20 R. H. Tiddeman — Formation of Eeef KnolU. 



upper clays along our coast than in the lower beds. But however 

 this may be, it is quite certain that the somewhat scanty drift that 

 reaches farthest up the valleys on our coast, and climbs the eastern 

 flank of the Yorkshire wolds, and the Oolitic moorlands, is, as far as 

 the foreign boulders are concerned, composed almost entirely of 

 rocks from the Cheviot area. The Scarborough district supplies 

 a good example of this rule. The comparatively low ground 

 adjacent to the sea is covered with thick drift full of boulders of 

 the usual types. On the other hand, Seamer Mooi', which is a mile 

 and a half west of the town and six hundred feet in height, is 

 capped by drift, the foreign pebbles of which are largely porphyrites. 

 It must not be understood from this, however, that other types are 

 entirely absent at high levels. Occasional specimens from probably 

 all the groups are found wherever the drifts extend. But the rule 

 is, that at high levels and along the western margin of the drift 

 generally, the porphyrites prevail. And if we follow that very 

 ill-defined line which separates the drift areas from the driftless, 

 it will be generally found that the outermost fringe of straggling 

 pebbles on the fields is largely composed of porphyrites. 



All the facts respecting the distribution of the boulders of 

 East Yorkshire, as far as I have seen, appear to agree vpith the 

 supposition put forward by Mr. Lamplugh in his paper on the drifts 

 of Flamborough Head,^ viz., that the North Sea ice-sheet attained its 

 maximum development and reached farthest inland before the ic& 

 flowing from the north-west had reached this part of the coast, and 

 that the North Sea ice dwindled away as the flow from the Pennine- 

 Chain and the Cheviots gained strength. 



VI. — On the Formation of Eeef Knolls.^ 



By R. H. TiDDEMAx, M.A., F.G.S. 



(Communicated by permission of the Director-General of the Geological Survey.) 



AT the meeting of the British Association at Newcastle in 1889 

 I brought out my interpretation of the probable origin of the 

 limestone knolls of Yorkshire.-^ 



It was shown that the Lower Carboniferous Eocks in the North 

 of England had two distinct types — that the Yoredale or Northern 

 type extended from the Craven Faults to the Tyne, and that the 

 Southern or Bowland type occupied the country from the Craven 

 Faults to near the Western Seaside plain and extended south as 

 far as Derbyshire. Without now recalling the two tables of the 

 succession there given, I mentioned specially the curious construction 

 of certain mounds of limestone which I called reef-knolls, gave my 

 reasons for supposing that they had been gradually built up on 

 a slowly sinking sea bottom by the gradual accretion of animal 

 remains somewhat in a similar manner to coral reefs. I also showed 

 that from the enormously disproportionate thickness of rocks in the 



1 QJ.G.S., vol. xlvii, p. 428. 



2 Eead before the British Association, Section C (Geology), Bradford, Sept., 1900. 



3 Eeport Brit. Assoc, 1889. 



