June 1st - “Green Island, Black Stones and Red Rocks”                  


After hearing a summary of the geological history of the last 500 million years, and in particular the events at the end of the Silurian period when there was a collision between a northern continent and southern continents which squeezed the “Iapetus” ocean out of existence, we descended to the beach and examined the rocks at Macedon Point - the Sherwood Sandstone, Mercia Mudstone, and intruded basaltic dykes.


This ancient ocean was orientated northeast - southwest and this is still reflected in the grain of counties Down and Donegal – and even in the orientation of Belfast Lough itself.

We descended to the beach and examined the Mercia Mudstone at Macedon Point.  This was formed in the Triassic period – a period when there was one enormous continent and the local area had a semi-arid climate.  As these red sediments are clay-grade they have a slippery feel between the fingers.  Here we are well below the level of the Palaeogene Antrim Basalts but there is plenty of evidence of the way in which the magma reached the surface, in the form of dykes - vertical cracks in the Mudstone filled with Basalt.  Being harder than the Mudstone these stand up from the shore as long straight reefs.  The majority run north-northwest to south-southeast.  Macedon Point owes its existence to a particularly thick dyke.  In 1899/1900 a member of the Field Club, Miss Sydney M. Thompson, published a paper on this locality describing a possible occurrence of “White Lias” – now known as the “Penarth Group”. There is now no sign of this in the area. 


We walked about 700 metres northeast to the next headland.  Here we saw several more dykes and the Sherwood Sandstone which underlies the Mudstone and which has here been brought above sea level along this stretch by a gentle fold. This rock is quite fine-grained but feels distinctly grittier in the fingers than the Mudstone.


In Hazelbank Park, as at many places along this coast, there is a bench a few metres above sea level, backed by a steep slope. This is a raised beach which was cut after the Ice Age, during the Flandrian Transgression about 8500 years ago.  The Estuarine Clay or “Sleech” which underlies central Belfast was also deposited at that time. Members admired a fine display of wild flowers growing on the slope.


We drove to Shorelands half a mile northeast of the University entrance and examined a ruined tower-house, Castle Lug. More Mudstone and dykes can be seen on the shore by crossing the main road here but as time was short and the traffic rather intimidating we continued to Carrickfergus.


At Carrick after evading a circus which was being set up in the car park we examined the rock on which the castle is built. This is another dyke, and a really large one. We discussed the conspicuous yellow Magnesian Limestone which is used extensively in the castle.  This outcrops at Cultra and doubtless was brought across the Lough by the Normans when they were building the castle.  The limestone was formed in the Permian period by an incursion of the sea.  Indeed both in the Permian and Triassic periods shallow inlets of the sea often formed and evaporation of these gave beds of gypsum and of course the famous Carrickfergus salt. The greatest mass extinction of all time happened 250 million years ago in the late Permian. What caused this is not known. Had it something to do with this single great arid continent?

Our final stop was at Island Park which is a short side road about halfway between Carrickfergus and Jordanstown.  This allowed us access to the tiny island which gave Greenisland its name.  The island is formed from a sill – a horizontal sheet of basalt – and typically of a sill, it has distinctive vertical jointing.  This site is also of archaeological interest because in the 1930s Nora Fisher McMillan found a Mesolithic occupation level on the islet with a layer of charcoal and two implements. 

The evening was steadily improving but as it was now 9 pm we decided to call a halt, having at last seen the real Green Island! 


Peter Miller







 
Field Trip Reports 2010Field_Trip_Reports.html

Carrickfergus Castle

©All images on this website are copyright

Greenisland Island

(intruded volcanic sill)

Home        Events        Programme        Membership        History        Contact        Field Trip Reports        Links     Archive