Cork’s Ancient Past

How old is Cork’s geology and its 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 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 how life and environments 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 out about 380 million years ago with the Devonian period:

The late end of the Devonian period

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 large mountain chain, with mountains likely as tall as the Alps or the Himalayas, stretched across what is now much of Connacht. These mountains were the sources for rivers and sediments here in Cork’s early floodplains. While it looks dry, occasionally huge floods would turn these rivers from steady flows to violent rapids.

The term ‘Devonian’ refers to this early time period in the Earth’s history, and it is during this time that we saw complex life take over the seas, but also 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 at the time.

The Devonian rocks are largely read in colour and a very visible part of Cork – both throughout our county and city. The dark red hues of the stone walls and buildings give Cork a distinct look and summarise the local geology perfectly.

Picture of Bothriolepis swimming in Devonian Cork river

The animals and plants that existed during this time were different from those we know of today. In these large river systems making up most Munster, small jawless fish swam around.

On land existed some of the first forests known. The presence of plants changed the surface and atmosphere on the planet radically.

It has been suggested that the success of plants during the Devonian might have contributed to a mass extinction that occurred at the end of the Devonian.

The late part of the Devonian represents many key events in the history of life. And we are luckily able to witness many of these events in the Devonian rocks of Cork.

The early part of the Carboniferous period

Over a “few” tens of millions of years everything changed. Cork looked very different by the time the Carboniferous Period began.

The Devonian time period ended 360 million years, and during the start of the Carboniferous period, the seas started to rise over millions of years. The area was still within the tropics and thus had a warm climate. Huge coral reefs dominated the seafloor at this time and the seas teemed with life! This is a very different looking landscape coming from the Devonian and going forward.

Mud and the remains of these organisms settled on the seafloor and became the limestone we see in many parts of the city.

While it harbours remains of an exciting part of Earth’s past, the Carboniferous limestone is the most common type of rock in Ireland. We know from many locations that sharks, numerous species of molluscs and forests of crinoids (also called sea lilies) covered these widespread coral reefs.

It is also from this time that we get the vibrantly coloured Cork Red Marble. Despite the name, this rock layer is not a marble, but also limestone. A suggestion to its origins could be that this red coloured limestone came from subterranean reworking of underwater sediments, perhaps through underwater landslides.

Picture of Carboniferous crinoids
Picture of Carboniferous volcano

To the immediate north and far west of Cork city, 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 Buttevant and areas in 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 dotted Cork city around the quarries. The remains of such a lime kiln can be visited along the river Curragheen 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 in 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 extrapolate (finding unknown information outside of the area of study).

The Quaternary

Quaternary Cork

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 and age, 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 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.

These sediments have helped founding the gravel industries in Cork, and there are several former pits dotted around the city, especially around the Glen near Mayfield.

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