OUGS FIELD EXCURSION TO THE MOURNES 23 May, 1998

At 11.30 on a blustery Sunday in the car park at Bloody Bridge, just south of Newcastle, a small group of regulars set out to examine part of a granite intrusion and its aureole. The Mourne Mountains are formed from a Tertiary Granite intruded into Silurian sediments. The granite has been mapped as having five main pulses (G1 to G5): G1, G2 and G3 being in the eastern part where we were, and G4 and G5 to the south and west toward Newry.

Alan Bell our leader produced an impressive 3-D computer print-out showing the topography with superimposed geology and gave us an introductory talk pointing out the various mountains such as Slieve Donard while indicating where the geological boundaries were. He also pointed out that the granite had been emplaced 4km down in the crust and therefore at least that thickness of rock had been eroded away over the last 65-70 Ma. We had visions of dinosaurs trundling 4km above where we now stood at the time the granite was intruded!

Down a short track from the car park on the shore were Silurian turbidites, deep-water sandstones and shales which formed as the Iapetus Ocean was closing, and interspersed with some volcanic debris. These were deformed and sheared and were steeply inclined toward the granite. It was suggested that this was caused by the granite being forcefully intruded up into these sediments and pushing them aside. It was noted how beds of different compositions deformed in different ways.

At the back of the beach about 10m of poorly sorted overburden lay above the rocks, forming a crumbling cliff, evidence of the work of ice and water in the more geologically recent past.

When we returned to the car park a chip van had materialised much to Máire's delight! I suppose it helped to make the decision to eat before setting off on the second part of the trip so we lunched at picnic tables beside our cars, very civilised.

After lunch we headed inland and upwards along the Bloody Bridge River. Evidence of the erosive power of water in even a relatively small stream was seen where the ~10m high bank had been cut back quite recently displaying at least three distinct layers of fluvio-glacial material. Moving upstream we came across one of the many intrusive dolerite dykes (i.e. of basaltic composition) to be seen in the area, this one cutting the sediments of the aureole was relatively fine-grained.

The next stop was the "contact". It was pointed out that although this was shown as a line on the map it was actually quite a wide zone. We saw fine-grained G1granite containing enclaves of the country rock.

Passing another dolerite dyke, this time in the granite, we continued upstream and saw a possible roof contact of the granite with the sediments along the far side of the stream. There was some evidence of late-stage hydrothermal activity in the granite and a description of the mechanisms and composition of such fluids was given.

We finally ended up at the contact between G1 and G2 granites just below spoil heaps from quarrying operations could be seen. The G1 was much lighter in colour and had more feldspars than the G2 granite which was more quartz-rich and appeared much darker (though just because they have more quartz doesn't necessarily make rocks darker!).

Having survived several showers, it now began to rain again and we decided to return to base camp.

Alan's area of expertise is in the field of dimension stone and he has some intriguing facts about rocks that other geologists don't tells us. For instance, did you know that 95% of larvikite, that deep blue-grey shimmery rock beloved of banks and building societies, goes to waste. (That's why only banks and building societies can afford it!!!) We are indebted to Alan for his enthusiasm and for making time for us on what was a most interesting outing.

Susan Pyne