316 B. Smith — Glacial Gravels of Cor wen. 



Amongst the boulders were some of subangular pink crinoidal 

 limestone up to 1 foot in length, fossiliferous rocks from the Bala 

 Beds, subangular cherts, and fragments of local Silurian rocks. 



About 300 yards west of Carrog Church there is a large natural 

 hollow or kettle-hole ; whilst on the south side of the river, west of 

 Nant Llechlog, the gravels are distinctly moundy and sandy, and 

 possess nothing like the nearly flat surface Ave should expect if they 

 were normal river deposits. The sand is greyish and the boulders are 

 usually of local rocks, with some of carboniferous limestone, and 

 a few of cherty rock. 



Coriven Graoels (Probable Origin). — The deposits described above, 

 in their general mode of occurrence, their arrangement and com- 

 position, resemble fluvio-glacial accumulations rather than those of 

 an ordinary river. They appear to have been formed during the 

 retreating stages of the Dee valley glacier as marginal and terminal 

 gravels that choked the valley, through which the glacial waters 

 swung from side to side in many interosculating channels. 



The general direction of glaciation at the height of the Ice Age, 

 as shown by striae in the neighbourhood, was from W.S.W. to E.N.E. 

 or S..W. to K.E., with local variations in direction due to the 

 topography of the lower ground. The Corwen amphitheatre, 

 formed by the confluence of the Dee valley with several tributary 

 valleys, was filled at a later stage in the glaciation with a mass of 

 ice from which tongues were thrust out eastwards down the Dee 

 valley, north-eastward up the Morwynion (Bala Fault) valley in 

 the direction of Bryn Eglwys, and northward past Gwyddelwern 

 over a low col into the Clwyd valley near Derwen. A prolonged 

 halt or very slow melting and retreat of the ice in the Corwen 

 amphitheatre favoured the accumulation of a broad outwash fan 

 covering an area of over two square miles, the drainage escaping by 

 the Dee valley. 



The Gwyddelwern Valley. — The deep trench-like valley, two 

 miles in length, crossing the watershed between the Dee and the 

 Clwyd north of Gwyddelwern, and now traversed by the Corwen- 

 Ruthin railway, is at first sight suggestive of a gorge cut by 

 overflow waters from a glacial lake impounded in the amphitheatre 

 to the south. However, no water carrying much gravel seems to 

 have entered the Clwyd valley ; and since the valley bottom now 

 forms a water-parting covered by boulder-clay, and the western side 

 is a natural escarpment capped by Wenlock Grit, it is more probable 

 that the valley is a natural pre-glacial feature 1 accentuated by the 

 scour of the lower layers of ice moving into the Clwyd valley, and 

 at. the time we are considering it was filled by a stagnant melting 

 mass of ice cut off from its base on the south. Had it been used 

 as a channel for overflow water, the boulder-clay at least would have 

 been cleared out and there should have been a steady descent of the floor 

 from Gwyddelwern to the Clwyd valley. The floor, on the contrary, 

 rises north of Gwyddelwern to a mile-long stretch at above 600 feet 

 O.D. before it descends to about 550 feet in the Clwyd valley. 



1 In pre-Glacial days a south-flowing tributary of the Dee must have risen 

 in this valley and have made a near approach to capturing the head-waters of 

 the Clwyd. 



