434 CARTER : GLACIATION OF DON AND DEARNE VALLEYS. 
as there are none of its distinctive rocks found in any of the 
drift deposits to the south of this line in the valley of the Don. 
The scattered pieces of slate and jasper reported by Dr. Sorby j 
in the valley of the Rother may be explained by the agency ] 
of icebergs from the lake -washed ice -front. 
When the ice at Staincross pushed down into the valley 
of the Dearne at Barnsley, as indicated by the Old Mill section ] 
(Fig. 2), the valley would be completely blocked, and the lake 
formed would have no escape except by the little col under 
Harborough Hill, Barnsley, at 310 feet, through which the 
Lancashire and Yorkshire Railway now runs. 
When the ice stood with its front at Adwick-on-Dearne, 
the overflow of the shallow lake in the valley of the Dove would 
be by a little valley at the 125-iOot contour, between Mexborough 
and Wath, through which the railway and canal now go. At 
this period it is probable that the valley of the Don at Mex- 
borough was blocked with ice, and the fine dry valley at Denaby 
excavated (Fig. 9). This valley is cut down from the 150- to the 
100-foot contour, but the old river channel was already cut to a 
lower level, and so on the retreat of the ice the river resumed 
its old course. 
There is also a little stream flowing nortliwards from Thry- 
bergh which appears to owe its present course to glacial diversion 
(Fig. 9). At Hooton Roberts it faces a wide, drift-encumbered 
valley, but, turning at right angles, flows through a much 
narrower and newer gorge to the Don, The explanation offered 
is that the old valley of Hooton Roberts was choked at the 
Denaby end by an ice-barrier, and that the lake formed drained 
over into the Don and commenced the present drainage vallc}^, 
which was cut below the level of the col of the Hooton Roberts 
deserted valley, which is at the 180-foot contour, before the 
disappearance of the glacier. 
The logical result of all this ice movement is the entire 
closing of the Calder vaUey. We cannot stop the movement short 
of Woolley Edge ridge, on the eastward slope of which, up to 250 
feet, are several drift gravel patches. A great lake would neces- 
sarily be formed in Calderdale, fed by the overflow from the Lanca- 
shire side by way of the Burnley and Summit vaUeys. This lake 
