How old is Cork’s geology and rocks?
Before we jump into talking about rocks, let us get a sense of the timescale we are dealing with. Getting an idea of geological time is very difficult for us to wrap our heads around. We are not talking hundreds, thousands or millions of years, but hundreds of millions of years ago! This is why we use the term ‘prehistory’, meaning ‘before written records’. In fact, we are further back in time than human history, which only covers the last 300,000 years.
Below is a timeline of Cork’s geological history. As the timeline shows, we only have evidence of Cork’s prehistoric past from the time periods called the Devonian, Carboniferous and Quaternary. Back then, Cork City’s looked incredibly different, as we are about to show you.
So what was it like back then?
By talking to experts such as palaeontologists (people who study what life and environment were like in the prehistoric past), we can piece together information about what Cork looked like millions of years ago.
The rocks tell us these stories, and that allows us to show you what Cork City looked like back then.
As you read the following from the top to the bottom, you are going forward in time, starting about 380 million years ago with the Devonian Period.
The late end of the Devonian Period
The Old Red Sandstone
As you can see, 380 million years ago, Cork looks very different than it is today. It was also much drier and warmer as it was in the tropical latitudes. A huge mountain chain stretched across what is now Counties Clare, Galway and Limerick. These mountains were the sources for rivers and sediments here in Cork’s early floodplains. While it looks dry here, occasionally huge floods would turn these rivers from steady flows into violent rapids.
The term ‘Devonian’ refers to this early time period in the Earth’s history. It is during this time that complex life took over the seas and the land. The tetrapod footprints on Valentia Island are from this time period, so we can be sure that even here in Ireland there was animal life on land.
The Old Red Sandstone is a very visible part of the city. The dark red hues of the stone walls and buildings give Cork a distinctive look and summarise the local geology perfectly.
The early part of the Carboniferous Period
The Limestones
Cork looked even more different by the time the Carboniferous Period began. The Devonian Period ended 360 million years ago, and during the start of the Carboniferous Period the seas started to rise over millions of years. The area was still very warm and within the tropics. Huge coral reefs dominated this time and area, and the seas teemed with life!
In the distance, island volcanoes grew from the sea floor, and as they erupted, volcanic ash rained down around the area. Remains of these volcanoes can be found in the rock layers of in Buttevant and areas of Kerry. There are also thin layers of ash in the rock layers (in both the sandstone and limestone) all across the county, including in the City.
Historically, the limestone has been used as a construction material, but also for the production of lime. Lime kilns, where limestone was heated and processed, were dotted around Cork City’s quarries. The remains of such a lime kiln can be visited along the Curragheen River in Bishopstown and another in Ballincollig.
What happens next?
We actually can’t be sure, because there’s a huge gap in the rock layers of Cork. In fact, we are missing 300 million years of history. Geologists have found evidence in Cloyne from the Jurassic period that at some point Cork was dominated by forests and had a warm climate. However, for this vast stretch of lost time we have to look at the geology of other areas of Ireland and the rest of Europe to speculate on what Cork might have looked like.
The Quaternary
The ice age deposits:
A mere 12,000 to 20,000 years ago: We are now in the latest ice age and the climate is much colder.
Here, we can revisit an early stage of the River Lee. At this point, Cork City resembled a fjord not unlike those seen in the West of Ireland. It is possible that the first humans in the area might have settled at this time, but we can’t be sure.
The term ‘Quaternary’ refers to the time period, the one we are in right now. It started 2.6 million years ago and is characterised by the expansion of polar ice as the climate cooled on and off for tens of thousands of years. In Cork, we see these sediments above the limestone and Old Red Sandstone, as it was deposited more recently.
When the climate became warmer 11,700 years ago the last of these ice ages came to an end, and all the ice sheets started melting. The melting ice led to torrents of water carrying a lot of sediments across the entire area, including filling up what is now the River Lee.
The presence of these sediments led to the birth of the gravel industries in Cork. There are several gravel pits dotted around Cork City, especially around The Glen, near Mayfield.