/. R. Bakyns — On the Drift of Derbyshire and Yorkshire. 63 



that stretclies from the Wharfe to beyond the Eibble; from this 

 point the Silurian erratics can be traced nearly aa far as Burnsall, 

 where the valley of the Wharfe again contracts into a narrow dale, 

 closely hemmed in by steep hills ; and they are found, singularly 

 enough, on the west side of the river only. They are not to be 

 found on ascending the hills west by north of Threshfield above say 

 800 feet, though this is the direct way to the nearest outcrop of 

 Silurian rock in place. Thus they occur solely in the Drift of the 

 plain, and would seem to have come on floating ice ; for it is pretty 

 clear that they were not pushed by an ice-sheet over the fells, as they 

 are not to be found anywhere on the high ground : and it is, in my 

 cJpinion, quite impossible that the Eibblesdale glacier should, after 

 debouching on the low ground at Settle, have sjDread out at right 

 angles to its course for the distance of twelve miles. 



The fact of these erratics being found only on the west side of 

 the river shows that something barred their passage ; this may 

 have been a current issuing from the higher part of Wharfedale, 

 which would then have been a fjord ; or possibly an ice-sheet 

 descended from the Grassington moors on the east; or an ice-foot 

 clung to the coast ; and thus the floating ice, laden with Silurian 

 blocks from Kibblesdale, would be unable to reach the present left 

 bank of the stream. 



In the narrow part of the dale below Burnsall I have not found 

 any foreigners ; for this fact I can only account by supposing that 

 that narrow channel was so blocked with ice descending from the 

 neighbouring fells that there was no passage. 



It is worthy of remark, with reference to these erratics being 

 found up to 700 or 800 feet, that the mass of the marine drift in 

 South Lancashire reaches an average height of 800 feet above the 

 sea, and that the Eskers of the dales are found generally up to 700 

 or 800 feet, and further that 700 feet is the amount of the last sub- 

 mergence recognized in Norway. Thus we have evidence of a 

 submergence to about the same amount in several localities. 



Before concluding, I may say that I have just seen Mr. Wilson's 

 interesting paper on the Formation of Eock Basins in Norway.^ I 

 think his suggestion well worth attending to ; but I fear it will not 

 help us much here, for we have piles of Drift where the beds could 

 have presented no edges to the moving ice ; and yet, from the position 

 and extent of the Drift, we know that the boulders could not have 

 been derived in all cases from the waste of cliffs falling on to the 

 ice, for such did not exist, but must have been torn out by the ico 

 from its rocky bed. It has often occurred to me that the bottom of 

 a glacier being at the temperature of 32° F., as is shown by the 

 existence of sub-glacial rivers, ordinary disintegration of the rocks 

 by means of frost may go on beneath a glacier as in the open air ; 

 and thus we may account for the stones in Drift where there could 

 have been no cliffs to afford ordinary moraine matter. The close 

 jointing and numerous alternations of hard and soft beds in the case 

 of the Carboniferous rocks would greatly facilitate such sub-giacial 

 1 See Geol. Mag. 1872, Vol. IX., p. 481. 



