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A POST-PERMIAN FAULT AT CUSWORTH, NEAR DONCASTER. 
BY H. CULPIN. 
The Permian rocks in the neighbourhood of Doncaster 
fall into three main divisions : — 
(1) The Lower Limestone, on the west, distinguishable 
by the massive character of the rock. Its bottom beds are 
fossiliferous. 
(2) The intermediate marls with gypsum. 
(3) The Upper or laminated Limestone, on the east, with 
strongly marked bedding planes, and with fossils in the topmost 
layers. 
In addition to the Don gorge, which is the principal and 
most imposing physical feature of the district, there are many 
small transverse valleys which take their rise in the Lower 
Limestone and then run eastwards, tapping longitudinal valleys 
in the marls. Several of these valleys are very charming, and 
one of the most attractive among them is that which commences 
about a mile nortli of the Sprotborough section of the Don 
gorge, and finally enters Cusworth Park. 
Exposures of rock on the northern slopes of this valley 
(Plate LXII., Fig. 1) show the Lower or massive Limestone in 
close contact at the same level with the Upper or laminated 
Limestone, and indicate an important fault not yet marked on 
the geological map. This occurs below a mound described on 
the ordnance map as the supposed site of a castle. 
The fault runs from south-west to north-east, and its hade 
is normal. The up-lift of the Lower or massive Limestone is 
on the north-west side, being the farther side from the Don. 
In the Edlington fault, about a mile beyond the opposite bank 
of the Don, the uplift of the Lower or massive Limestone is 
on the south, being again on the farthei side from the river. 
These two faults, therefore, form a trough, in which lies the 
Don gorge. At their inception, the uplifted strata probably 
towered at a great height above the intervening surface, thus 
forming a deep wide valley, out of which the Don gorge has been 
