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Founded: November 1960
The Tyrone Igneous Complex
This area lives up to its title. A glance at the much simplified map shows its
complexity.It is made up of two types of igneous rock, (4,5) Plutonic and (33)
Both began life as molten magma deep in the earth. The plutonic rocks solidified in
large chambers before they reached the surface around 520 million years ago, they
cooled slowly and grew large crystals, while the volcanic rocks were erupted on to
the earth’s surface, cooled quickly, and grew very small crystals, around 420 million
There are two types of plutonic rock (4) and (5)
The Volcanic Rocks
These are found west of the Plutonic rocks and are bounded
on the NW. by the Omagh fault and on the SE by the Davagh
fault. Extruded from volcanoes on the seabed when the sea
covered this area, the reaction of the hot lavas with the sea-
resulted in piles or tongues or pillow shapes formed when the
lava cooled quickly in contact with the cold sea. Copney, near
the Creggan cross-roads is a good site. The only use is for
An introduction to the geology of the Cookstown district
(Thanks to Jim Rutherford for providing excellent information for this short geological history of
Click to enlarge map.
The geology of the Cookstown district is complex, as the accompanying map with a spider’s web
network of fault-lines and a rainbow of colours for rock types shows. Hopefully people will see
this as a challenge to get to grips with and understand the physical characteristics of our
Geology is important in the economic development of a region and the rock types, present or
missing, can be equally important. Geology has not dealt a very favourable hand to the
Cookstown area, or indeed to Ireland. Most rocks that made Britain “great” are missing, removed
by millions of years of erosion. We can only make the best of what we have.
We hope these notes will stimulate curiosity in a less well-known aspect of the Cookstown
As you look through these pages you will find the Map of the Geology of Northern Ireland (1/250
000) very useful.
Going through the Geological periods, correlate the colours and numbers with the key.
The Central Inlier
These are the oldest rocks in Ireland ( except for Inishtrahull rock,
north of Malin Head), schists and gneisses dating back as much
as 1000 million years.
Going this far back, dates can only be a reasonable guide. They
have been through the geological mill several times as the earth’s
plates converge and diverge and as a result have been folded and
refolded several times in a process of metamorphism and are now
extremely hard and thus are in great demand as road metal.
They are best seen in Corvanaghan quarry, some 5 miles west
Pale coloured, with large pink and glassy crystals of feldspar
and quartz respectively. It is used for decorative cladding on
buildings and pillars and best seen naturally on Slieve Gallion.
Like granite has large crystals, very dark in colour, nearly black or blackish green,
and is denser. Gabbro and granite are often found close together; the lighter
granite is always found on top of the gabbro.
Its main uses are decoration on buildings, for headstones, and road metal,
however in this area it is not of decorative quality.
There are several quarries, Orritor quarry, Bell’s quarry at Tullycoll and Black
Rock just south of the Beaghmore Circles.
Ordovician and Silurian sedimentary rocks.
These form a small rectangular area just east of Pomeroy and consist of sandstones,
siltstones and mudstones laid down
around 440 million years ago. They contain a variety of fossils, including graptolites,
but notoriously difficult to find. The rocks
are covered with boulder-clay which produces quite good grazing land.
Devonian rocks; c. 420 million years.
This period saw one of the periodic closures of the Atlantic Ocean and the collision of the North American and European plates which pushed up
a great mountain range, the Caledonian Mountains, at least as high as the present Himalayas. Time has eroded them down to remnants of their
former glory in the Sperrins, North West Ireland, the Highlands of Northwest Scotland, Scandinavia and Newfoundland. Severe erosion under
tropical conditions set in and vast accumulations of sandstone built up.
Soils are often poor and badly drained in these areas, producing poor farmland.
Devonian rocks are found south of Pomeroy
During the Devonian ( around 400 million years ago) there was an eruption of an acid lava in the Cappagh area. Contact with iron oxide has given
it a distinctive reddish colour. This is a hard rock used for road metal, which has lost popularity as it fractures with sharp edges causing
considerable wear on vehicle tyres. They are best seen at Cappagh or Barrick Hill quarry.
