1911 Encyclopædia Britannica/Millstone Grit

From Wikisource
Jump to navigation Jump to search

MILLSTONE GRIT, in geology, a series of massive sandstones, grits and conglomerates with alternate shales, the whole resting directly upon the Carboniferous Limestone or upon intervening shales (Yoredale, Limestone Shales), usually in stratigraphical continuity. Its occasional coal-seams show that conditions of coal-formation had already begun. In Great Britain its outcrop extends from the Bristol Coalfield through South and North Wales to its fullest development in the north-midland counties, Lancashire and Yorkshire, and thence to Scotland, where the Roslin Sandstone of the Lothians and the Moor Rock of Lanark and Stirling are considered its equivalents. Characterized by grits and sandstones of the same general type, though individually variable, as sandbanks formed on the shoaling of the Carboniferous sea, yet often persistent over wide areas, the formation, estimated as 5000 ft. thick in Lancashire, contains typically the following grits in descending order: First, or Rough Rock; second, or Haslingden Flags (Lancashire); third, or Chatsworth Grit (the last two being the Middle Grits of Yorkshire); fourth and fifth, or Kinderscout Grits and the Shale Grits. The first and third, the most persistent, are often coarse and pebbly, like the Kinderscout Grits. In the north of England these grits lose their identity. In South Wales the Millstone Grit, immediately succeeding the Carboniferous Limestone, consists of 450 ft. of grit and shale, its upper member being the massive pebbly Farewell Rock. It extends into the Bristol Coalfield, though not recognized in the Devonshire Culm. In Ireland certain grey grits and flags are assigned to it. In northern France and Belgium it loses its individuality and is merged in the Coal-measures. It reappears east of the Rhine, but is unrecognizable in the somewhat different Carboniferous succession of eastern Europe. In America the Pottsville Conglomerate, 1500 ft. thick in the south Appalachians, with workable coals, and widely unconformable upon the Mississippian, introduces the Pennsylvanian (Upper Carboniferous) system, and approximately represents the Millstone Grit of western Europe, as does the red conglomerate of Nova Scotia.

The shales of the Millstone Grit include thin beds of marine goniatites (Glyphioceras bilingue, Gastrioceras carbonarium), Pterinopecten papyraceus, and Lingula mytiloides, while the grits contain Lepidodendron, Stigmaria and calamities. In Scotland plants and estuarine fishes differ markedly above and below the Roslin Sandstone.

The English Millstone Grit produces a characteristic scenery of wild moorland plateaux, or alternations of shale-valleys and rugged grit-ridges. The grits furnish valuable building stones, and grindstones. They also afford an excellent water supply. (C. B. W.*)