398 CARTER : THE EVOLUTION OF THE DOK RIVER-SYSTEM. 
in one period to suffer movements at intervals in subsequent 
periods,* there seems no valid objection to the supposition that 
there may have been oscillations along the line of the Don faults 
as late as Cretaceous times. This suggestion may also help 
to give plausibility to the supposition that the Sheaf valley 
was from the beginning easier to erode than the adjacent valleys, 
owing to the denuding agents having an old valley filled with 
Chalk to commence upon. This would account for its occupying 
a commanding position in the struggle for supremacy from 
a very early period. This supposition may also account for 
the unusual direction of the Sheaf, which follows the faulted 
syncline of the Middle Coal Measures and flows north-east instead 
of east, as is the case with the other consequent streams of this 
district. (Fig. 3). 
A comparison of the level of the Sheaf at Sheffield (150 feet) 
Avith that of the Don at Penistone (700 feet), each about the 
same distance as the other from its source, shows how great 
must have been the advantage possessed by the southern river 
over the northern in the struggle for the Wharncliffe watershed. 
A reason for this may be found in a comparison of the rocks 
over which the two streams flow. The Penistone Don has to 
deal with the MiUstone Grit and the Grenoside Rock, whereas 
the Sheaf flows entirely over Lower and Middle Coal Measures, 
and massive grits are not nearly so predominant in this area 
as they are northwards. 
For instance, the Grenoside Rock, which is recognisable 
but not a conspicuous bed in the valley of the Sheaf, swells out 
northwards to 100 feet above Sheffield, and is 75 feet thick at 
Penistone. t The Wharncliffe Rock also, which is an ordinary 
sandstone in the Shefiield district, swells out at Wharncliffe 
into a great wedge of highly siliceous grit 90 feet thick, forming 
the most prominent rock of the neighbourhood. { 
Coming now to the consideration of the Wharncliffe water- 
shed, I venture to suggest that on going back say to Miocene 
*As noted by Professor Kendall in Rep. of Royal Comm. on Coal 
Supplies ; also in the case of the Cleveland anticline in a paper read at 
the Yorks. Geol. Society's meeting, Thirsk, June 2nd, 1905 (not yet pub- 
lished). 
t Geology of the Yorkshire Coalfield, p. 135, &c. +Ibid, p. 117. 
