in rock of the same kind. 

 Nor can the disparity of development be ascribed to difference of 

 .stream-volume, as might normally be the case if the main valley 



