87 



SHEFFIELD'S TROUGH FAULT. 



COSMO JOHNS, M.I.Mkch.E., F.G.S. 



Sheffield is remarkable for the many striking geological 

 features which occur in its vicinity. Reference has been made 

 in the pages of this journal * to the small patch of boulder 

 clay found at Crosspool, at an elevation which places it in a 

 position of splendid isolation so far as current theories of the 

 glaciation of Yorkshire are concerned. The famous Wharn- 

 cliffe Gorge has supplied the key j to one of the interesting 

 examples of river diversion in this island, while the Red Rock 

 of Rotherham has puzzled a couple of generations of geologists, 

 and, despite several praiseworthy attempts,! still remains with 

 its exact stratigraphical position undefined. 



It only requires a striking example of the effect of com- 

 plicated earth-movements to complete the series, and this is 

 supplied by the narrow strip of faulted ground that forms the 

 northern side of the Don Valley between Sheffield and Rother- 

 ham. A reference to the sketch map (Plate IV) will help to 

 make clear the structure of the rocks intersected by the 

 Sheffield, Northerly Don, and Southerly Don Faults. East of 

 the last mentioned fault the coal measures dip gently to the 

 N.E., while the various coal seams crop out with a strike 

 ranging N.W. and S.E., this being the normal dip and strike 

 for this portion of the coal-field. North of the Northerly Don 

 Fault, and west of the Sheffield Fault, the measures have again 

 the normal dip and strike, but are lower in the series. In the 

 country between the two main faults we find the so-called 

 middle coal measures, from the Silkstone up to the Barnsley 

 seams, exposed, dipping steeply in places up to 50''', and with a 

 strike at right angles to the rest of the country. The late Prof. 

 Green, when making his survey of the district, drew attention 

 to this striking feature, and, after describing the present lie of 

 the rocks, remarked as follows : — § 



" The way in which this block of strata has been torn from 

 the beds to which it was once united, and twisted round through 

 an angle of nearly go'' is very striking, but we know so little 



•Johns, 'Naturalist,' Aug. 1905, p. 243. 

 t Lower Carter, ' Rep. Brit. Assoc.,' 1904, p. 55S. 



J Green, 'Yorkshire Coal-field,' p. 482. Kendall, 'Rep. Royal Com. on 

 Coal Supplies', 'Geological Report,' p. 22. 

 § Green, ' Yorkshire Coal-field,' p. 498. 



1906 March i. 



