A short visit to Scrabo

A small party of 28 (of whom only 6 had actually booked) met in the main car park at Scrabo Tower on May Bank Holiday Monday afternoon for one of the Irish Geology Week events. We made our way to the South Quarry admiring the nesting kestrels and the bluebells on the way.

The main rock here is Triassic sandstone, deposited 220Ma -200 Ma ago, overlain in parts by horizontal layers of mud, cross cut by 3 dark grey sills of cooled lava (about 50 million years old) and several dykes. There are three different colours of sandstone here - the Bunter sandstone which is a rich red colour, a pink sandstone, and where the sandstone has come into contact with the sills a much paler, harder and very light coloured sandstone. Until the early 1960s, much of it was quarried out for building and examples of Scrabo sandstone can be found in the Albert Clock, Belfast, in Trinity College Library and some was exported to New York.

There are many fine examples of beautifully carved sandstone from Scrabo on coffin lids in Movilla Abbey nearby and at the Old Cross in Newtownards. At the entrance to the quarry, we stopped first of all at a red sandstone cliff where we happened upon 2 geologists from Queens University who were using a permeameter to assess the permeability of the rock by testing the porosity of the grains. This is a non-destructive method which forces gas into the rocks. In the metre tested there appeared to be a great variation in the permeability measured.

Closer to the ground in that same outcrop there was a very distinct head shape with eyes, nose and mouth carved into the sandstone. Although it is obviously very old, it had been noticed by a young schoolboy in January this year for the first time and a photograph is to be sent to Museum Headquarters for dating.

Further into the quarry we stopped by ‘Lion Rock’, a piece of dolerite with a distinctive lion shape. Here we were able to see the sandstones, mudstones, sills and dykes very clearly. In 1938, 3 and 4-toed trace fossil footprints had been discovered in the sandstone beds. These were ‘proto dinosaurs’ or reptiles which pre-dated the Jurassic dinosaurs.

Cross-bedding was clearly visible here and in places there were exposed ‘beaches’ complete with undulating ripple marks. As we looked left, there was evidence of a sizeable vent which had been filled with agglomerate. The period over which the volcanic action took place here is thought to be the same as that which formed the Giant’s Causeway in North Antrim.

During the last period of glaciation, the dolerite on top of the uppermost sill protected the rest of the underlying rock (until the quarrymen got at it!) , and much of the softer rock around was eroded giving Scrabo its distinctive crag and tail formation. From the top of Scrabo Tower further glacial features in the form of drumlins can be seen stretching for many miles around.As we left the quarry, overhead the peregrine falcons were screeching because their quiet afternoon had been disturbed.

An enjoyable jaunt on a beautiful afternoon!

Mary Cromey