User:Geopersona/Millstone Grit Group

Millstone Grit is the name given to any of a number of coarse-grained sandstones of Carboniferous age which occur in the north of England. The name derives from its use in earlier times as a source of millstones for use principally in watermills. Geologists refer to the whole suite of rocks which encompass both the individual sandstone beds and the intervening mudstones as the Millstone Grit Group. The term Millstone Grit Series was formerly used to refer to the rocks now included within the Millstone Grit Group together with the underlying Edale Shale Group.

The term gritstone describes any sandstone composed of coarse, angular grains though, it is frequently used (with an upper case ‘G’) by British rock climbers specifically to refer to such sandstones within the Peak District, Pennines and neighbouring areas of northern England.

Geographical occurrence
Rocks assigned to the Millstone Grit Group occur over a wide area of northern England and parts of northeast Wales. They comprise of a succession of sandstones, mudstones and siltstones, the specifics of the sequence varying from one area to another.

Peak District
They are the major landscape-forming rocks of the northern part of the Peak District and of its eastern and western flanks in the counties of Derbyshire, Staffordshire and Cheshire. The ‘eastern edges’ of the Peak District’ comprise a broadly north-south arranged series of west-facing cliffs from Bamford Edge in the north through Stanage Edge, Burbage Edge, Froggatt Edge, Curbar Edge, Baslow Edge, Gardom’s Edge, Birchen Edge, Dobb/Chatsworth Edge, Harland Edge and Fallinge Edge in the south. To the east of these edges is a broad band of relatively flat moorland.

Towards the western margins of the Peak District are a rather more broken series of edges, facing in a variety of directions, from those surrounding the high plateaux of Kinder Scout and Combs Moss to the imposing crags of The Roaches, Hen Cloud and Ramshaw Rocks in the south.

Pennines
These rocks extend northwards through the South Pennines of Lancashire and West Yorkshire and westwards into the Forest of Rossendale and West Pennines and the Forest of Bowland, also in Lancashire. They also extend across much of the Yorkshire Dales, most extensively in the east of this area but forming cappings to the hills of the central and western parts of this area.

Northeast Wales
A small area of Millstone Grit Group rocks stretches through Flintshire and Wrexham into the northwest corner of Shropshire near Oswestry.

South Wales
The term 'Millstone Grit' was also adopted in South Wales where rocks of similar age and lithology are found though the Millstone Grit Series of this region has recently been formally re-named by the British Geological Survey as the Marros Group. The thickest bed of sandstone within it was known as the Basal Grit. This has now been renamed as the Twrch Sandstone. The Farewell Rock was formerly considered to be the uppermost unit of the Millstone Grit Series of South Wales though it is now included within the overlying South Wales Coal Measures. Main article: Marros Group

Origins
The Millstone Grit dates from the Namurian stage of the Carboniferous period. At this time a series of isolated uplands existed across the British Isles region. One particular east-west aligned landmass stretched from Wales through the English Midlands and East Anglia to the continent and is now known as the Wales-Brabant High though was formerly referred to as St George’s Land. Other uplands the erosion of which would provide the source material for the Millstone Grit lay to the north and northeast of the region. The Pennine basin received input of sand and mud largely from southerly directed rivers from these northern landmasses.

Rivers running north off the Wales-Brabant High deposited material in the southern parts of the Pennine basin from northeast Wales to the Peak District. Southerly flowing rivers from this same landmass were responsible for the Millstone Grit/Marros Group succession in South Wales.

During much of the Carboniferous period, world sea-levels were fluctuating in response to the growth and decline of a series of major ice-caps over the continents then clustered around the South Pole. Britain lay in the equatorial region. At times of high sea-level, silt and mud accumulated within the Pennine basin whilst at times of low sea-level, major deltas prograded across the region, their legacy being the thick sandstone beds of the Millstone Grit Group.

Stratigraphy
The Millstone Grit Group comprises over thirty individually named sandstones, some of regional extent, others more local in their occurrence. The intervening mudstones and siltstones are not generally named though important marine bands within them are named.

The oldest, and hence lowermost in the succession is the thick Pendle Grit of central Lancashire. It is succeeded by the sandstone known variously as the Brennand Grit, Warley Wise Grit and Grassington Grit. These are all of Pendleian (E1) age – the lowermost stage of the Namurian.

The Lower Follifoot Grit, Silver Hills Sandstone, Nottage Crag Grit, Marchup Grit, Red Scar Grit, Ward’s Stone Sandstone, Cocklett Scar Sandstones and Dure Clough Sandstones are all assigned to the following Arnsbergian stage. The Kinderscoutian includes the Kinder Grit, Longnor Sandstones, Shale Grit, Todmorden Grit, Parsonage Sandstone, Heysham Harbour Sandstone, Eldroth Grit and Ellel Crag Sandstone.

The next stage of the Namurian succession is the Marsdenian and it is to this that the Chatsworth Grit, Huddersfield White Rock, Holcombe Brook Grit, Greta Grits, Roaches Grit, Ashover Grit, Gorpley Grit, Pule Hill Grit, Fletcher Bank Grit, Brooksbottom Grit, Five Clouds Sandstones and Sheen Sandstones are assigned.

The closing stage of the Namurian, the Yeadonian includes the Lower Haslingden Flags and the last sandstone in the entire Millstone Grit succession known as the Rough Rock. It is a widespread unit which attains a thickness of around 45m though is more generally 15m thick.

Landscape
The Millstone Grit sandstones are a hugely important landscape-forming element of the rock succession. They give rise both to a number of ‘edges’ and a series of high plateaux throughout the region, many of which are of considerable cultural significance. The great extent of country within the northern part of the Peak District underlain by rocks of the Millstone Grit Group is known as the Dark Peak on account of both the colour of the rocks and the peaty moorland which these rocks support. The great expanses of bleak moorland around Bleaklow and Black Hill and fringed with broken outcrops of gritstone are characteristic of the area.

The landscapes of the South Pennines and the West Pennines are similar though the edges are less pronounced than within the Peak.

Rock climbing
The gritstone edges of West Yorkshire and the Peak District provide one of the classic areas in Britain for rock climbing. Public access to these edges for climbing developed at much the same time as access for walkers to the moors of the Peak District and Pennines was established during the first half of the twentieth century. Their proximity to large centres of population resulted in their rapid development as climbing venues. Today many thousands of climbers are found throughout the year enjoying the several thousand recorded rock climbs on the dozens of individual gritstone crags in the region.

See Rock climbing in the Peak District

Economic importance
Various of the sandstone beds of the Millstone Grit have been quarried for building stone, paving flags and roofing material. It use in the construction of dry stone walls across the areas where it outcrops is considerable. In neighbouring limestone areas, gritstone is often preferred for use as gateposts and lintels. The very name of the rock derives from its widespread use within cornmills where it proved suitable for grinding stones.

The sandstones also serve as aquifers into which numerous wells and boreholes have been sunk to provide local water supplies.

Crushed gritstone is also used as aggregate in path and road construction.

Millstone Grit in culture
A millstone shaped from Millstone Grit quarried in the area has been adopted as the emblem of Peak District National Park. As an image, the millstone is widely visible on literature but use is made of the objects themselves at many of the entrances to the National Park.

Millstone Grit is the name of an album released in 1973 by folk singer/songwriter Michael Chapman.

Millstone Grit was the name of an autobiographical work constructed around a Pennine journey and published in 1975 by Middlewich-born poet and writer, Glyn Hughes.