CARTER : GLACIATION OF DON AND DEARNE VALLEYS. 421 
of ordinary conditions was out of the line of stream denudation, 
and thus their contents have been preserved intact. 
The Rev. C. T. Pratt, M.A., records a boulder of volcanic 
ash, weighing 3 cwt., from Rawgreen, a quarter of a mile west of 
Cawthorne. at the 300-foot contour. Also a larger boulder of 
volcanic ash from Banks Bottoms, Nobelthorpe.* These I should 
explain as conveyed from the Staincross glacier-lobe by icebergs. 
The scattered pebbles of foreign rocks, many resembling 
compact slate rocks or jaspers, and the boulder near Renishaw, 
recorded by Dr. Clifton Sorby,t from the valley of the Rother, 
could also be accounted for by the agency of small icebergs from 
the glacier which closed the mouth of the Rother. 
I have not attempted to explain or to include in the present 
survey the puzzling, isolated patch of boulder-clay with Lake 
Country erratics found at Crosspool, at an elevation of 730 feet 
west of Sheffield. I believe this will be found to be a relic of 
another phase of the Glacial Period than the one I have described 
here, and may be found to link on to the Crich Stand and other 
southern patches of boulder-clay at much greater elevations 
than those of the Don and Dearne. 
II. — Conditions of Formation. 
To explain the conditions of the formation of these puzzling 
deposits one must turn to the Vale of York, and try whether 
the drifts of the Don area can be linked on to those north of 
the Aire. Stretching across the Vale of York in a semi-circular 
curve, which passes through York, is a Hne of mounds which was 
recognised by Professor Carvell Lewis i as the terminal moraine 
of a glacier advancing from the north. Parallel to this moraine 
is another and parallel ridge passing through Escrick, about 
five miles south of York. This Professor Kendall described || 
as a second terminal moraine of the same glacier. The left 
lateral moraine of this glacier has been traced along the western 
♦Report Yorks. Boulder Com., 1896-97. 
t Letter to Professor Green, January 7th, 1868. 
X Glacial Geology of Great Britain, p. 30. 
Proc. Yorks. Geol. and Polytec. Soc, XII., 310. 
