240 Proceedings of the Royal Irish Academy. 



steeply for some miles as a cross-contour valley.' ThK lake and channel 

 remained in action for a long time, as a considerable retreat of the ice from 

 the hillslopes bounding the lake would uncover no other outlet for the escape 

 of the impounded waters. Only when the ice-barrier had completely with- 

 drawn from the month of the recess did this lake cease to exist. The long 

 persistence of this channel is in marked contrast with the more or less rapid 

 changes of marginal drainage which were effected on the north during the 

 same time. The Altnaheglish overflow continued in operation until the 

 whole valley, to a point well below its confluence with the Glenedi-a, had 

 become ice-iiee, and the Altnasheskin valley (A.S.) no longer carried oflf the 

 watere of 'Lake Glenedra." The sti-atified sand and gravel, frequently 

 cnrrent-bedded, exposed in the deep sections in the streams now draiuiug 

 this great depression- N.-W. of Drapei-stown, doubtless represent the 

 lacustrine deposits fonned on the floor of " Lake Altalaeky." 



Hence this high and prominent watershed of the Spenin Mountains, 

 •one of the most important watersheds of Jforth Lreland, was cut across by 

 a series of seven overflow channels, eaiTying off the surplus waters from 

 extra-glacial lakes held up in the recedes to the south of it. 



The ■' Tamnagh " (T.) and " Cloghomagh " (C.C.) channels discharged into a 

 large lake, " Lake Dreen," supported by ice resting on the flanks of Leaimoun.t 

 (1,615) and Mullaghash (1.581). The surplus waters escaped by a broad valley, 

 south of the latter hill, in taking just below 1,000 feet, O.D., and dischai^ed into 

 the large " Lake Glenedra," which extended Snger-Hke up the Fingeen, Glenedra 

 and AltuaheglLsh valleys. Tliese were blocked across their mouths by an iee-lobe 

 thiTist into the recess between Teeavan Hill (1,085) and Mullaghash (1,581). 

 The lacustrine deposits, situated near the confluence of the Altnaheglish and 

 Glenedra rivers, and derived chiefly from material swept out of the ice, were 

 lai-gely dissected and re-distributed by the glacially-directed stream eourees 

 when later the lake level was lowered. Water-worn sands and gravels are 

 exposed in the fine sections all along the sides of the Glenedra valley.' 

 "Lake Gleuedi-a" received the waters of ''Lake Altalacky," and fonned an 

 exit by the large diy glen (-A.S.) east of Teeavan Hill.* It poured its waters . 

 and carried its detrital matter into the huge lake, "' Lake Benady," held up 

 in the Benady Glen. The height of this lake at this stage was roughly 800 

 feet, O.D., i-ather below it than above it, as shown by the magnificent terrace^ 



■ The site of the proposed reservoir for Londonderry City lies in its lower part^ 

 - E.g., in the banks of the Dunlc^n River, near Moneyneany. 



^ Those occurring near Glenedra Bridge, at an altitude somewhat exceeding l,00(i feet, 

 possibly represent morainic accumulations. 



* The Altnasheskin valley of the six-inch O.S. maps. 

 '" The Mumee Hills of the six-inch O.S. map. 



