THE CAUSEWAY COAST - 19th - 20th JUNE

by Andrew Sheridan

SATURDAY Our field trip to the Causeway Coast began on a drizzly overcast morning in Portrush at the Landsdowne Crescent car park. Here we met up with Paul Lyle, our leader for the day, and had our first sight of a coarse dolerite sill which intruded into overlying mudstones and shales giving rise to fine-grained brittle hornfels at the contact. Ammonite fossils are plentiful here. The sill extends offshore, dipping at approximately 15 degrees and forms the Skerries islands 4 km to the North-East. We then travelled eastwards to our next stop at Ballintoy Harbour. In the car park, the chalk forms a raised beach which exhibited faulting, producing a fault breccia zone. Further west, sea caves, arches and stacks were more evidence of a raised coastline. At the present shoreline, a thin pale green layer of volcanic agglomerate underlies a red layer of volcanic dust which in turn is overlain by some thin Lower Basalt flows. Above this again are the Causeway lava basalts. Between the Lower and Causeway basalts are interbasaltic laterites. Ballintoy Harbour is divided by the fault line - to the NW is the basalt and to the SE, the chalk. We continued along the shore to the chalk cliffs at Whitepark Bay. The chalk sits above the Lias indicating an unconformity. There are also signs of a change in sedimentary conditions Just above the high water mark the lowest of the chalk beds are exposed. These contain pebble and rock fragments which are distinctly speckled and sometimes green due to the presence of the mineral glauconite, a hydrous silicate of iron and potassium. The coarse grained beds resulted from the gradual flooding of the Jurassic land surface in the Cretaceous period (145-65Ma).

Height of the basalts indicated by the cliff path snaking under the red lateritic layer.

After a short lunch at the Causeway Hotel, we visited the Giants Causeway. For many it was the highlight of our weekend field trip (except me - I have never been so wet, cold and miserable at the Causeway in my life. Thanks for bringing the foul weather north lads! Ed) Along the road from the Visitor’s Centre to the Causeway there is an outcrop of the Lower Basalts containing red laterite. Further along were some excellent examples of spherical or onion-skin weathering. On to the Causeway itself, and the famous basalt columns formed as a result of shrinkage when the lavas cooled. The first basalt flows formed regular shaped vertical columns known as a colonnade. The change from colonnade to entablature jointing.is evident with different column styles and widths. A grassy ledge separates the first and second basalt flows. Between Port Noffer and Port Reostan, a dyke visible from sea level cuts through these Lower Basalts. Further east again is an isolated structure known as the Chimney Tops which formed from the colonnade of the second flow. We concluded our day’s events with an evening of discussion and a well-earned meal in the Magherabuoy House Hotel. Columnar basalt with jointing

SUNDAY

Tony Lee whom many of you will have already met, was our Sunday leader. We travelled to the White Rocks, just off the Portrush to Bushmills Road.

The cliffs here are chalk (98% calcium carbonate) formed from the fossil remains of marine algae with calcareous shells during the Cretaceous. Interruption in this coccolith sedimentation, possibly the sea shallowing, caused hardening and mineralisation of the sediment, forming what is known as North Antrim hardground (85-90Ma), similar to what is presently occurring in the Indian and Caribbean Oceans. Flint nodules, some containing fossils can also be found within some of the chalk, along with evidence of faulting. Lower Basalts are exposed on top of the chalk. Here again is evidence of a raised beach with sea caves, arches and stacks.

The chalk has been disrupted, shattered and re-cemented by explosive volcanic action. The margins of a volcanic pipe can be seen, together with the contact between the chalk and basaltic agglomerates.

A mystery remains over the presence of a basaltic agglomerate outcrop situated on the sea side of this volcanic vent. There are three distinctly different types of boulder in the agglomerate - one with mineral filled vesicles (gas cavities), one of similar appearance without vesicles and a third consisting of blocky material. If anyone has any theories as to the process which created this outcrop I know Tony Lee would be glad to hear from you.

To end the trip, most of the group decided to visit Carrick-a-rede Island, west of Ballycastle, and its well-known rope bridge (for the salmon fishermen). Despite the rope bridge being closed in the interests of safety, because of the prevailing wind strength, it didn’t dampen our enjoyment of this volcanic site.

We had splendid views of the scenic coastal area, of Rathlin Island just off the coast, and the Scottish islands of Islay, part of Jura and the Mull of Kintyre on the Scottish mainland.

I would like to thank Paul Lyle and Tony Lee for a very enjoyable and informative trip and finally all members of our Branch, from other Branches and others in the group who were able to attend and who made it such a successful field trip.