Marr : The Rigidity of North-west Yorkshire. 65 



valley-bottoms which fringe the block. The rocks are of 

 various dates anterior to that of the Coniston Limestone. 

 R. H. Rastall has claimed the old rocks of Ingleton as of 

 pre-Cambrian age, and though this is disputed by J. F. N. 

 Green, I regard the evidence as decidedly in favour of Rastall's 

 views. But an interesting feature of these rocks is the occur- 

 rence of pebbles therein, which have been derived from an 

 ■earlier rock-series. These have been described by Rastall, who 

 draws attention to the frequency of pebbles of schistose and 

 gneissose rocks, similar to those of an Archaean complex. 

 He states that ' the portion of the Archaean complex from which 

 the material of the Ingletonian rocks was derived must have 

 been close at hand. . . . This Archaean massif probably under- 

 lies the Pennine area of the north of England at no great 

 depth.' It seems then, that the nucleus of rigid material 

 which is responsible for the rigid block of the northern Pennines 

 is a crystalline mass now buried under the newer sediments ; 

 that over it were deposited later sediments, some probably 

 pre-Cambrian, others of Lower Palaeozoic age, and that these 

 were subsequently bent down against the old block, being 

 compressed, and adding to the size of the rigid mass which 

 gradually grew outwards. It is conceivable that such bending 

 occurred at the end of Lower Palaeozoic times, leaving these 

 rocks relatively undisturbed over the crystalline nucleus, but 

 the system of fractures which limits the rigid mass must be 

 studied much more fully before this can be determined. The 

 little mass of old rock rising up in Teesdale rather tells against 

 this view, as the rocks appear to be much disturbed. 



It is, however, clear that after the deposition of the Car- 

 boniferous rocks, no folding of importance took place, for as 

 before stated, the Carboniferous rocks are practically horizontal. 



Let us turn now to consideration in further detail of the 

 changes that occurred outside the rigid mass. Evidence as 

 to this is obtainable all along its borders, but is most clearly 

 presented to us in a tract north of Yorkshire along the flanks 

 of Edenside in Westmorland and Cumberland. We have 

 seen that rocks newer than the Coniston Limestone are found 

 in those tracts, so that they were depressed, forming geological 

 troughs of varying degrees of complexity.. The hollow fronts 

 of earth-waves broke against the rigid block along its western 

 and southern margins, with the ridges of the waves behind. It 

 is also clear that a similar wave broke against the northern 

 margin, for in Northumberland and the Southern Uplands of 

 Scotland are slate rocks of date posterior to the Coniston 

 Limestone, which latter must therefore turn eastward some- 

 where between its last northerly appearance on the western 

 side of the Pennines and the outcrop of the later slate rocks to 

 the north. Possibly its trend is somewhat south of Halt- 



1921 Feb. 1 



