Field Trip Reports 2011Field_Trip_Reports.html

13th August - Geology of the Mournes          

Down where the Dykes Swarm

Leader: David Kirk

This study began at Glassdrumman and proceeded northward to Bloody Bridge. The short section of shoreline at Glassdrumman opens a window on the beginnings of the violent tectonic events that tore a continent apart and eventually created the Mourne Mountains.

Walking north from the access lane we found a shore consisting of almost vertical Silurian shales (usually known as greywackes). They were formed on a deep sea floor which was compressed, folded and turned on end more than 400 million years ago in the collision between two wandering crustal plates.

We then come to a dramatic outcrop of pink
granite rocks, beautifully water-sculpted, emerging from the sand in a cluster of sensually curved shapes and distinctly varied in its textures, from very fine to patterned with large crystals—some up to 25 mm across---of quartz and feldspar. This is a porphyritic micro-granite and it forms the floor of the core of a 16m thick “ring” that cuts through the shales and encircles the Mourne granites and which marks the beginning some 60mil years ago of the tectonic turbulence that led to their huge granite masses,  and the Atlantic Ocean, being created.

The ring - not continuous because of later earth movements - can be traced in small exposures at various places round the mountains, but none show the structures as clearly as here. Following a  lengthy period of stretching and fracturing of the crust, as pressure split the continental mass, and which is evidenced by the hundreds of dykes around the Mournes, upward pressure of the magma chamber forced the kilometres of rock above to rise, causing a circular fracture which spread upwards and outwards. Molten rock, first basalt and then successive pulses of granitic magma forced their way up through it at enormous pressure, melting and remelting and mixing with each other to create what we now see at Glassdrumman. The result is an inverted cone, with its point of origin an estimated 5 km below the present surface. At the time it was emplaced there were probably at least another 3 km of granite above the present surface---including the deposits of later periods---Triassic, Jurassic, and Cretaceous.

The Bloody Bridge River

The bed of the river displays the structural details of the greywackes and granites. Here we were mainly interested in the junction of the two rock types. This junction is to be found 600m upriver from the bridge and displays the changes in the greywacke shales when they were in contact with the hot magma as it surged upwards 56 million years ago. The shales have been baked by the heat making them hard and splintery. The granite itself is different as it cooled more quickly at this point against the cold shales making its crystals smaller than in the main body of this rock.

Coastal Dyke Swarms

Between Newcastle and Annalong there are some 130 dykes along the 10 miles of coast. This demonstrates the level of violent tectonic disturbance in this area. In fact it has been calculated that the aggregate thickness of all the dykes represents a stretching of the earth’s crust by 2.5%.

Their minerals show a greater variety than other dyke swarms, producing basalts, andesites and granites. Some of the dykes are multiple (different intrusions of the same magma) and composite (intrusions of different magmas through the same vent).

Some dykes are harder than the host rock so they stand proud and sometimes they are softer and are eroded away leaving dramatic gullies, eg. at Maggie’s Leap.

Glacial Deposition  

Finally, parallel to the coast are extensive deposits of glacial till and moranic material. These are moulded at several points into raised beaches. They are products of changes in sea level. Immediately after the retreat of the ice, sea levels were higher in relation to land surfaces and with little or no vegetation cover, wave action rapidly eroded this unconsolidated material left behind by the ice sheets. Gradually freed of its ice burden the land rose - isostatic lowering of sea-level - until land and sea were in their present relative positions.

The BNFC is indebted to David Kirk for compiling and conducting this interesting field study. Many thanks David.

James Rutherford (Geological Sectional Secretary)


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Sharp junction of basalt and granite

Water sculpted granite

Cross section through Glassdrumman composite sheet (Paul Lyle)

Granite meets greywacke

A basalt dyke thrusts up through the weaker Silurian shales

Here the dyke is softer than the shales.

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