A study of rocks in Co Cork reveals that millions of years ago Ireland had a Bahamas-like climate, writes Dick Ahlstrom.
Azure skies, bright sun, balmy temperatures and warm water lapping at your toes. The south of France? Majorca? How about Co Cork 360 million years ago?
Stones don't say much but they do nonetheless have stories to tell. They can provide a great deal of information about past environmental conditions.
A highly detailed study of bedrock outcrops across large swaths of Co Cork reveal the region enjoyed a wonderful climate and once lay south of the Equator. It also shows that the rocks we see today spent millions of years at the bottom of the ocean.
Dr Ivor MacCarthy, senior lecturer in the Department of Geology at University College Cork, has published a 30-year study of the county's rocks in map form. The huge undertaking has required decades of work, given that it involves a detailed examination of rock outcrops across a 3,500 square kilometre area from Kenmare in Co Kerry, west to Macroom towards the east and all points south of there.
The study began back in 1974, says MacCarthy. "It is a lot of work done over a long period of time. I have had quite a number of research students working with me over time," he says.
"It gives detailed information on the distribution of different rock types across the region on a detailed scale. It gives very factual information about the rock types, but also the physical features of the rock which are important in engineering terms," he adds.
While most people might be interested in what the rocks say about past climate, engineers will find the study invaluable because of what it tells them about rock condition underfoot today. The area has many fractures and cracks that criss-cross the rock surface making it unstable, says MacCarthy.
"They have quite a bearing on ground water and stability. The area is very heavily fractured. That gives the rock porosity and allows water to flow through but it also weakens the rock."
His study has helped identify where rock instability is likely to occur in regions of bedrock hidden under the soil. The fractures can be up to 10 metres across and these gaps are filled with a variety of soil deposits.
With flooding an increasingly likely prospect, given climate change, it is important for engineers, hydrologists and planners to know areas where flood induced ground collapse and rock instability are likely to occur, hence the value of MacCarthy's map.
The study, "The Geology of the Devonian-Carboniferous South Munster Basin, Ireland", includes both a map and associated booklet. This provides explanatory notes and a detailed reference list of the findings, MacCarthy says.
"The bulk of the rocks are sandstones and mudstones and some limestone." The rocks were laid down as layer after layer of sediments at a time when the region was between five and 10 degrees of latitude south of the Equator, he says. "The area was a little like the Bahamas."
The terrain was low-lying and so a shallow basin formed into which sediments were periodically washed. The conditions were generally arid during the Devonian period with infrequent, but occasionally heavy, rains and flooding. These filled the basin and it would then dry out.
"These rocks range in age from about 360 million years down to about 350 million years old," MacCarthy says. "This is a really short period of time in terms of the overall age of the Earth."
Things changed at the start of the Carboniferous period, however, with a changing environment causing a rapid rise in sea level. The then freshwater basin that is now south Cork was flooded out by the sea to produce marine conditions.
"Eventually the depth of water in the basin rose dramatically to produce a relatively deep basin hundreds of metres deep," he says. "These major environmental changes are recorded in the rocks."
Also there for those who can read them are the signs of what followed at the end of the Carboniferous 290 million years ago.
The Earth's ever-restless crust began shifting the slab that now carries Africa northwards. This in turn crushed into the part of the crust carrying the rocks that now cover Cork.
The collision of the two great slabs threw up a colossal mountain range as high as today's Alps. North America was connected to Europe then and the range started at the US Appalachian Mountains, ran through Newfoundland and on across Ireland into southern Germany, says MacCarthy.
The collision lifted Cork's rock cover out of the sea and also caused many of the fractures we see today, he says. Further cracking and splitting were caused by the 20 or so glaciations that occurred over the region since the Carboniferous.
These glaciers also deposited soil and rock that filled the cracks and left the topography we see today.
The sedimentary rock are about 8.5 kilometres deep under south Co Cork, says MacCarthy. Sedimentary deposition continues apace in the North Sea, another sedimentary basin with rocks 10km deep. What is remarkable is that MacCarthy's study can interpret the clues that tell us the history of the stones we tread today.
Copies of the map and report are available for €25 by e-mailing phegarty@ucc.ie and further information about UCC's geology department is available at www.ucc.ie and clicking "departments".