65o [Proo. B.N.F.C, 



basalt of the plateau, and obviously the products of well washed 

 Boulder-Clay — are to be found under some 1 2 feet of an entirely 

 different kind of Boulder-Clay to be afterwards described. The 

 washed basaltic debris rests upon New Red Sandstone, about one 

 mile and a half west of the plateau-basalt, its source ; and both of 

 the instances here given testify to the former existence of a 

 westerly moving ice-sheet. 



The occurrence of Ailsa Craig erratics, in the gravel-pits near 

 Limavady and at Moys,* is the strongest corroborative evidence 

 obtainable of a westerly flowing ice-sheet. The erratics were 

 probably deposited, in the first instance, higher up the valley 

 southward, and have been carried downward to their present site s, 

 partly, perhaps, by a subsequent ice-sheet moving northward, and 

 partly by torrential waters from melting glaciers; but of their carriage 

 westward from their original source there can be no question. 



BOULDER-CLAY OF THE SECOND STAGE. 



In the Drift Memoir of the Londonderry District,! ample 

 evidence was presented of an inflow of thick ice from the sea over 

 the northern parts of this country, as already mentioned, in the 

 form of red shell-bearing calcareous Boulder-Clays, in some of the 

 present valley bottoms. Portlock traced those clays up to 450 

 feet above datum on the west side of the Roe Valley J — assuming 

 them to be marine deposits of Tertiary age, and the shells to have 

 been formed nearly in situ. But it is rare indeed to find any 

 whole shells : all practically are fragmentary, and must have 

 suffered intense crushing in so yielding a matrix as the Boulder- 

 Clay, to be in so fragmentary a condition. In an ice-sheet, how- 

 ever, the shells would of course have suffered fracturing.. 



On the east side of the valley I have traced the red shell- 

 bearing Boulder Clay, from the stream bed of the Castle River, to 

 450 feet above datum in the bank of a stream bounding Gortnarne 

 on the south, about 12 miles inland. The ice-sheet bearing 



* See Geological Section Report ante. 



f" Geology of the Country around Londonderry,'* p. 54. 



$" Report of Londonderry and Tyrone," p. 158. See Note C at end. 



