E. Greenly — The River Cefnl, Anglesey. 263 



largest is that through which the railway has been taken, just 

 beyond the little market town of Llangefni. 



The Eiver Cefni is the name given to a very complicated system 

 of streams, which, with all its branches, drains some 60 square miles 

 of the interior of Anglesey. Entering the great hollow of the 

 Malldraeth Marsh, which, as is well known, belongs to the dominant 

 system, on its north-west side, at a point about a mile below 

 Llangefni, the combined waters then turn round to the south-west, 

 and find their way out to sea in Malldraeth Bay. 



The Malldraeth Marsh is practically at sea-level, and until the 

 year 1788^ was flooded by the sea at spring tides. It was then 

 reclaimed, and the river embanked and straightened for a distance 

 of some six miles from its mouth ; but the deserted meanders of the 

 old river can be traced in many places as a series of stagnant, 

 crescent-shaped pools along the alluvial plain. 



Between the Marsh and Llangefni the river runs a little east 

 of south in a comparatively open valley, chiefly cut in Carboniferous 

 rocks. But at Llangefni it emerges, rather suddenly, from a ravine, 

 which is excavated in schists and quartzites of the ancient complex, 

 with a general N.E.-S.W. strike. This ravine extends for about 

 1^ miles, almost exactly north-west, and is the most striking valley 

 of the kind in the island. It is 140 feet deep at the wood's end by 

 Pandy,^ and in places less than 400 feet wide at the top, with 

 precipitous and rocky sides, and it describes bold and sweeping 

 curves in its course. In short, it is a perfectly typical water-cut 

 valley, and differs in the most striking manner from the valleys of 

 the dominant type. 



But, at its upper end, instead of proceeding from an upland 

 hollow in the same direction, we find that it issues, almost at right 

 angles, from the side of a N.E.-S.W. valley of the dominant type, 

 which extends for several miles to right and left of the exit. 



Now this hollow, which may be called the Trefollwyn Valley, 

 after a farmhouse in it near the railway, does not open out to the 

 south-west like the Straits, the Malldraeth, and most others of the 

 class. It is a closed oval basin, and its waters all converge to 

 the head of the Cefni Ravine, and pass out that way at a point 

 midway along its south-east side. 



The further upper waters are very complex, and raise problems of 

 their own, which cannot be dealt with in this paper, and which, 

 indeed, I do not feel that I understand. But the anomalous system 

 here described seems to throw some light on the relation of the 

 two types of valley and their mode of origin. For the 

 Trefollwyn Valley could not have been in existence when the Cefni 

 Eavine was in course of excavation. Its south-west barrier, at Tai 

 Mona, on the Holyhead Eoad, is less than 150 feet above sea-level, 



1 Information kindly supplied by Mr. Thomas Prichard, of Llwydiarth Esgob, 

 Anglesey, from the records of the Drainage Commissioners. 



2 I am indebted for this measui'emeut to the kindness of Mr. Tobias Clegg, F.G.S., 

 of Llangefni County School, who very ingeniously determined it by means of a kind 

 of extemporized theodolite. 



