Yol. 60.] ANNIVERSARY ADDRESS. lxxxix 



rock-shelves. Certainly throughout this wonderful assemblage of 

 lake-shores, there is nothing for a moment to be compared to the 

 incised platforms of rock so abundant as part of the raised beaches 

 of the western coast of Scotland. We must remember also that the 

 production of such ice-dammed lakes took place as a mere episode in 

 the retreat of the ice. No means are available to determine what 

 may have been the length of time during which the water stood at 

 the level of any one of these Parallel Roads. We may probably 

 infer, from the absence of well-marked and continuous intervening 

 shore-lines, that the shrinkage of the ice and the consequent lowering 

 of the level of the water were somewhat rapid. 



The Parallel Roads of Lochaber, although the most imposing, 

 are not the only examples of the shore-lines of ancient glacier- 

 lakes in this country. Another striking case is that of Strath Bran 

 in Ross-shire, where the glaciers descending from the mountains 

 on each side ponded back the drainage of the valley, and sent it 

 across the present watershed of the country at a height of about 

 600 feet above the sea. The conspicuous gravel-terraces at Achna- 

 shean are a memorial of this vanished sheet of water. 1 



Now, with these undoubted records of ancient lakes, let us com- 

 pare the structure and distribution of our Raised Beaches. These 

 shore-lines are found, on both sides of Scotland, at approximately 

 the same heights above the level of the sea. They are partly 

 terraces of deposit, and partly true seter or platforms cut out of the 

 solid rock, the same beach presenting frequent alternations of both 

 structures. In general, it may be said that the detrital terraces 

 are found chiefly in bays, sea-lochs, or other sheltered places ; while 

 the rock-terraces are conspicuous in more open sounds and exposed 

 parts of the coast, where the tidal currents and wind-waves are most 

 powerful. 



As the highest terraces are the oldest, they have been longest 

 exposed to the influences of denudation, and are thus the faintest 

 and most fragmentary. But the dimensions and perfection of a 

 raised beach do not depend merely on age, but in large measure on 

 the length of time that the water stood at that level, and the varying- 

 local conditions that favoured or retarded the planing-down of solid 

 rock or the deposition of littoral sediment. 



That these beaches unquestionably mark shore-lines of the sea 

 may be inferred on three grounds : — (1) Their position on both 



1 ' Summary of Progress of the Geological Survey for 1898' pp. 17r>. 17G. 

 VOL. LX. (J 



