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a 
220 Dr. Irving—Surface- Chanyes in London Basin, 
Valley and the Pangbourne Gorge, the ice-rafts which conveyed 
it to the spot having floated in succession into one of those back- 
waters of the “Thames Straits,” which were caused by some details 
of surface-contour long since swept away. 
This paper has already grown to considerable proportions. I will 
therefore describe but one more instance of surface-change. The 
deposits referred to were laid open a short time ago in widening the 
road-cutting below the church at York Town, Surrey, as indicated 
in the accompanying diagram :— 
Fic. 2. 
Roap-secTion AT YorRK Town, SURREY. 
(Over-grown slope of the hill.) 
> POLS 
me ee E 
SE 
Line of road. 
A. Ferruginous Upper Bagshot Sands, part of the terrain of the district. 
B. Reconstructed materials of the Plateau-gravels, on an ancient hill-slope (talus). 
C. Fine irregularly-bedded quartz-sand with occasional discontinuous Zones 
marked by oblique lamination, and here and there some small inclusions (a, d@) of 
fine angular flint detritus; the whole extending many yards along the road-side. 
D. Quaternary gravel of the ordinary character filling eroded hollows on the top 
of bed C. 
The massive bed, A, calls for no remark. The talus, B (with just 
that crude stratification which distinguishes a true stream-wrought ~ 
“talus” from a “ scree”), seems to record a very early stage of the 
sculpturing of the present upland valley-system. But it is to the bed 
C that the chief interest attaches. I do not hesitate to call it a 
truly lacustrine deposit, and perhaps its position with reference 
to the overlying secondary gravel and its altitude would justify one 
in assigning to it a date as old at least as the Glacial Period. Any- 
how there could be no possible mistake as to its being a good deal 
younger than the Upper Bagshot Sands (A) at the northern end of 
the section. Yet, were it not for the small inclusion or two of fine 
flinty detritus (a, d), which might be absent altogether if the 
excavation were carried a few feet further into the hill, I have little 
doubt that (standing by itself) it would, on lithological grounds alone, 
have been claimed by certain writers on Bagshot stratigraphy as a 
“characteristic section of Lower Bagshot Sands.” Cases of this 
sort are seen to discount largely the value of the purely lithological 
method, on which so much reliance is placed in certain quarters. 