This is one of the most extensive rock sequences in the geological table and takes up by far the largest area of all the rock
types in Ireland. It covers the Central Plain and extends north into East Tyrone and in a narrower strip to near the North Coast. Carboniferous
rocks were laid down from 360 to 290 million years ago
Three rock types occur, always in the following order:
This is the basal sandstone and conglomerate forming the transition between the Devonian and the Carboniferous
A. These make up the limestone series, which in this context will be treated as a unit.
They are present from Cookstown to Dungannon (and beyond) usually concealed by boulder clay which makes up the
For exposures of the limestone we are largely dependent on quarries, the two principal ones being: 'Cummings’ at
Tullyhogue, and the Cement Works at Ballysudden.
Other good outcrops occur along the Ballinderry river at Loughry College.
The strata dip north at 15 degrees, and add up to an overall thickness of 100m., representing a period
of sedimentation of 7 million years. The accumulation took place in warm, shallow tropical seas which
provided optimum conditions for the relevant fauna. These were mainly brachiopods, corals, crinoids,
bivalves and bryozoans.
From time to time, the process of accumulation was disrupted by changes in sea level.
This resulted in deposition of sometimes substantial layers of sandstone,(e.g) Carland quarry and near
Newmills. These sandstones have been used as good quality building stone in important buildings in
B Sandstone (often called Millstone Grit)
Unfortunately there are no exposures of these rocks in East Tyrone as a considerable depth of more recent rocks conceals
them. They are down-faulted and can only be reached by shafts. There are many such shafts in the Dungannon-Coalisland
area as there were several good coal seams associated with these sandstones but are now worked out.
C Coal Measures. Being the highest of the three series of rocks in the Carboniferous Period they have been most
seriouslyaffected by erosion and are only preserved in down-faulted strata in the Coalisland area.
Coal-mining here was never easy. Poor quality coal, difficult mining conditions and intensely faulted seams which dipped
steeply (50 degrees) made mining dangerous and uneconomic. Mining began in the 16th century and continued sporadically
until it petered out in the 1960’s. Surprisingly enough there are still reserves of a quarter of a million tons of coal in the area.
Another resource associated with the coal measures is clay, including the Rossmore Mudstones, suitable for making cement,
pottery and bricks.
The clay pit in Gortnaskea (at Coalisland) has an impressive array of plant fossils.
The Permian Period Age: 270 million years
Only very small outcrops of these rocks are to be found in Ireland and we have one at Grange in the Cookstown area. The
outcrop is poorly exposed, badly overgrown and limited in scope, so a visit to the site is disappointing. The main rocks present
are magnesian limestone, and a coarse sandstone forms the basement.
The Triassic Period, Age: 250 to 200 million years
These rocks cover a considerable area just east of Cookstown and consist of pinkish sandstones and mudstones.Not surprisingly many
soils and sub-soils in the area are also red. These rocks were laid down in tropical deserts. Due to weathering, iron oxides
accumulated, hence the colour. They are not very accessible and usually covered by boulder clay laid down at the
end of the Ice Age.
The best exposures are at Drapersfield quarry (H 841 767) which unfortunately has been used as a dump and
become overgrown making access difficult. However there is a good rock face of pink sandstone showing cross-
bedding and ripple marks.
It has been in demand as a building material as it is easy to cut into rectangular blocks.
The “Red Row” terrace at Drapersfield is a fine example. Another quarry, with good outcrops exists within
Killymoon Estate quite close to the river.
The presently active Carmean quarry near Moneymore also has good outcrops of Triassic (Sherwood)
sandstone and mudstones.
The Cretaceous Period. Age: 140 to 65 mil. years.
The main rock is chalk also known as Ulster White Limestone.
