E. Oreenly — The River Cefni, Anglesey. 265 



Nor does underground structure help us in any way. Though it 

 is likely enough that there may be some fault or line of weakness 

 along the hollow, there is certainly none of any importance.^ 



Let us consider the relation of the two valley systems to the 

 greatest of Post-Tertiary events, the Glacial Period. The valleys 

 of the dominant system coincide, as is well known, with the general 

 direction of glaciation, and contain abundance of Glacial Drift. They 

 are either of Glacial age or older, and if we admit the possibility of 

 glacial erosion, may have been produced by the ice, as was suggested 

 by Ramsay, or, at any rate, deepened and enlarged by it. 



The Ravine, lying directly across the path of the ice, cannot have 

 been produced by it, and must be either Pre- or Post-Glacial. It 

 must therefore be Pre-Glacial, as we have seen that it must be older 

 than the Trefollwyn Valley. Confirmation of this is found in the 

 presence in its bottom of Boulder-clay, undisturbed as far as I can 

 see, at the south point of the great bend west of Pandy. 



The Cefni, then, is a Pre-Glacial river. 



If now we suppose that it received two small 'subsequent' 

 tributaries from north-east and south-west, which met near the 

 farmhouse of Trefollwyn, and that their combined hollows were 

 deepened and greatly widened by glacial erosion to form the large 

 valley of that name, we shall be able to understand the severance of 

 the two portions of the Pre-Glacial Cefni, the production of the 

 curious closed oval hollow, and the anomalous configuration and 

 drainage of the present day. 



Indeed, it seems probable that in Pre-Glacial Anglesey the 

 drainage to north-west or south-east was much more continuous and 

 systematic than it is now, and that the history of the present 

 anomalous drainage cannot be understood without reference to the 

 events of the Glacial Period. 



A curious and very interesting consideration in connection with 

 the ancient river Cefni is that, in the earlier stages of its history, 

 far back in Pi-e-Glacial time, there must have been a fine waterfall 

 close to where is now Llangefni Station. For, at that point, the river 

 has cut through a lenticular band of very hard quartzite, about 

 100 feet thick, which is conspicuous from the Station, as of it are 

 composed the white crags below the fir-trees just opposite. Resisting 

 erosion much more strongly than the schist on either side of it, 

 it gives rise, even now, to a rapid. In the early stages of erosion 

 this rock, while freely allowing the river to cut hach to it from 

 below, must have retarded cutting doion in its rear for a very long 

 time : and consequently for long ages a fine cataract must have 

 poured over a crag some 80 or 100 feet in height. Like Gray's 

 flower in the famous Elegy, it came into being, and then dwindled 

 gradually away, unwatched by any but the old wild beasts, and 

 perhaps by some men almost as wild. 



^ A fault which partly coincides with the course of the Cefni between the town 

 and the marsh does not pass into the Rayine, but somewhat to the eastward. 



