264 



E. Greenly — The River Cefni, Anglesey. 



whereas the highest part of the plateau trenched by the Ravine 

 is about 200 feet (see Map), so that the waters of a lake lying in the 

 basin would have chosen the south-west exit and deepened that 

 channel. 



Nor can we postulate in this case an ice-dam such as those that have 

 been appealed to with such striking success in Cleveland and other 

 regions. There is not the smallest evidence of such a dam, the 

 glaciation being steadily from north-east to south-west, with mere 

 local variations of direction. 



All the evidence goes to show that the Trefollwyn Valley is 

 the later. 



Now, nearly opposite to the exit at the Ravine's Head, a small 

 stream enters the Trefollwyn Valley on its further side, after a course 

 of about a mile from the north-west. It has no name on the maps, 

 but the railway has been carried quite near to its south-west banks. 

 It is cut in rock, not merely in drifts. 



Sketch-map of part of course of River Cefni. 



This channel is in the natural position for the apparently vanished 

 upper portion of the Cefni. The fall would be slight, but there 

 would be a fall, enough to make such a continuation possible, and 

 ample if we allow for cutting back as well as cutting down of the 

 lower part of the course. How, then, did it come to be so strikingly 

 severed from its lower portion by the broad valley of Trefollwyn ? 



Admitting that 'subsequent' tributaries of the Cefni may have 

 initiated that hollow, it is not easy to see how, with so short a course 

 and so slight a fall, especially from the south-west (if, indeed, there 

 was not a fall to the south-west, which seems much more likely), 

 they could have produced one of such dimensions, for it is both deep^ 

 and broad. 



1 It is floored by alluvium, and its real bottom may even be below the level of the 

 rock at the exit. 



