CHAP. X.] CRETACEOUS PERIOD. 189 



eastern escarpment of the South Welsh coal-basin. It may 

 be that the south of England has been affected by subse- 

 quent subsidence to a greater extent than Wales, but the 

 difference can hardly be more than 300 feet, so that we 

 might guess the level to which Greensand formerly reached 

 in Monmouthshire to be about 900 feet. 



But in speculating on the possible course of this coast- 

 line further north, and on the probable limits of the sea in 

 which the Upper Greensand was formed, we must remember 

 that the Lower Greensand or Vectian sea seems to have 

 extended much farther west in the south of England than 

 it did over the Midland counties (see p. 186), that its 

 shore-line had a general north-easterly direction, and that, 

 consequently, for a certain length of time, the shore of the 

 Upper Cretaceous sea would have a parallel direction. We 

 may assume that the recession of this coast-line proceeded 

 at nearly the same rate so long as it lay along the strike of 

 the Jurassic strata, but the superficial extent of these strata 

 being so much greater in the Midlands than in Dorset or 

 Somerset, it is probable that when the sea reached the 

 borders of Devon its shore between latitude 52 and 53 

 did not lie farther west than the line of the Warwick and 

 Leicester coalfields ; and again, when the south-western 

 shore was carried back to the Palaeozoic rocks, the waves 

 may still have been attacking the mass of Triassic marls 

 and sandstones which must then have enveloped the coal- 

 fields of Stafford and Shropshire. 



These Triassic beds would form a tract of land uniting 

 the Longmynd district to the Carboniferous region of 

 Derbyshire, and constituting a barrier that would for some 

 time resist the advance of the Cretaceous sea. We have 

 some reason, therefore, for supposing that from the hills 

 of Monmouth the shore of the Upper Greensand sea 

 trended north-eastward to Derbyshire, and passed up the 

 eastern side of the Pennine range, without gaining access to 



