243 



A YORKSHIRE GLACIAL PROBLEM. 



COSMO JOHNS, M.LMech.E., F.G.S., 

 Sheffield. 



The Erratic Blocks Committee of the British Association 

 described in their report for 1883 the very remarkable patch 

 of Boulder Clay with erratics then being- worked at Crosspool, 

 near Sheffield. The place is about miles west of the city 

 on the ridge that divides the Rivelin and Porter Valleys. The 

 striking feature of the deposit is the altitude, which is 730 feet ; 

 for the clay, which is now worked out, was of limited extent, 

 and its depth did not exceed 12 feet. The hollow in which it 

 lay, and to which it owed its preservation, is now partly filled 

 in. The rocks here are Lower Coal Measures, with the Mill- 

 stone Grit cropping out a little to the west. 



The majority of the boulders found were Grits and Coal 

 Measure Sandstones, but included among- them was a remark- 

 able assemblage of far-travelled rocks, many of them striated. 

 Among those identified by Prof. Bonney were Quartz-felsite, 

 Porphyritic tuff, Felstone, Quartz-felsite with Hornblende, Fel- 

 site. Indurated tuff, decomposed tuff, grey Magnesian Lime- 

 stone, Felstone with vein stuff, slaty rock, Felstone without 

 quartz, vesicular Felsite, Rhyolite, Quartzite, Carboniferous 

 Chert, Porphyrite, Tuff and Porphyritic Ash. There was much 

 Magnesian Limestone and many boulders of Red Sandstone. 



A glance at the list indicates a Lake District origin for some 

 of the rocks and the general character of the series rather 

 suggests the Vale of York glacier. Still, there is nothing 

 against the view that the Teesdale ice, after being diverted by 

 the Scandinavian, ran along its face, and that it was this British 

 fringe of the foreign ice that laid down this deposit. When, 

 however, we consider its altitude and the lie of the neighbour- 

 ing land, the problem becomes somewhat complicated, for it 

 is difficult to imagine a tongue of ice being pushed up longi- 

 tudinally along this ridge without having filled the two valleys 

 and interfering with the drainage. But both these valleys are 

 free from drift with the exception that a large Felstone boulder 

 was found much lower down in the Porter Valley. That the 

 deposit was much greater in extent originally is very probable, 

 and that the deposit we are now discussing is but the relic left 

 after the denudation that removed the rest is a reasonable con- 

 clusion. It is the almost complete obliteration of the evidences 



1905 August I. 



