Geology of County Monaghan
Geology Matters No. 7
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THE GEOLOGY OF COUNTY MONAGHAN

Introduction
The geology of a county can often define the nature of the landscape, farming, industry and its architecture. In this synopsis of the geology of County Monaghan we will briefly describe the underlying bedrock geology and its links to its landscape and historic buildings.

Bedrock Geology
The geology of County Monaghan exhibits a wide variety of geological formations, recording ancient environments ranging from mud and sands being deposited in a closing ocean basin, limestones of tropical seas and the desiccated coastal plains similar to the modern day Arabian Gulf. The environment of the time the rocks were deposited, whether on land or in the ocean as well as the prevailing climate at the time, all contribute to the type of rocks that were deposited, and are used by geologists to unravel the earth's history through time. The time span of rock formation recorded in county Monaghan ranges from the Ordovician Period c.500 million years ago to the Triassic Period some 200 million years ago.
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Some geological facts
Oldest Rock Units: The Coronea Formation of the Northern Belt of the Longford-Down Inlier, shown as Mid to Upper Ordovician greywacke, sandstone and shale on the map. These were deep marine sediments and minor lavas and are approximately 470 million years old.

Youngest Rock Units: The Kingscourt Succession, shown as Triassic sandstone on the map. These are approximately 200 million years old. Within this succession are commercially valuable deposits of the mineral gypsum, which is mined for use in plasterboard and plaster products for the construction industry. The mineral formed as seawater dried out leaving a salt-like deposit, hence these types of rocks are known as evaporate deposits.

Gold deposits: The area around Clontibret, in the townlands of Lisglassan and Tullybruck is actively being explored for economically viable gold deposits. Gold was first recorded in the area as far back as 1799!

The most common rock packages in County Monaghan are: (as on Bedrock Geology Map of Ireland, 1:500,000 scale, GSI 2006)

    1. Silurian sandstone, greywacke, and shale (514km2 or 40% of the county).
    2. Visean shelf limestone, shale (340 km2 or 26% of the county).
    3. Mid-Upper Ordovician greywacke, sandstone and shale (273 km2 or 21% of the county).

Geology of Co. Monaghan


Architecture
One of the finest buildings in Monaghan town is the Courthouse in Church Square. The building was destroyed by fire during the 1980s but has been fully restored. Built in 1829, the stone comes from the townland of Eshnaglogh some 15km to the northwest of Monaghan. The original quarry is now disused but stone from the same formation is now quarried at Bragan a few kilometres to the northeast of Eshnaglogh. The stone is a yellowish sandstone from the Carnmore Sandstone member of the Meenymore Formation. It comes from the Asbian Stage of the Carboniferous Period which ranges in age from 337.5 to 333 million years ago. The Carnmore Sandstone is a medium to very coarse grained sandstone containing scattered pebbles within its matrix.

Monaghan Courthouse

 The image to the left shows faint layering and a structure known as cross bedding where the layering appears at different angles in different parts of the rock (almost horizonatal in the upper part and sloping downwards to the left in the lower part).   The image to the right shows a close up of the sandstone with a larger grey coloured fragment in the centre of the photograph.
Close up photographs of the Carnmore Sanstone in the Monaghan Courthouse. The image to the left shows faint layering and a structure known as cross bedding where the layering appears at different angles in different parts of the rock (almost horizontal in the upper part and sloping downwards to the left in the lower part). The image to the right shows a close up of the sandstone with a larger grey coloured fragment in the centre of the photograph.

Suggested further reading (GSI publication):
M. Geraghty, 1997, Geology of Monaghan-Carlingford, A geological description to accompany the Bedrock Geology 1:100,000 scale map series 8/9, Geological Survey of Ireland, 60 pages incl. map enclosure.

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