GLACIAL BEDS. 



491 



of Llanberis, in Nant Francon, and other valleys of Caernarvonshire. Llyn 

 Llydaw and Llyn Idwal are clearly dammed up by moraines of Glaciers.' (See 

 Fig. 84.) 



Fig. 84. — Section through Llyn Idwal, North Wales. 

 (Sir A. C. Ramsay.) 



1. i\Ioraine. 



2. Grooved and polished surfaces formed by more ancient glaciers. 



3. Drift at an elevation of 2300 feet. 



Prof. Hughes describes the Drifts of the western part of North Wales under 

 two heads, as follows : - — 



2. The Newer or CIvvydian Drift, or that due to the destruction of the older 

 glacial deposits by marine action, during which boulders were carried on floating- 

 ice from the north, and flints travelled in the shingle round the coast. All the 

 shells found in it are of species still living on the adjoining coast ; but some of the 

 shells found in what he considers part of the same series of deposits in neighbour- 

 ing districts, are of a more arctic tvpe, and may belong to an earlier part of the 

 same epoch. This is also termed the St. Asaph Drift. 



I. The Older or Arenig Drift, or that in which boulders were transported from 

 Arenig into the Vale of Clwyd. 



It is not always possible to separate these Drifts ; but in the main the older 

 Boulder Clay is made up of local detritus, and the newer of material from the 

 north-west of England, etc. Boulder Clay is well shown at Llandudno, Colwyu 

 Bay, and other places. The newer Drift skirts the margin of Flintshire and 

 Denbighshire, extending into the Vale of CTvvyd, and over much of Cheshire. 



The striated surfaces of the rocks in this area are connected with the Boulder 

 Clays, and in Mr. Strahan's opinion the striae below the Drift which came from 

 the north, were produced by floating-ice, driven by tidal or oceanic currents, 

 during a period of submergence. He considers that the marine origin of these 

 newer and erratic drifts is indicated by their well-marked stratification as a whole, 

 by the alternations of sands and gravels with the Boulder-clays and by the 

 occurrence through all the beds of marine shells.^ 



In 1831 Joshua Trimmer discovered marine shells, for the most part broken, in 

 sand and gravel on Moel Tryfaen. about five miles south-east of Caernarvon. Their 

 occurrence suggested a great submergence in Post-Pliocene times. Fossils have 

 since been obtained at heights of 1330 to 1360 feet. The Dritt as described by 

 Mr. Mackintosh consists of 35 feet of irregularly stratified gravel and sand, which 

 rise from the floor of the highest excavation of the Alexandra Slate Quarry on the 

 north side of the hill, to within a few yards of its summit.* The following section 

 was noted by Mr. Darbishire : — 



Feet ins. 



Peaty soil o 9 



Irregular sandy and stony clay I 9 



Gravel and sand with shells 26 O 



Slate, much broken and disturbed beneath the Gravel. 



1 Q. J. viii. 373, and The Old Glaciers of Switzerland and N. Wales, 1S60. 



- Brit. Assoc. 1886, Q. J. xliii. 



3 A. Strahan, Q. J. xlii. 369. See also D. Mackintosh, G.Mag. 1S72, p. 15 ; 

 T. G. Bonney, G. Mag. 1867, p. 289 ; Miss Eyton, G. Mag. 1868, p. 349 ; 

 H. F. Hall, G. Mag. 1870, p. 509. 



* J. Trimmer, Proc. G. S. i. 331 ; Journ. Dublin Geol. Soc. i. 286, 335 ; Practical 

 Geology, pp. 396, 406, 491 ; C. Darwin, Phil. Mag. (3), xxi. 180 ; 1). Mackintosh, 

 Q.J. xxxvii. 352 ; R. D. Darbishire, Mem. Lit. Phil. Soc. Manchester, 1863, iii. 177. 



