Mr Lyell on former Glaciers in Forfarshire. 199 



II. On the Geological Evidence of the former existence of Glaciers 

 in Forfarshire. By Charles Lyell, Esq., F.R. S., M. G. S., &c. 



Mr Lyell on the 18th of November and 2d December 1840, after the 

 reading of the memoir of Dr Buckland, laid before the Geological Society 

 a series of observations on the ancient glaciers of the county of Forfar. 

 Mr Lyell, after some preliminary observations, remarked that he had, by 

 the use of the theory of M. Agassiz, become convinced that glaciers not 

 only existed a long time in the Grampians, but extended even into the 

 low country ; and, further, that in his opinion, immediately antecedent 

 to the present epoch, there may have been great oscillations of climate 

 in the Northern Hemisphere. 



The county of Forfar, Mr Lyell divides geologically into three princi- 

 pal districts, — that of the Grampians, formed of granite, gneiss, mica- 

 slate, and clay-slate, flanked by a lower range of old red sandstone 

 associated with trap — that of Strathmore, composed of old red sand- 

 stone — and that of the Sidlaw Hills, constituted of the inferior beds 

 of the same formation, usually accompanied by trap ; and, he adds, 

 that the district may be considered to represent on a small scale, both 

 geologically and physically, that part of Switzerland where the pheno- 

 mena of erratic blocks are most remarkable, for the Grampians with their 

 crystalline rocks are comparable to the Alps, the Sidlaw Hills with their 

 secondary formations to the Jura, and Strathmore to the great valley 

 of Switzerland ; and the masses of Grampian rocks in the Strath and at 

 considerable heights on the Sidlaw Hills, recall to mind the erratic blocks 

 of the Pays du Vaud and the Jura. The detritus spread over Forfar- 

 shire, Mr Lyell divides into three deposits, presenting distinct characters ; 

 — 1st, the thin covering on the tops and sides of the Grampians, and de- 

 rived from the disintegration of the subjacent formations, with a slight 

 intermixture of pebbles traceable to rocks not far distant and at higher 

 levels ; 2dly, the impervious till and boulders, with other unstratified, 

 transported materials, disposed at various heights in the glens and the 

 Strathmore ; and, 3dly, the stratified gravels, sands, and clays, which 

 overlie the unstratified. The accumulations belonging to the second di- 

 vision occur on both sides of every glen, frequently arranged in terraces 

 with a nearly flat top, and sometimes with two taluses, one towards the 

 river, and the other of less height towards the mountain. These terraces 

 or lateral mounds generally increase in width and depth as they descend 

 from the higher to the lower glens, attaining in the latter sometimes a 

 thickness of 100 feet. In the inferior part they consist of large angular 

 and rounded fragments, imbedded in unstratified mud and sand, the com- 

 position of the mass increasing in complexity as the mounds of the late- 

 ral glens unite with those of the main glen. In the higher part they are 

 often composed of forty and even eighty feet of gravel and sand of the 

 same nature, but stratified. These mounds acquire occasionally, as in 

 the glen of South Ksk, so great a volume as to block up the valley, leav- 



