180 MR. L. J. WILLS ON LATE GLACIAL AND [June I912, 



10. Late Glacial and Post-Glacial Changes in the Lower Dee 

 Valley. By Leonard Johnston Wills, M.A., F.G.S., Fellow 

 of King's College Cambridge. (Read February 28th, 1912.) 



[Plate XI— Map.] 



Contents. 



Page 

 I. Introduction 180 



II. The Composition of the Grlacial Drifts 181 



(a) The Irish Sea Drift. 



(b) The Welsh Drift. 



III. Post-Glacial Changes in the Topography of the 



District 18^ 



(1) The Llantisilio Diversion. 



(2) Holyhead Road Overflow Channel. 



(3) The Pengwern Diversion. 



(4) The Argoed Diversion. 



(5) The Chirk Diversion. 



(6) The Story of the River Ceiriog. 



(7) The Pre-Glacial Valley near Holt and Chester. 



(8) The Relationship of the two Buried Valleys. 



(9) Points of Theoretical Interest. 



IV. Conclusion 197 



I. Introduction. 



This paper is the result of observations made in the Lower Dee 

 Valley by myself and other members of H.M. Geological Survey, 

 in the course of mapping in the Denbighshire Coalfield and neigh- 

 bouring areas. My thanks are especially due to Mr. G. W. 

 Lamplugh, F.R.S., for the help that his knowledge of Glacial 

 geology has afforded me and for the generous way in which it has 

 been offered ; and also to Mr. D. A. Macalister and Mr. C. B. Wedd 

 for observations relating to part of the Dee Valley. 



Dr. Strahan's ^ memoir on the Geology of Flint, Mold, and 

 Ruthin, chapt. xii, appears to be the only work that has dealt 

 directly with the post-Glacial history of the Dee. His observa- 

 tions relate to that part of its course which lies in the neighbour- 

 hood of Chester, and will be referred to in the sequel. 



In § II of the present paper a general account of the Drift 

 deposits of the critical area around Llangollen and Chirk is given ; 

 while § III deals with post-Glacial changes in the topography 

 and in the drainage of the Dee from near the former place to its 

 mouth. 



^ A. Strahan, ' The Geology of the Neighbourhoods of Flint, Mold, & 

 Ruthin' Mem. Geol. Surv. 1890, chapt. xii, p. 149. 



