Charlesavorth — Glacial Geology of North-West of Ireland. 189 



are largely overgrown with vegetation and obscure ; and in the banks and 

 bed of the Castle lliver. 



In a stream bounding Ardinarrive, two miles west of the Eiver Eoe, this 

 eastern drift also occurs as a hard compact brown boulder-clay, composed 

 almost wholly of basalt boulders and pebbles, but containing also some 

 chalk-flints and chalk-fragments. 



The location of these occurrences of pure eastern boulder-clay is indicated 

 by asterisks on the map (PL IX). 



The great chalk-basalt escarpment of the Antrim plateau lies well to the 

 east of all these localities, so that this boulder-clay is to be regarded as the 

 deposit of an ice-sheet moving off the plateau westerly or south-westerly. 

 As J. E. Kilroe has expressed it : — 



" That a westward ice-movement existed prior to the latest glaciation, 

 is at least consistent with the occurrence of Scottish boulders on the 

 ground, and of a boulder-clay in the glens, with stones from easterly 

 sources. The- sparse yet general occurrence of this boulder-clay, its 

 preservation only in glens, the presence in water-worn gravels washed 

 from the glens of such travelled stones as the boulder-clay is known to 

 contain, and the present prevalence of grey boulder-clay of local origin, 

 are all facts consistent with the supposition that the purple clay once 

 enjoyed a wide distribution, to be since almost entirely swept from the 

 ground by the ice-sheet which, as we have seen, moved from the south 

 and south-west later in the period." (Londonderry Memoir, p. 5i.) 



This view of an easterly or north-easterly origin is confirmed by the 

 presence of shells in the deposits. These shells scattered promiscuously 

 throughout the matrix are generally comminuted, though a few whole ones 

 are to be met with ; some are in fair condition and quite recognizable. 

 Those collected, by the Geological Survey were identified as Tcllina hattica, 

 Ledii pernvJa, Mya truncata, Saxicava, and the crustacean Balanns. I have 

 been unable to add to this short list. 



At Bovevagh and other places in the valley of the Eoe, a Turritelkt 

 bearing-clay occurs; TurritcUa, as Portlock observed, has been dredged in large 

 numbers from Lough Foyle. These shells were without doubt scooped up 

 from the bottom of the adjacent seas by the invading ice and dumped on the 

 land in the sub-glacial deposits, as has now so often, in the study of the 

 British drifts, been shown to have obtained where ice moved from sea to land. 

 These shelly clays were discovered by Portlock, but were assumed by him ' 



' Report of Londonderry and Tyrone (1843), p. 157. 



R.I. A. PBGC, VOL, XXXVI, SECT. B. [" Cj 



