The Stirling Sill is an outcropping of a large

quartz-dolerite Quartz dolerite or quartz diabase is an intrusive rock similar to dolerite (also called diabase), but with an excess of quartz. Dolerite is similar in composition to basalt, which is volcanic, and gabbro, which is plutonic. The differing crystal ...

intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

or sill that underlies a large part of central

Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...

, and may be contiguous at great depth. The sill is of very late

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

age or more probably

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...

, as it penetrates the coal measures, often in

bedding planes In geology, a bed is a layer of sediment, sedimentary rock, or volcanic rock "bounded above and below by more or less well-defined bedding surfaces".Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds., 2005. ''Glossary of Geology'' (5th ed ...

between the various strata. In places, it rises through fractures in the strata to a new level, forming features that, at the surface, would be called

dikes Dyke or dike may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), formations of magma or sediment that cut through and across the layering of adjacent rocks * Dike (mythology), ''Dikē'', the Greek goddess ...

. At its

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth and other terrestrial planets. Features Outcrops do not cover the majority of the Earth's land surface because in most p ...

on the west, the sill generally lies towards the base of the Carboniferous sequence, and just above the

calciferous sandstone Calciferous sandstone is an obsolete geological term relating to strata at the base of the Carboniferous formation, below the coal measures. The rocks of this formation are now considered part of the Inverclyde Group and Strathclyde Group. Typ ...

deposits and lavas, as can be seen in the upper part of the valley of the

Bannock Burn The Bannock Burn () is a stream (''burn'' is Scots for stream) which rises at about on Touchadam Moor, NS715891, just to the north of Earl's Hill in the Touch Hills to the south-west of Stirling in central Scotland. The Bannock flows eastward ...

. However, to the consternation of miners, it occupies a higher position amongst the useful strata towards the east, and in places has destroyed the coal, while in others the effect of the sill has been to convert the

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal i ...

to more valuable

anthracite Anthracite, also known as hard coal and black coal, is a hard, compact variety of coal that has a lustre (mineralogy)#Submetallic lustre, submetallic lustre. It has the highest carbon content, the fewest impurities, and the highest energy densit ...

. The slope of the Abbey Craig, or the

Stirling Castle Stirling Castle, located in Stirling, is one of the largest and most historically and architecturally important castles in Scotland. The castle sits atop an Intrusive rock, intrusive Crag and tail, crag, which forms part of the Stirling Sill ge ...

rock, gives a general idea of the angle of dip of the coal measures at the extremity or the coalfield, and the thickness of the sill can also be seen, which is approximately 100 metres. Geologists have had various theories about the formation of anthracite, as the coal has been most effectively baked near the

Ochil Fault The Ochil Fault is the geological feature which defines the southern edge of the Ochil Hills escarpment in Scotland. North of the fault, Devonian lava flows and pyroclastic deposits slope gently down, thinning towards the north. These are in p ...

, however the theories are not necessarily incompatible. The source of the sill, where the molten rock rose from below, is not known; however it is known that there are risers in the Ochil Fault, although these are inconsistent with the overall structure. The main origin of the sill may be in the region of the eruptive centre that formed the

Ochil Hills The Ochil Hills (; ) is a range of hills in Scotland north of the River Forth, Forth valley bordered by the cities of Stirling, Perth, Scotland, Perth and the towns of Alloa, Kinross, and Auchterarder . The only major roads crossing the hil ...

in a much earlier period, which geologists postulate is somewhere to the east of

Stirling Stirling (; ; ) is a City status in the United Kingdom, city in Central Belt, central Scotland, northeast of Glasgow and north-west of Edinburgh. The market town#Scotland, market town, surrounded by rich farmland, grew up connecting the roya ...

, possibly in the region of Kincardine. This ties in with subsidence following the first round of eruptive activity that lead to the formation of the coal basins, with a much later outbreak of intrusive activity from the same magma feeders, however the truth will remain unknown because it would take very many deep bores in

hard rock Hard rock or heavy rock is a heavier subgenre of rock music typified by aggressive vocals and Distortion (music), distorted electric guitars. Hard rock began in the mid-1960s with the Garage rock, garage, Psychedelic rock, psychedelic and blues ...

to probe the depths and find the necessary evidence.{{Citation needed, date=February 2011 The western extremities of the sill can be seen at the

Abbey Craig The Abbey Craig is the hill upon which the Wallace Monument stands, at Causewayhead, just to the north of Stirling, Scotland. Physical geography The Abbey Craig is part of a complex quartz-dolerite intrusion or sill within Carboniferous st ...

, Kings Park, Gillies Hill, Sauchieburn and various points to the south. The

was once quarried at several places including Cambusbarron and Murrayshall, but is quarried today at Northfield, primarily for roadstone.

References

Hills of the Scottish Midland Valley Geology of Scotland Mines in Scotland Sills (geology) Geography of Stirling (council area) Landforms of Stirling (council area)