Charleswokth — Glacial Geology of North- West of Ireland. 237 



1 . The Retreat from the Sperrin Mountains. . 



In the Sperrin Mountaina a suite of channels was observed, which bears 

 witness to the former presence of swollen torrents, linking chains of lakes, 

 which, resting between the hill-slopes and the temporary support of the ice- 

 barriers, were the result of special conditions imposed upon the laud drainao-e 

 of this hilly country. 



During the retreat of the ice from the Sperrin Mountains, their highest 

 parts were tlie first to be uncovered. The drainage of these ice-free surfaces 

 was unable to escape normally, as the main valleys and the lower parts of 

 the tiibutaries were full of ice. In consequence lakes were formed in the 

 unoccupied heads of the main depressions, the accumulated waters held up by 

 the ice-barrier running in finger-like projections up the tributary valleys. 



These temporary lakes were drained by channels cut across intervening 

 cols and spurs ; lower and lower cols were opened, and lower and lower 

 channels brought into use as the retreat progi-essed. 



There is abundant evidence in these hills to permit of the marking of the 

 position of the ice front throughout the region during the various stages of 

 the retreat, though the location of the overflow channels would seem to have 

 been determined as much or even more by the general configuration of the 

 country where left exposed by the ice-retreat as by the detailed nature of the 

 retreat itself. A few of the more important of these stages are represented 

 on the ]nap (PI. VIII). 



These channels occasionally indicate a small local advance of the ice- 

 margin, though no general advance along the whole line would seem to have 

 taken place. 



Magnificent deltaic terraces were formed in these extra-glacial lakes, and 

 were best developed where tributary streams, pouring out of higher lakes 

 and transporting excessive quantities of sand and gravel, entered lower 

 lakes. To a much less extent they fringe the -lakes round their landward 

 margin. The material of which these huge deltas are composed is well 

 water-worn, varying in coarseness from fine sand to boulders a few feet in 

 length. 



The higher or older terraces were largely destroyed during the building 

 up of the lower and newer ones, the lowest beilig, iu consequence, best 

 preserved, the highest least recognisable and most effaced. Hence, the 

 highest terraces of all appear now as a mere collection of detached remnants, 

 while few occur on the same grand scale as the comparatively low terraces 

 found in the vicinity of Claudy. 



Weeks spent among these terraces impress one with the great extent and 



K.I.A. PKOC, VOL. XXXVI, SECT. B. [2 -CJ 



