342 prof. w. morris datis on [^- u g- 1909,- 



The possibility of the overflow and destruction of a pre-Glacial 

 divide that crossed the present valley of Afon Gwarfai where the- 

 porphyry-spur of Mynydd Mawr has been truncated deserves 

 further consideration than can be given to it here. 



XXIV. Historical Note. 



It has been by intention that references to the many writers on 

 the problem of glacial erosion have here been reduced to a minimum. 

 It would be easy to fill page after page with citations from many 

 European and American writers. There is abundant material in 

 articles by the Geikies, Jamieson, Bonney, Marr, Harker, Garwood, 

 and others in Great Britain ; by Helland and Reusch in Norway ; by 

 de Margerie, de Lapparent, Kilian, Lory, and Martonne in Prance; 

 by Partsch and Steinmann in Germany ; by Heim, Brunhes, Priih, 

 and many others in the Alps ; by Johnson, McGee, Gilbert, Matthes, 

 Tarr, Pairchild, and others in the United States ; but there is no 

 space left for citations in this already long paper. Yet it is only 

 fitting that explicit acknowledgment should be made of the work 

 accomplished by certain students of the subject to whom my indebted- 

 ness is particularly great. Mention cannot be omitted of Henry 

 Gannett, of Washington, my class-mate of forty years ago, who, in 

 1898, first set forth clearly and explicitly the illuminating analogy 

 between the hanging channels (valleys) of lateral glaciers over the 

 deeper channel of the main glacier, and the hanging channels of 

 lateral streams over the deeper channel of the main river ; of the 

 late Eduard Richter, of Graz, who was among the earliest to bring 

 forth clearly the great importance of retrogressive glacial erosion 

 in determining the form of mountain-peaks and crests that rise 

 high over neve-reservoirs ; or of Prof. Albrecht Penck, of Berlin, 

 and Prof. Eduard Bruckner, of Vienna, joint authors of that com- 

 prehensive treatise ' Die Alpen im Eiszeitalter.' Those who know 

 the writings of these investigators will find many of their opinions 

 in the preceding pages. 



A word may also be given concerning the choice of North Wales 

 as the field for such an investigation as is here presented. I was 

 attracted to it by the peculiar features shown on the Snowdon 

 sheet of the Ordnance Survey map. On the ground these features 

 were found to be even more pronounced than the maps had 

 indicated ; they are admirably developed for field-study. There 

 is nothing in the United States east of the Rocky Mountains that 

 can be compared with North Wales in the way of vivid exhibition 

 of cwms, hanging valleys, and their associated abnormal features, 

 all the more striking because of their immediate association with 

 normal features. The Rocky Mountains, to be sure, contain 

 abundant examples of essentially similar abnormal features, as in 

 the Sawatch and other ranges of Colorado, in the Big Horn range 

 of Wyoming, and in the Pront range of Montana; but all of these 

 have the disadvantage of development on a rather inconveniently 



