312 B. Smith — Glacial Gravels of Cor wen. 



continued easterly twist in successive lines, the direction being 

 about N. 25° E. through the Stafford foci, N. 28° E. through the 

 Derby foci, and N. 30° E. and N. 42° E. through the Leicester foci. 



The existence of this deep-seated double system of corrugations 

 may perhaps explain another peculiarity of British earthquakes. 

 Dividing them into three classes of strong, moderate, and slight 

 earthquakes, according as the areas embraced by the isoseismal 4 

 are greater than 5,000, between 5,000 and 1,000 or less than 1,000, 

 square miles, it appears that the average length of focus for 

 strong earthquakes is 12J miles, and for moderate, earthquakes 

 13 miles. Slight earthquakes are divisible into two groups, in 

 one of which the focus is 9 miles or more in length, and in the 

 other 6 miles or less in length. The average length of focus for 

 the former is 12 miles, and for the latter (omitting a large number 

 of very slight shocks) 4J miles. If we leave out of account the 

 second division of slight shocks, which are of the nature of local 

 creeps, it follows that the average length of focus in all three 

 classes of earthquakes is very nearly the same, about 12J miles. 1 



III. — The Late Glacial Gravels of the Vale op Edeyrnion, 

 Corwen, North "Walks. 



By Bernard Smith, M.A., F.G.S. 

 (WITH A MAP.) 

 rjIHE Corwen gravels were described by D. Mackintosh, forty-three 

 J_ years ago, in a paper " On the mode of Occurrence and 

 Distribution of Eeds of Drifted Coal near Corwen, North Wales ". 2 

 He attributed the gravels to an interglacial marine submergence and 

 claimed that the coal found in them was derived from the outcrop of 

 Coal-measures near Ruabon, twelve miles due east of Corwen. 



In the present communication a correlation is made between the 

 Corwen gravels and certain gravels of Late Glacial age formed during 

 the melting stages of the North Welsh valley glaciers. A brief 

 description of these gravels, as a preface to our correlation, seems 

 advisable in view of the fact that no general account of them has 

 been published. ' 



lteasons are given for assuming that the drifted coal, found at 

 Corwen, was derived from a local concealed outcrop. 



Late-Glacial Gravels. — In many of the valleys of North Wales, 

 such as the Dee, Ceriog, Tanat, Vyrnwy, and Severn, especially in 

 the more mature parts of their courses through the mountains, the 

 recent alluvium is flanked at intervals by terraces of gravel, 3 derived 

 chiefly from the boulder-clay which previously clung to the valley 

 slopes, and still floors the valleys over long stretches. These terraces 

 have usually been regarded as post-glacial, and mapped as ordinary 



1 "Characteristics of British Earthquakes": Geol. Mag., Vol. VII, 

 pp_. 410-19, 1910. 



2 Quart. Journ. Geol. Soc, vol. xxxii, pp. 451-3, 1876. 



s Summary of Progress for 1914 (Mem. Geol. Surv.), 1915, p. 17; and for 

 1915 (Mem. Geol. Surv.), 1916, pp. 11, 12. 



