18 



J. W. Stather — Sources of Torkshire Boulders. 



After eliminating all the local boulders from the lists, at the 

 above-mentioned localities, the relative proportion between the 

 several groups of far-travelled boulders is as follows : — 



It will be seen from the above table that among the far- 

 travelled boulders of the East Yorkshire drift deposits, Carboniferous 

 rocks (group 1) take numerically the leading position ; and the 

 Carboniferous area west and north of the Tees is generally regarded 

 as their place of origin. Group 2 consists of boulders of black 

 basalt, very plentiful in East Yorkshire and very easy to distinguish. 

 The source of these basalts is undoubtedly the Whin Sill. It is 

 clear, then, that groups 1 and 2 have travelled into our district 

 from practically the same area; nevertheless, it will be seen, on 

 referring to the above table, that the relative proportions of the 

 boulders from the two groups vary considerably from point to 

 point. Thus, while both groups probably decrease numericalhj 

 southwards, the percentages show that the basaltic group increases 

 relatively from Saltburn southwards. The obvious explanation seems 

 to be that large boulders of basalt bear transjDort better than similar 

 masses from the Carboniferous sedimentary rocks. 



The Magnesian limestone (group 3) occurring in the East York- 

 shire boulder-clays is matched by the rock found in situ at Eoker, 

 near Sunderland. This limestone is rarely found except as pebbles 

 in South Holderness, but these grow perceptibly in numbers and 

 size northwards. Large boulders begin to appear north of Scar- 

 borough, and at Whitby and Saltburn they form from 5 to 7 per cent, 

 of the foreign boulders present in the clays. 



Besides the above-mentioned rocks, the East Yorkshire drifts, 

 especially in the southern parts of Holderness, are rich in boulders 

 of igneous rocks of widely diverse types, and these are included in 

 group 4. Phillips long ago pointed out that in the drifts of the 

 Yorkshire coast there were rocks from the English Lake District ; 

 and it is now certain also that the Cheviots and Scandinavia are 

 well represented ; but the source of by far the greater number of 

 the rocks included in this group is not yet known. The table 

 shows that the boulders of group 4 diminish both numerically and 



1 This group was not represented by any boulders of the requisite size in the cliff 

 sections when this table was compiled, but several large boulders of Shap granite 

 were seen in the gardens and about the town, which had probably been derived 

 from the neighbouring drifts. 



