64 Marr : The Rigidity of North-west Yorkshire. 



which are physiographically one with it. If one were asked 

 what is the distinguishing feature of this area, one would 

 probably name the prevalence of limestone ; but it seems to 

 me that a much more important characteristic is the general 

 horizontality of the beds which occur therein, and it is upon 

 this that I wish to dwell at some length. 



It is well known that certain blocks of the earth's crust have 

 maintained a state of rigidity through geological ages. Many 

 such blocks have a foundation of highly altered crystalline 

 rocks, which date from a very early period, though, as other 

 regions which are underlain by similar rocks have undergone 

 much folding, the mere presence of these altered foundation- 

 stones is not in itself the whole reason for rigidity. 



One of the best examples of a block which has remained 

 rigid for a long period is the area occupied by the Baltic prov- 

 inces of Russia. There the crystalline rocks are overlain by 

 those ancient sediments known to geologists as the Lower 

 Palaeozoic rocks, and these are still horizontal and so little 

 changed that their organisms are beautifully preserved. It is 

 clear that this region has not undergone folding since pre- 

 Cambrian times. Such rigid tracts are not conspicuous in 

 Great Britain. The most striking is that with w^hich we are 

 now concerned, of which North-west Yorkshire forms the 

 southern portion. In this block we find evidence of resistance 

 to the folding-stresses since pre-Carboniferous times, and 

 possibly for a longer period. Let us briefly consider the 

 geological structure of the tract. The greater part of it is 

 occupied by Carboniferous rocks, which are nearly horizontal, 

 though inclined slightly downward to the east. Folding of 

 these rocks is practically non-existent, save locally, though 

 fractures are common, as is the case with similar blocks else- 

 where. 



Beneath the Carboniferous rocks are the ancient slates of 

 the region, which must be considered rather fully. For the 

 present, however, I wish to insist on the fact that one division 

 of these rocks, known as the Coniston Limestone, follows the 

 border of the block country, on its western and southern 

 sides. It is well known that this block is bounded by three 

 great earth-fractures, namely, the Pennine Fault to the west 

 along its northern extent, the Dent Fault to the north-west 

 along the middle portion, and the Craven Fault to the south, 

 forming its southern boundary. Along the country affected 

 by these faults, the Coniston Limestone can be traced, and 

 it is always underlain by older rocks on the side of the rigid 

 block and succeeded by newer rocks on the sides away from 

 that block. It is clear, then, that a core of rock older than the 

 Coniston Limestone underlies the Carboniferous rocks of the 

 rigid block, and these older rocks are often exposed in the 



Naturalist 



