254 Proceedings of the Royal Irish Academy. 



A coiuparalively sliort wiilulrawal to the west of "Windy Hill caused the 

 excavation of the small glen at Eallylaw falling uoith. It intakes at 

 243 feet, OD} 



A later phase is represented by the Stonypath channel, along which runs 

 the Donegal railway (about halfway between Strabane and Donemana Station). 

 The height of the intake is about 180 feet, O.D. The irregular country to the 

 west, about Lisdoo, may mark the inoraine of this time. During the period 

 ot operation of the " Stonypath overflow," a lobe of ice must have been 

 thrust well into the embaynient of the Glenmornan valley to cause the east- 

 ward deflection .of the overflow valley at its intake and tlie deposition of 

 the Holly Hill ridge of sands and gravels, representing the moraine of 

 this halt. 



The Glenmornan valley is choked full of current-bedded lacustrine sambs 

 and gravels which form perfectly flat terraces, e.g. the extensive terrace at 

 Keenaghan." 



By the lecessiou of the ice-front to the approximate site of the \illage 

 of Dunnamauagh, the waters of "Lake Dunnamanagh " were lowered; the 

 '■' Lough Ash overflow " (L.A.) was thrown out of action and the cutting of the 

 " Drain channel " initiated. This broad valley, intaking roughly one mile S. of 

 Dunnamanagh at about 450 feet, O.D., discliarged into a lake ' held up in the 

 Liscloon valley, flowing then via the Liseloon overflow (L.O.) and the 

 Gleurandal valley into " Lake Claudy," as described above. A further retreat, 

 though comparatively slight, caused the excavation of the magnificent " Black 

 Park ( ;hanuel," just E. of Dunnamauagh, which intakes at about 370 feet, O.D., 

 and falls north. The detritus transported by this stream doubtless contributed 

 to the terraces which occur immediately to the north of its debouchure into the 

 depression of the Burndennet, including the beautiful terrace (altitude about 

 320 feet, O.D.) on which Dunnamanagh Church now stands. This channel 

 carried the waters of the lake held up in the valley to the south of 

 Dunnamanagh into a similar lake, " Lake Burndennet," to the north of that 

 place, the two lakes later becoming continuous on a slight westward retreat 

 of the ice-front. 



The surplus waters of " Lake Dunnamanagh " were carried off at three 

 distinct stages by separate outlets, the " Lough Ash channel " (Ty.A.) intaking 

 justbelow600 f eet, O.D., the "Drain channel "intaking at about 450 feet, O.D. , 



' Measurements of altitudes were made by a small pocket aneroid aud are in 

 consequence only approximate. lu many cases, as here, spot levels supply the exact 

 height. 



-They are also well developed, among other jilaces, at Gorticum Irish, Gorticiim 

 Scotch and Grove Hill. 



' A corrom marks the outfall of this valley at Liscloon. 



