Field Trip Reports 2011Field_Trip_Reports.html

Tuesday 31st May - A Geological study of the North Down Coast

from Horse Rock to Grey Point

Leader: Dr.Bernard Anderson

This stretch of rocky coastline on the western side of Helen’s Bay is an excellent site to study volcanic rocks of the oceanic crust and the overlying oceanic sediments. The geology of this area is the product largely of Plate Tectonics. In the Ordovician period the North American (Laurentian) and European (Gondwana)  plates pulled apart forming the Iapetus Ocean. About 100 million years later, in the Silurian, they converged again resulting in the closing of the Iapetus. While the Iapetus Ocean existed deep marine sediments were deposited by turbidity currents deep on the ocean floor forming an extensive belt of sandstones, shales and mudstones. They formed rock and trend northeast to southwest across the counties of Down and Armagh into Monaghan and Cavan and are a continuation of the Southern Uplands of Scotland.

Turbidity currents occur when accumulations of sediments on continental shelves become unstable and slide down the continental slope as a dense current of mixed sediments. When accumulated on the sea floor they are graded from coarse at the bottom to fine upwards and are known as turbidites or greywackes. These deepwater sediments solidify into shales and are folded and fractured by converging continents.

At the same time there was igneous activity on the ocean floor, producing pillow lavas. As the name implies they resemble a pile of pillows formed as the moving lava made contact with the cold seawater. Good examples of these rocks are to be found just west of Helen’s Bay, at Horse Rocks, just west of a block of apartments at the west side of the beach. Examination of the pillow outlines shows that they are inverted. These examples are classics of their type, probably the best in Ireland.

About 50 meters northwest towards Grey Point is a good exposure of laminated shales and turbidites in the form of coarse-grained sandstone, interbedded with thin shale beds and showing well-developed sole markings and graded bedding. With such a good range of rocks and structures this site is a valuable teaching resource.

The BNFC owes a big debt of gratitude to Dr. Anderson for his careful research and compelling presentation.

James Rutherford


©All images on this website are copyright

Rhomb-shaped clast (olistolith)

of Greywacke, west of Horse Rock

Small olistolith of serpentine in fine siltstone, 30m north of Horse Rock

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