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26th June - Gibson’s Quarry, Moy Road, Portadown 

Leader: Leslie Wortley

There are three main exposed rock types in Gibson’s quarry, the Ulster White Limestone (Chalk), the Clay–with-Flints which is overlain by the Antrim Lava group. At the top of the succession is glacial till.

The Ulster White Limestone represents a period when Ireland was covered by warm, clear, shallow seas. The composite thickness of the formation is approx. 133 m and at least 38 m have removed by erosion. This limestone is much harder than the English Chalk group. Fossils to be found in Gibson’s Quarry include Crinoids and Belemnites.

The clay-with-flints is part of the 10million year gap between the limestone and the earliest Basalt lavas. This deposit is thought to have been formed as a fossil soil combining the weathering products of limestone and flint, an aeolian component and alluvium from the overlying basalt lavas. Contemporaneous volcanism contributed much of the clay fraction. The clay with flints comprises a soft, highly-weathered clay matrix consisting of fine quartz clay minerals, opaque oxides and haematite. Flints range from grey to white, fawn purplish to deep reddish brown. A cataclysmic volcanic explosion produced ash at a minimum of 750ºC. which along with the haematite changed the flints from grey to an intensely coloured jasper.

The basalt lava flows cover the Clay-with-Flints (over 50 mil. years ago). It has been estimated that up to 1000m of basalt lava has been eroded indicating that the lavas extended far beyond their present outcrop. In the quarry the depth of the basalts varies from 5 to 20m.

This is a working quarry with mobile crushing and screening plant producing mainly road metal. Near the surface the basalt is rubbly producing a very poor stone the use of which is limited to infill. As depth increases it becomes massive giving a very good aggregate.

The Glacial Till is not consolidated and is relatively soft and contributes to modern soils. As glacial deposits cover most of the older rocks in Ulster we can see the importance of quarries to geologists both professional and amateur.

All and all a very interesting quarry and as several people remarked, they have lived and worked in the Portadown area, unaware that such a geological gem existed. 

James Rutherford (Geology Section Sec.)


Fossil echinoid (Sea urchin)

The K-T boundary.


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