258 Proceedings of the Royal Irish Academy. 



These two overflows clearly denote a temporary halt during the retreat off 

 the hills to the north of the Biu-ndennet valley, and while the lake impounded 

 in that valley stUl drained by Bond's Glen into " Lake Faughan." A short 

 retreat, to the westward threw these small high-level channels and also the 

 main outlet out of action, and opened the escape by the large Burngibbagh 

 valley. 



The latter runs for nearly six miles parallel with and to the east of Ihe 

 Eiver Foyle from well south of Cullion to Drumahoe in the Faughan valley.^ 

 Tiie highest part of the valley lies about three-quarters of a mile north of 

 Cullion railway station at an altitude of about 130 feet, O.D. Here the valley 

 is as well marked and developed, it-s sides as steep and its floor as broad as 

 in the lower stretch. With the exception of one or two small valleys farther 

 west, the Burngibbagh clianuel furnishes the last e^^dence of constrained 

 drainage to the east of the Eiver Foyle in this area. 



At the time of this overflow, the Foyle valley to the west was still 

 occupied by ice, which continued to produce a lake in the valley of the Burn- 

 denuet: this is the last phase of "Lake Burndennet," the successor of "Lake 

 Dnunamanagh." The irregular and moundy country ui this wide depression at 

 Duiiamana Station marks the floor deposits of this lake. The sands and 

 gravels were derived in part from the streams pouring into the lake from the 

 south, i.e., the large Inver overflow river entering at Dunnamanagh and the 

 smaller streams flowing through the Stonypath- and Ballylaw channels, in 

 part from the sheets of material swept out of the ice standing to tlie west. 

 The bulk of the detritus earned into the lake from the south of Dunna- 

 managh forms the flat teiTaces of sand and gravel which occur X.-W. of this 

 village and about Mount<;astle. 



The size of this lake, of its deltaic teiTaces, and of the streams pouring 

 into it across the Sperrin Mountains via the "Inver Channel," and round 

 them from " I.ake Glenmornan " by the " Sionypath " and "Ballylaw" 

 channels, taken in conjunction with the great dimensions of the Btirngibbagh 

 valley, the outlet of the lake, furnish some measure of tlie volume of the 

 waters which drained along tliis valley. With the exception of that 

 comparatively small part draining normally by the Eiver Eoe, it carried off 

 the whole of the drainage of the Sperrins and the meli waters of a great 

 length (over 40 miles) of ice-front.' 



' In its upper part it is traversed by the Donegal raUvray. 



- This overflow, though by no means small, is quite diminutive when compared with 

 the huge Burngibbagh valley, and i>roves cle.-\rly the great volume of the river which 

 entered -'Lake Burndennet," south of Dunnamanagh itself, and which escaped by the 

 Burngibbagh outlet. 



3 Professor H. J. Seymour suggests, iu the Londonderry Memoir (p. 7), that " The 



