Page:VCH Derbyshire 1.djvu/70

 A HISTORY OF DERBYSHIRE surmounted with piles of rock weathered into fantastic shapes. Robin Hood's Stride and the Rowtor Rocks near Rowsley exhibit the characteristic weathering of this grit. The Black Rocks near Cromford, so named from their black surface, probably due to the action of vegetation, form part of an escarpment of the Kinder Scout Grit. The third, or Chatsworth or Rivelin Grit, varies greatly in charac- ter. It is a coarse conglomerate in the centre of the district, and further south becomes a fine grained sandstone. It is sometimes called the Escarpment Grit, because its outcrop forms escarpments which often extend for miles along the county, and make a distinctive feature in the scenery. Amongst these edges or escarpments are Crow Chine and Stanage Edges. The rocks between the third and first grits are made up of shales and sandstones which vary in thickness and horizontal extent, beds of gannister and thin coals. The first or topmost grit, known as the Rough Rock, is the most constant of the Millstone Grit group, both in thick- ness and in character. Its average thickness is 100 feet. It is a massive coarse grit with a large proportion of felspar, the decomposition of which renders the rock loose and crumbly. The same bed of grit often varies greatly in character from a coarse conglomerate to a fine grained sand- stone, and no particular grit in the series can be identified by its litho- logical appearance. The sequence can only be found by tracing the successive beds along the country. Traces of coal are found on the top of each of the five grits of this series. COAL MEASURES The Derbyshire coalfield forms part of the largest coalfield in England, viz. that which occupies portions of the counties of Derby, Nottingham and York. It is bounded on the east by an escarpment of magnesian limestone, which lies above the Coal Measures and extends from near Nottingham in a northerly direction, far beyond the limits of the great coalfield. On the west it is underlain by the Millstone Grit, Yoredales and Mountain Limestone, which with their easterly dip rise into the ridge of the Pennine Chain. On the western side of the Pennine Chain are the south Staffordshire and north Staffordshire and Cheshire coalfields. A comparison of the carboniferous rocks on the east and west side of the Pennine anticline has shown that though there is on the east a general diminution in the thickness of the strata, they can be correlated from the Millstone Grit up to the lower beds of the middle Coal Measures. This correlation of the beds on opposite sides of the anticline, taken in connection with the fact that there lies between them an elongated dome of Mountain Limestone, is sufficient to lead to the conclusion that originally the Coal Measures extended over part of the north of England almost from the coasts of Lancashire and Cheshire into Nottinghamshire, and perhaps as far east as the river Trent. The denudation which has taken place since the Pennine upheaval has 22