290 



PROF. W. MORRIS DAVIS ON 



[Aug. 1909. 



proceeds in so orderly a manner as to give assurance that, when a 

 given stage of development is attained, a certain reasonable asso- 

 ciation of hill-and-valley forms must be produced. Irresponsible, 

 arbitrary assumptions are thus excluded ; the details of a normally 

 carved landscape come to have an organic consistence ; and every 

 item of form has its reasonable place and significance. 



XIV. Abnormal Forms op the Snowdox District. 



Imagine Snowdon to be visited by an observer acquainted with 

 mountains of normal form, and persuaded th;.t the theory of the 

 cycle of normal erosion (in which glacial erosion plays no part) is 

 essentially true, but unacquainted with the forms of glaciated 

 mountains. Such an observer would be forcibly struck by the 

 frequent occurrence of peculiar features of whicb he had not pre- 

 viously seen examples, and for wbich he could give no normal 

 explanation. The valley-heads and many of the ridges between 

 them, the valley-sides, and the valley -floors would appear to be of 

 abnormal form. It is true that the smoothly graded slopes of the 



Fig. 3. — Diagram of a cwm in a subdued mountain, based on a 

 sketch of Cwm Du in Mynydd Mawr, looking southwards. 





Welsh moels would correspond, as far as they go, with portions of a 

 normally carved mountain-group, such as ought to have been drained 

 by a maturely organized river-system ; but elsewhere the departures 

 from normal form would seem very pronounced. 



Close alongside of the graded summit and slopes of a moel, stand 

 the head cliffs of a rock-walled cwm, that by some peculiar process 

 has been excavated irrespective of rock-structure in one side of the 

 dome-like mass, as in fig. 3 ; the cwm-head cliffs are so steep that 

 the fall of fragments from their upper part is rapidly forming a 



