Rocks of Cork

Welcome to the Rock Museum!

As mentioned, the two main types of rocks (which we in geology call ‘lithology’ or ‘lithologies’) are the Old Red Sandstone and the limestone, each representing Cork’s red and white colours. Both of them show incredible evidence of what Cork looked like hundreds of millions of years ago!

But don’t just take our word for it. Have a look at our rock museum below. You can interact with the rocks of Cork, read fascinating facts about them and see what they looked like when they came into being.

Old Red Sandstone

The Old Red Sandstone shows us a world that is alien to us today. Back then, rivers and ponds dominated expansive floodplains. In between grew huge forests of some of the earliest ‘true’ trees.

This rock is generally made of sand and sediments that come from the erosion of mountains and are transported by rivers to a basin. The sand has then been compacted through years of compression and high temperatures. They are found all around the world, and the Old Red Sandstone or ORS we see in Cork is common throughout southern areas of Ireland.

This specific sandstone comes from the Gyleen Formation in North Mall, Cork City, and can be observed along the northern side of the River Lee.

Archaeopteris hibernica fossil - one of the first 'true' trees on Earth

Archaeopteris is the name of the species of one of the first true trees in Earth’s history. At the end of the Devonian period, these early trees began to dominate the surface of the Earth. The fact that we see evidence of this in the Old Red Sandstone layers here in Cork City is impressive in itself! This plant fossil was collected from Glanmire, but it is unclear if it means from Glanmire itself or Lower Glanmire Road, where a sandstone quarry was located, known as Tivoli quarry.

From this, we can imagine a Cork with these primitive trees growing in between the massive Devonian floodplains.

Limestone - fossilised coral reef

Limestone is quite spectacular. While the Old Red Sandstone is made of sand and fine rock material, limestone is made entirely by marine organisms and mud. As in the modern-day Bahamas and the Great Barrier Reef, countless numbers of sea creatures such as corals grow on the seafloor, constructing their skeletons and platforms from the chemical compound calcium carbonates. As they die, all that is left is the calcium carbonate structures. 

This is what we find today as limestone, making up a huge part of Cork’s foundation. Imagine all that limestone once was a great ocean with beautiful coral reefs teeming with life. The limestone in Cork dates back from the Carboniferous period, and can tell us a great deal about what the area was like back then. The grey cleaved limestone above is from the Little Island Formation, a layer of limestone rock particular to Cork.

Blackrock Diamond - not a diamond, but an amethyst

Blackrock Diamond is unfortunately not a diamond. The name has been given to these large amethyst crystals found in the Carboniferous limestone of the Blackrock area of Cork City. Amethyst is a form of quartz (SiO2), which is also what we use to make a lot of things, including window glass, quartz clocks and microchips in computers.

The specific place where these crystals were found is known as Diamond Hill or Diamond Quarry, and can be seen from Monahan Road. The amethyst crystals grew in gaps in the limestone where mineral-rich waters deposited loads of SiO2, which then remained as the water disappeared.

The name amethyst comes from the Greek word amethystos, which means ‘that which pushes away drunkenness’. It was believed to be a great hangover cure when powdered (please don’t try this, though!). The crystals were used extensively by jewellers in Cork City as a semi-precious gemstone in the 18th Century, and are still used by crystal healers for all sorts of supposed cures.