The BGS Lexicon of Named Rock Units — Result Details

Millstone Grit Group [See Also MIGR]

Computer Code: MG Preferred Map Code: MG
Status Code: Full
Age range: Namurian Stage (CN) — Namurian Stage (CN)
Lithological Description: Fine- to very coarse-grained feldspathic sandstones, interbedded with grey siltstones and mudstones, with subordinate marine shaly mudstone, claystone, coals and seatearths.
Definition of Lower Boundary: A conformable base, defined at the base of the lowermost thick feldspathic sandstone in a sequence of Namurian strata; e.g. Mam Tor Sandstones, Longnor Sandstones, Ashover Grit, and Pendle Grit lying above the dark grey mudstone-dominated succession of the Bowland Shale Formation (Craven Group).
Definition of Upper Boundary: A conformable top, located at the incoming of thick coal-bearing Coal Measures facies, at the top of the Rough Rock, or more precisely at the base of dark grey shaly mudstone of the Subcrenatum Marine Band, with eponymous fauna where present (Earp and others, 1961, p.104).
Thickness: Thickest in the north of the Central Pennine sub-basin, with 1225 m in Wharfedale, Bradford. The unit broadly thins towards the southern part of the basin, with 150 m thickness in North Staffordshire.
Geographical Limits: Central Pennines and the Midlands.
Parent Unit: Not Applicable (-)
Previous Name(s): Shale Group [May Be Obsolete = Part Of Millstone Grit] (SH)
Millstone Grit (-2071)
Alternative Name(s): Millstone Grit Formation (Offshore) [See Also MG]
Reference Section  Base: Stream sections at Edale; e.g. Crowden Brook [SK 1021 8578], Grinds Brook [SK 1208 8649] and the River Noe [SK 1665 8586]. Stevenson and Gaunt, 1971, p.210. 
Reference Section  Base: Tansley Borehole (SK35NW/13) at 462 ft 7 ins depth, which shows the base of the siltstone and sandstone of the Ashover Grit, resting on Edale Shales. Ramsbottom et al., 1962. 
Type Area  Numerous natural sections, central and south Pennines, in SD, SE, SJ and SK National Grid map areas. 
Reference Section  Base: Little Mearley Clough, where a waterfall displays the base of the Pendle Grit, resting on dark grey shaly mudstone. Earp et al., 1961, fig. 8 and p.118. 
Reference Section  Base: Blake Brook, which displays the base of the Longnor Sandstones, resting upon dark grey mudstones with common marine shaly mudstones, and beds of turbiditic proto-quartzitic sandstone of the Bowland Shale Formation. Aitkenhead et al., 1985, p.84, and also fig. 28 which displays measured section SK06SE/17. 
Aitkenhead, N, Bridge, D M, Riley, N J and Kimbell, S F. 1992. Geology of the country around Garstang. Memoir of the British Geological Survey, Sheet 67 (England and Wales) 
Aitkenhead, N, Chisholm, J I and Stevenson, I P. 1985. Geology of the country around Buxton, Leek and Bakewell. Memoir of the British Geological Survey, England and Wales Sheet 111. 
Ramsbottom, W H C, Rhys, G H and Smith, E G, 1962. Boreholes in the Carboniferous Rocks of the Ashover district, Derbyshire. Bulletin of the Geological Survey of Great Britian, No.19, p.75-142. 
Earp, J R, Magraw, D, Poole, E G, Land, D H and Whiteman, A J, 1961. Geology of the country around Clitheroe and Nelson. Memoir of the Geological Survey of Great Britain, Sheet.68, England and Wales, pp.346. 
Phillips, J. 1836. Illustrations of the Geology of Yorkshire, Part II. The Mountain Limestone District. (London:John Murray). 
Stevenson, I P and Gaunt, G D, 1971. Geology of the country around Chapel-en-le-Frith. Memoir of the Geological Survey of Great Britain, England and Wales, Sheet 99. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E024 E020 E059 E067 E077 E070 E141 E041 E040 E051 E069 E099 E111 E112 E124 E125 E050 E060 E061 E062 E075 E076 E084 E085 E110 E123