This is a distinctive and familiar rock type. It is an unusually pure form of calcium carbonate, laid down in a warm shallow sea,
allowing a vast accumulation of marine organisms. There is a great wealth of fossils eg. ammonites, bivalves, belemnites,
sponges, corals and echinoids. The chalk probably covered most of Ireland, but has been largely removed by erosion, except
in the North East, where it has been protected by a covering of basalt lava. Consequently the chalk is found outcropping
around the edge of the basalt. This gives us the familiar Antrim coastal scenery of a black layer of rock on top of the white
chalk. On the landward side of the plateau the chalk outcrop is intermittent and occurs randomly from the Downhill area to
Moneymore and Stewartstown. There is even a small amount of chalk at the top of Slieve Gallion. Locally the best location for
the chalk is Carmean quarry at Moneymore, where all the fossils listed above may be found.
Chalk is always in demand for cement making and for agricultural purposes, with minor usage in pebble-dashing, as poultry
grit,road markings etc. Flint is usually associated with chalk, in nodules that occur in layers parallel to the bedding planes. It is
hard and usually grey in colour: it breaks with a curving (conchoidal) fracture and has a very sharp edge; hence its interest to early man.
Some flints are a bright red colour (Carmean quarry) due to iron-staining from the overlying basalt.
Basalt. Age: about 60 million years
Basalt is a basic lava, an extrusive igneous rock, occurring extensively in the North East of the province and extending
southwards on the western side of Lough Neagh to the Stewartstown area.
The lava was erupted from fissures and volcanoes eg. Slemish mountain is the plug of a volcano. Basalt is a fluid type of
lava which flowed out in horizontal layers (eg) cliffs at the Giant’s Causeway or the black cliff along the Magherafelt Road
just north of Moneymore. Once the lava reaches the surface it cools rapidly forming microscopic crystals.
Given time, the lava, rich in basic minerals such as iron, pyroxene and bauxite, weathers down to a reddish fertile clay
highly valued by the farmer. Its other main use is road-metal, but in the past it was used as a building stone in many
towns(especially in Co. Antrim.)
The Quaternary Period
This is the most recent geological period and represents what has happened in the last few million years. It includes the Ice Age and its
consequences The last Ice Age ended about twelve thousand years ago.
The ice performed three main functions; on high ground there is chiefly erosion; it then transported the eroded material down to lower ground where
it was deposited as the ice melted. The Cookstown area is mainly lowland so the most conspicuous function of the ice was deposition with the
effects obvious all around us.
The main feature of our lowlands is the thick mantle of boulder clay covering the bed-rock, this is the material which forms the soil on which our
farming depends. It sometimes takes the form of elongated mounds or drumlins. These have been moulded into streamlined forms which have
offered the least resistance to the moving ice, and drumlins now reflect the direction in which the ice moved. They are widespread in the Cookstown
Where a lot of melt-water was present the deposits consisted of sands and gravel, often taking the form of eskers and deltas. Eskers are sinuous
ridges of ready-washed sand and gravel found eg. on both sides of the Omagh road in the Dunamore and Cam Lough areas. The Lough Fea area
is a huge delta complex of the same type of material and the makeup of the sands and gravels are of geological interest. They reflect the sources
of the material. They can be traced back to the Sperrins, Scotland, Donegal etc. but most will naturally be local.
Begin this investigation in your own garden: examine what you dig up. You could find pieces of granite, basalt, sandstone, flint, limestone etc.
SOME NOTES for GUIDANCE
A useful site-find out shapes and positions of continents in different geological times.
A most informative and comprehensive site, giving detail on almost all sites worth visiting in N.I.
Spaces have been left for photographs - unfortunately never added.
Movement of plates related to geological time.
Some useful books:
A Story Through Time: Patrick McKeever: (1999): Very good introduction
Regional Geology of Northern Ireland: H.E. Wilson (1972): GSNI
The Geology of Northern Ireland : W.I. Mitchell (ed.) GSNI: more academic
The Geological Map of Northern Ireland: (1/250 000): GSNI : essential
If you intend to take geology seriously you will want to examine outcrops in the field. Remember that river valleys, quarries and sea
cliffs are dangerous and you should proceed with extreme caution. There is no right of access to private land. Permission to enter
private land should always be obtained from the landowner.