Vol. 65.] GLACIAL EROSION IN NORTH WALES. 281 



18. Glacial Erosion in North Wales. By Prof. William Morris 

 Davis, For. Corr. G.S. (Eead March 24th, 1909.) 



[Plate XIV— Map.] 



Contents. 



Page 



I. The Form of Snowdon 281 



II. Excursions around Snowdon 282 



III. Welsh Terms, Local Place-Names, and Illustrations 283 



IV. Geological Structure of the Snowdon District 284 



V. Indifference of Form to Structure 285 



VI. General Erosion of the Snowdon District 286 



VII. Ramsay's ' Plain of Denudation ' 288 



VIII. Date and Altitude of Ramsay's Plain 290 



IX. Dissection of the Uplifted Plain 291 



X. Pre-Glacial Form of Snowdon 292 



XI. Subdued Mountains of Normal Erosion 293 



XII. Texture of Dissection 295 



XIII. Interdependence of Parts 295 



XIV. Abnormal Forms of the Snowdon District 296 



XV. Abnormal Forms and Normal Processes 300 



XVI. Association of Abnormal Features with Glaciation 303 



XVII. Various Methods of Discussing Glacial Erosion 304 



XVIII. Glaciers as Protective Agencies 308 



XIX. Special Features of the Theory of Glacial Protection 311 



XX. Glaciers as Destructive Agencies 313 



XXI. Special Features of the Theory of Glacial Erosion 318 



XXII. A Personal Section 320 



XXIII. Confrontation of the Deduced Consequences of the Two 



Theories, with the Facts of Observation in the Snowdon 



District 322 



Valley-Heads: Cwms 322 



Valley-Floors: Lakes 329 



Valley-Floors: Rock-Steps 330 



Valley-Sides 333 



Hanging Lateral Valleys 339 



Glacial Overflows 341 



XXIV. Historical Note 342 



XXV. Closing Suggestions 343 



XXVI. Bibliography , 343 



' Few subjects in physical geology would possess greater interest than a 

 complete account of the denudations by which, after the disturbance of the 

 strata, Wales assumed its present form.' — Ramsay, Mem. Geol. Surv. Great 

 Britain, vol. iii (1866) p. 238 ; or, with verbal modifications, 2nd ed. (1881) 

 p. 330. 



I. The Form of Snowdon. 



An excursion around Snowdon, the highest of the Welsh mountains, 

 in September, 1907, led me to the conclusion that a large-featured, 

 round-shouldered, full-bodied mountain of pre-Glacial time had 

 been converted by erosion during the Glacial Period — and chiefly 

 by glacial erosion — into the sharp-featured, hollow-chested, narrow- 

 spurred mountain of to-day. Or, to phrase it in somewhat more 



