146 Henry Deiuey — Land-forms in Caernarvonshire. 



foot-hills, the crags and pinnacles rising precipitously from the level 

 land ; or, in other words, tlie plain cuts as it were a shelf in the 

 mountainous masses. 



Two years ago I communicated to the Geological Society of 

 London a paper on the " Origin of Iliver Gorges in Cornwall and 

 Devon ",^ and therein described an upland plateau that attains 

 a maximum altitude of 430 feet above sea-level. In discussing that 

 paper Mr. E. Greenly and Professor Fearnsides called attention to 

 the existence in North Wales of similar coastal plateaux, but at 

 different heights above sea-level from the one I had described. At 

 the time those comments were made I was under an impression, 

 gained during a short visit to North Wales in the spring of 1915, 

 that a precisely similar feature terminating at the same height above 

 sea-level occurred in both Cornwall and Caernarvonshire. But I was 

 not sufficiently versed in the land-forms of the latter county to feel 

 justified in asserting their practical identity. I therefore resolved 

 to revisit the district to inquire more particularly into these land- 

 forms, and in consequence spent some weeks during the summers of 

 1916 and 1917 in making a close investigation of the points to be 

 solved. As a natural consequence other problems arose, and one in 

 particular that cannot be settled in Cornwall or Devon, namely, the 

 effects of glacial conditions upon this upland plain and the amount of 

 denudation which has taken place since those conditions terminated. 

 Hestricted railway facilities more or less confined work to the district 

 around Bethesda, and in consequence I chose for detailed investiga- 

 tion the valley of the Ogwen and the country lying between 

 Bethesda and Llanberis, and extending westward to Caernarvon and 

 Bangor. 



Previous Literatxtrk. 



This district is classic ground. Darwin '^ recognized the glacial 

 features of parts of it and described in detail the valley of Llyn 

 Idwal. Many years afterwards Sir Andrew Ramsay^ (in spite of 

 great difficulties, especially with regard to inadequate topographical 

 maps) fully and accurately described the mountainous tract. His 

 work will be referred to frequently, but it may here be said that its 

 accuracy is such that it needs but little revision, except where 

 additions and refi^nements made possible by more precise methods 

 and the general advance in geological science have necessitated 

 modifications of nomenclature. His inferences, however, are open 

 to question, and have already drawn into controversy many 

 observers, including Watts,* Marr and Adie,^ Dakvns,^ Jehu,'' and 

 W. M. Davis.« 



1 Quart. Journ. Geol. Soc, vol. Ixxii, for 1916, pp. 63-76, published 1917. 



" Phil. Mag., ser. Ill, vol. xxi. 



^ The Old Glaciers of Sioitzerland and North Wales, London, 1860. 



** "Notes on some Tarns near Snowdon " : Geol. Mag., 1895, p. 565. 



^ "The Lakes of Snowdon " : Geol. Mag., 1898, p. 51. 



^ " Some Snowdon Tarns" : Geol. Mag., 1900, p. 58. 



'' "The Lakes of Snowdonia and Eastern Carnarvonshire" : Trans. Eoy. 

 Soc. Edinburgh, vol. xl, pt. ii, pp. 419-67, 1902. 



* " Glacial Erosion in North Wales" : Quart. Journ. Geol. Soc, vol. Ixv, 

 pp. 281-350, 1909. 



