244 Proceedings of the Royal Irish Academy. 



surface on the southern flanks of tlie range probably marking the edge of the 

 ice. Consequent upon its withdrawal, the most westerly of the great recesses in 

 the northern side of the Sperrin Mountains, north and north-west of Mullagh- 

 clogha (2,088), became ice-free. The large lake in the recess drained over the 

 col to the south of Mullaghanimma, and into the lake (L.G-.) held up at the 

 head of the Glenrandal valley. A later and more important stage is 

 represented by the deep, straight channel falling northwards along the face 

 of the hill. A doubtful overflow valley occurs below this. 



"Lake Glenrandal" (L.G-.), drained firstly by the deep and narrow Doalt 

 Glen (D.), intakiug at 960 feet, later by the Carnanreagh channel, which in- 

 takes at about 750 feet, O.D. Both are cut in rock. 



These two streams have contributed to the sands which occur as moundy 



-features over the floor of " Lal^e Dreen," e.g. N.-W. of Dreen itself, though 



the bulk of the material was probably washed out of the ice. The detritus 



brought down by the Clogliornagh Channel (C.C.) from tlie south side of the 



Sperrin watershed has formed a terrace in this lake south of Dreen. 



The surplus waters of "Lake Dreen" flowed through the Glenmoyle 

 valley (G.M.), near Glenmoyle Lodge. It is cut in rock, and intakes at 

 about 800 feet, O.D. Morainic ridges on the lower jST.-E. slopes of Learmount 

 (S.-W. of Dreen) probably mark the position of the ice-fiont at this stage. 



With the withdrawal of the ice from Meeny Hill, and the opening of the 

 Doalt Glen channel (L.), the drainage of " Lake Dreen " escaped by the lower 

 and larger valley of the Ballydonegan Liver (B.D.). This glen runs along the 

 western foot of Mullaghash (1,581), and has its intake at roughly 550 feet. 

 A moraine, probably corresponding with this stage, runs north-west of Dreen 

 roughly east-west, while the fine series of mounds extending south of Feeny 

 from just east of Umrycam to Fincarn mark, without doubt, the position of 

 the ice-edge along this part of its course. The deep dry Banagher valley (B.), 

 intakiug at about 540 feet, O.D., is carved out of " solid." The greater part of 

 the deltaic and lacustrine sand and stratified gravel occurring to the south of 

 Fincarn, and above the intake of the Banagher valley, represents the sedi- 

 ments deposited in the lake by the large streams entering it from the Finglen 

 and adjacent valleys. The Ballydonegan valley (B.D.) only began to operate 

 as the overflow of " Lake Dreen " after the Banagher valley was thrown 

 out of action. 



By the retreat of the ice from Teeavan Hill (1,085) to its position west of the 

 Banagher valley, the large lake extending up the Altnalieglish Eiver ceased to 

 exist, as the normal drainage down the Owenrigh was now open. The " Lake 

 Fincarn," to the west, prior to its drainage by the Banagher valley, discharged 

 its surplus waters by the " solid " glen of the Cushcupal Water, and later by 



