Geological Heritage Sites of Mayo

Mayo’s geological heritage spans nearly two billion years of Earth’s history. Its coastal regions are home to some of the oldest rocks exposed in Ireland. The powerful erosive force of the wild Atlantic coupled with other powerful geological processes e.g. the last glaciation, are visible in the county’s landscapes and landforms. Unique exposures of bedrock display well-preserved fossils, minerals and erosional features. The Mullet Peninsula, Dún Briste sea stack at Downpatrick Head, the volcanic rocks of the Kilbride Peninsula, the famous drumlin landscape of Clew Bay and the iconic quartzite profile of Croagh Patrick are but some of Mayo’s notable geological heritage sites.

The folded metamorphic rocks at Cross Point are called the Cross Point Gneisses (pronounced nice). They have been polished by wave action to reveal striking pale yellow and dark grey banded and folded patterns. The Cross Point Gneisses are 1.3 billion year old granites that were folded, deformed and metamorphosed between 1,177 million and 960 million years ago.

Doolough Valley separates the Mweelrea Mountains (814 m) to the west, and the Sheeffry Hills (762 m) to the east. With hanging valleys and corries, an elongated lake on the valley floor, steep mountainsides, kame terraces and glacial striations visible on the bedrock surfaces, the Doolough Valley is a classic ice carved U-shaped valley.

This green rock, formed some 475 million years ago, is composed of the green mineral serpentine. It is exposed on Croagh Patrick’s Pilgrim Path. Serpentine was named in 1564 by Georgius Agricola from the Latin "serpens" meaning "snake", alluding to the mottled green appearance of the mineral suggesting the resemblance to some snakes. The presence of this snake like rock close to St Patrick’s statue, makes for an interesting coincidence.

Croagh Patrick is formed almost entirely of metamorphosed sedimentary rocks (originally formed during the Silurian Period some 435 million years ago) including quartzites. The well-worn Pilgrim Path ascends from Murrisk up the north-east slopes of the mountain. Every year, tens of thousands of people, especially on the last Sunday in July, follow the route to the summit, 764 m high.

Over time, water and trapped water vapour dissolves limestone to form centimetre-scale circular pits (eggbox pitting) and vertical tubes (tube karren). The shores of Lough Mask and Castle Lake are renowned for their limestone pavements that display these water-dissolved features.

The deep amphitheatre-shaped Glencullin Corrie opens out into the Doolough Valley. A glacier resided in this ladle-shaped depression during the last glacial advance before melting some 12,000 years ago.

The entire drumlin landscape of Clew Bay is a great example of the effects of rising sea level since the end of the last glaciation. Geologists regard Clew Bay and the surrounding landscape as being of major importance in uncovering the story of the last glaciation, off the west coast of Ireland some 16,000 years ago. Clew Bay’s iconic drumlin landscape prompted cartographers charting Ireland’s Atlantic coast 500 years ago to include the drumlin islands as a navigational aid for seafarers.

The shoreline around Carrickahowley Castle is the one of best locations to examine fossiliferous Carboniferous age rocks up close. Fossils of marine fauna (called Syringopora) that lived in warm shallow tropical seas around 330 million years ago can be seen on the limestone strata. These disc-shaped fossils (up to 30 cm in diameter) consist of hundreds of individual pencil-size coral tubes, joined together at the base of the coral.

Killary Harbour is a spectacular example of a glacial fjord, and is the only fjord on Ireland’s west coast. Once a deep, U-shaped glacial valley, the valley floor was flooded and drowned by the rising sea levels after the last glaciation ended, some 12,000 years ago.

Keem Conglomerate Formation -- 650 million year old seabed deposits

The rocks of Keem Bay Island were originally formed by deposition of layer upon layer on an ancient seabed. Called conglomerates, these sedimentary rocks were later subjected to high temperatures and pressures to form the cobble-size meta-conglomerates visible today. The conglomerates are best viewed at the north end of Keem Strand at low tide.

At Cummer, to the west of Finny, world-renowned exposures of pillow lavas occur. These volcanic rocks formed when basaltic lavas were erupted onto the ancient Ordovician seafloor some 480 million years ago. The outer “skin” of the lava cooled quickly in the cold seawater, and then solidified into round pillow-shaped forms. Over time, repeated lava extrusions formed mounds of pillows which can be seen at many locations in the Finny area. Similar lavas form today off shore from volcanic islands such as the Hawaiian Islands

Sheer cliffs, the 20 m high sea stack of Dún Briste, and the gaping blowholes that erupt sea spray from underground caves are just some of the remarkable features to be seen at Downpatrick Head. Layers of sedimentary rock in the cliffs tell the story of a time when Ireland was located to the south of the equator in a warm tropical climate, during the Carboniferous Period (~340 million years ago).