Stirling Sill
Encyclopedia
The Stirling Sill is an outcropping of a large quartz-dolerite
intrusion
or sill
that underlies a large part of central Scotland
, and may be contiguous at great depth. The sill is of very late Carboniferous
age or more probably Permian
, as it penetrates the coal measures
, often in bedding planes 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
.
At its outcrop on the west, the sill generally lies towards the base of the Carboniferous sequence, and just above the calciferous sandstone
deposits and lavas, as can be seen in the upper part of the valley of the Bannock Burn
. 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
to more valuable anthracite.
The slope of the Abbey Craig, or the Stirling Castle
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
, 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
in a much earlier period, which geologist postulate is somewhere to the east of Stirling
, 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 to probe the depths and find the necessary evidence.
The western extremities of the sill can be seen at the Abbey Craig
, Stirling Castle
, Kings Park, Gillies Hill, Sauchieburn and various points to the south.
The quartz-dolerite
was once quarried at several places including Cambusbarron
and Murrayshall, but is quarried today at Northfield, primarily for roadstone.
Quartz-dolerite
An intrusive rock, similar to dolerite, but with an excess of quartz. Dolerite is similar in composition to basalt, which is eruptive , and gabbro, which is plutonic. The differing crystal sizes are due to the different rate of cooling, basalt cools quickly and has a very fine structure, while...
intrusion
Intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
or sill
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
that underlies a large part of central Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
, and may be contiguous at great depth. The sill is of very late Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
age or more probably Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
, as it penetrates the coal measures
Coal Measures
The Coal Measures is a lithostratigraphical term for the coal-bearing part of the Upper Carboniferous System. It represents the remains of fluvio-deltaic sediment, and consists mainly of clastic rocks interstratified with the beds of coal...
, often in bedding planes 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
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
.
At its outcrop on the west, the sill generally lies towards the base of the Carboniferous sequence, and just above the calciferous sandstone
Calciferous sandstone
Calciferous sandstone is a geological term relating to strata at the base of the Carboniferous formation, below the entire sequence of coal measures...
deposits and lavas, as can be seen in the upper part of the valley of the Bannock Burn
Bannock Burn
The Bannock Burn is a stream which rises in the Touch Hills to the south-west of Stirling in central Scotland. The Bannock flows eastward and enters the River Forth to the east of Stirling, close to the site of the Battle of Bannockburn...
. 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
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
to more valuable anthracite.
The slope of the Abbey Craig, or the Stirling Castle
Stirling Castle
Stirling Castle, located in Stirling, is one of the largest and most important castles, both historically and architecturally, in Scotland. The castle sits atop Castle Hill, an intrusive crag, which forms part of the Stirling Sill geological formation. It is surrounded on three sides by steep...
rock, gives a general idea of the angle of dip
Strike and dip
Strike and dip refer to the orientation or attitude of a geologic feature. The strike line of a bed, fault, or other planar feature is a line representing the intersection of that feature with a horizontal plane. On a geologic map, this is represented with a short straight line segment oriented...
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
Ochil Fault
The Ochil Fault is the geological feature which defines the southern edge of the Ochil Hills escarpment, Scotland.North of the fault, Devonian lava flows and pyroclastic deposits slope gently down, thinning towards the north. These are in part overlain by Old Red Sandstone rocks formed later in the...
, however the theories are not necessarily incompatible.
The source of the sill, where the molten rock
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
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
Ochil Hills
The Ochil Hills is a range of hills in Scotland north of the Forth valley bordered by the towns of Stirling, Alloa, Kinross and Perth. The only major roads crossing the hills pass through Glen Devon/Glen Eagles and Glenfarg, the latter now largely replaced except for local traffic by the M90...
in a much earlier period, which geologist postulate is somewhere to the east of Stirling
Stirling
Stirling is a city and former ancient burgh in Scotland, and is at the heart of the wider Stirling council area. The city is clustered around a large fortress and medieval old-town beside the River Forth...
, possibly in the region of Kincardine
Kincardine
Kincardine or Kincardine-on-Forth is a small town located on the north shore of the Firth of Forth, in Fife, Scotland. The town was given the status of a Burgh of barony in 1663. It was at one time a reasonably prosperous minor port...
. 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 to probe the depths and find the necessary evidence.
The western extremities of the sill can be seen at the Abbey Craig
Abbey Craig
The Abbey Craig is the hill upon which the Wallace Monument stands, at Causewayhead, just to the north of Stirling, Scotland.The Abbey Craig is part of a complex quartz-dolerite intrusion or sill within carboniferous strata, at the western edge of the Central Coal Field, known as the Stirling Sill...
, Stirling Castle
Stirling Castle
Stirling Castle, located in Stirling, is one of the largest and most important castles, both historically and architecturally, in Scotland. The castle sits atop Castle Hill, an intrusive crag, which forms part of the Stirling Sill geological formation. It is surrounded on three sides by steep...
, Kings Park, Gillies Hill, Sauchieburn and various points to the south.
The quartz-dolerite
Quartz-dolerite
An intrusive rock, similar to dolerite, but with an excess of quartz. Dolerite is similar in composition to basalt, which is eruptive , and gabbro, which is plutonic. The differing crystal sizes are due to the different rate of cooling, basalt cools quickly and has a very fine structure, while...
was once quarried at several places including Cambusbarron
Cambusbarron
Cambusbarron is a village in Stirling, Scotland. In the 2001 census, it had a population of 3,224. There is evidence of settlement at the site since the Bronze Age, and several forts dating from the Iron Age have been found near the village...
and Murrayshall, but is quarried today at Northfield, primarily for roadstone.