An introduction to the geology of the Cookstown district
(Thanks to Jim Rutherford for providing excellent information for this short geological history of the area)

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 neighbourhood.
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 district.
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.

 

 
Tertiary Dyke, through Cretaceous chalk -Moneymore
 

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 the Corvanaghan quarry, some 5 miles west of Cookstown.

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) Volcanic.
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 years ago.
There are two types of plutonic rock (4) and (5)

Granite. 
It is pale coloured, with large pink and glassy crystals, feldspar and quartz respectively. It is used for decorative cladding on
buildings and pillars and best seen naturally on Slieve Gallion.

Gabbro.
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 Tullycall and Black Rock just south of the Beaghmore Circles.

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-water has
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 road metal.

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.
 

Carboniferous Period    

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 farmland.
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 local towns.

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 seriously
affected 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 16^th 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 very 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
very 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 area.
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 of course it is all of great
value to the building trade.
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

                 

 Geological Time Sequence. 

    A useful site-find out shapes and positions of continents in different geological times.

 Habitas Geology site.           
 
    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.

 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.