314 B. Smith — Glacial Gravels of Corwen. 



fairly close together. At some points, where a valley was winding, 

 narrow and constricted, the escaping glacial waters distributed the 

 debris irregularly, the resulting terrace being of varying height and 

 uneven surface and occasionally pitted with kettle-holes. 1 "Where 

 the valleys debouched on the plain, for example where the Vyrnwy 

 leaves the mountains at Llanyrnynech, the gravel is spread out as 

 a huge apron or fan several miles across, 2 the grade of material 

 becoming finer and finer, and the fan progressively thinner, towards 

 its margins. 



Near the mountainous headwaters of the rivers these gravels are 

 usually thin and patchy, and their surfaces are not far above the 

 present alluvial level, whilst the floor (boulder-clay or "solid") 

 upon which they rest is also sometimes above this level. As we 

 fellow them down stream towards their lower mountain course the 

 terraces rise higher and higher above the alluvium. Where the 

 rivers debouch on the plain the terraces descend quickly to stream- 

 level. Thus the gradient of the late-glacial streams was flatter 

 than that of the present stream. The gravels themselves also are 

 actually thicker in the middle or lower part of the mountain 

 course than near the headwaters or in the fan. This result was 

 undoubtedly due to the aggrading power of the streams, which were 

 choked with more debris than they could conveniently carry. The 

 dissected terraces that now rise prominently in bold bluffs above the 

 present rivers are not primarily due to any uplift of the land, but 

 to the sinking of the thalwegs of the present streams to a curve 

 consonant with their present volume and rate of flow. During this 

 process the late-glacial gravels have been attacked and redistributed 

 as post-glacial terraces at one or more lower levels. 



The gravels are varied in grade and character, in places being 

 fairly clean boulder beds with good-sized erratics, which gradate 

 through finer gravel and silts or clayey gravel into a more clayey 

 deposit that cannot be distinguished easily from the parent boulder- 

 clay. Usually the gravels are roughly stratified, the constituent 

 beds often dipping at steep angles ; but in some places they appear 

 to be tossed together iri disorder. 



The Corwen Gravels ( General). — At Corwen there lies an open plain, 

 surrounded by an amphitheatre of mountains, near and north of the 

 confluence of the River Alwen, which flows from west to east, with 

 the River Dee, which flows from south-south-west to east-north- 

 east and then turns due east past Corwen and traverses the famous 

 Vale of Edeyrnion. 



The plain is made up of two physiographical units. On the south 

 it consists of the recent flood-plain of the above-mentioned rivers, 

 north of which is a gently undulating plain of gravel extending 

 1£ miles in the direction of Gwyddelwern and averaging about 

 550 feet above sea-level, or 100 feet above the alluvial plain of 

 the Dee at Corwen. The two features are separated by a steep 

 bluff. 



The gravel plain is about 1^ miles wide near the rivers, but 



1 Summary of Progress for 1915 (Mem. Geol. Surv.), 1916, pp. 11, 12. 



2 Op. cit., p. 12. 



