Charlesworth — Glacial Geology of North- West of Ireland. 29 1 



The " Finn Glacier " gradually shrank towards the west and from the 

 watersheds bounding the valley on north and south, impounding the drainage 

 to form a fine series of lakes and overflow valleys. The successive stages of 

 the retreat of the northern edge of the glacier are represented on the map 

 (fig. 3). 



Fig. 3. 

 Map representing the retreat of the iicrtliern edge of tlio Finn Glacier. The 

 varioiisly-shndetl ureas represent tlie size of the extra-glacial lakes at different 

 stages of the retreat. Scale — 4 miles = 1 inch. 



The earliest evidence of this retreat is furnished by the Sruffannagallagh 

 glen, running to the south of Cark Mountain. This broad valley (S.Gr.), 

 streamless in its upper part, intakes at an altitude just above 800 feet, O.D., 

 and poured its waters into a " Lake L)eele," held up in the Deele valley. 

 Sections in the lower part of the Sruffannagallagh valley, about three- 

 quarters of a mile above Cark Bridge, show boulder clay in the lower 

 portion of the section, and in the upper, several feet of stratified gravel and 

 lenticles of coarse sand. These represent the deltaic deposits thrown down 

 in the lake by the waters flowing through the " Sruffannagallagh channel." 

 " Lake Deele " at this stage discharged into the Swilly valley by the broad 

 and flat-floored depression (D.), situated S.-E. of Drumanaught Hill, and 

 intaking at about 660 feet, O.D. 



The " Sruffannagallagh overflow " served as the outlet of a lake held up 

 by an ice-barrier resting on the slopes of TuUytresna and Cronaglack. A 

 small retreat from the latter uncovered the col over the watershed between 

 it and Altinierin, and put out of action tlie " Sruffannagallagh channel." Its 

 successor, the " Farnoge channel " (F.), has an intake just below 600 feet, O.D., 

 and falls rapidly northward into the Swilly valley. It continued to act as 



