February 03, 2005
Rock of ages
In hard rock terms, this is Lemmy from Motorhead - gneiss rock on Annagh Head, the Mullet, Co. Mayo.
When Ireland was a land of saints and scholars, monks travelled frequently between Ireland and the isolated islands of Scotland's north Atlantic coastline. When St. Brendan sailed across the North Atlantic to Newfoundland, the windswept land may have reminded him of his homeland - at least the western seaboard of Ireland that is continually battered by sea and wind. The learned monks may have noticed the similarity of the barren rocks in all three places but could hardly have imagined that the rock was, in fact all part of the same landmass. Or rather, it had been, about 600 million years ago.
When I arrived at Annagh Head, on the Mullet peninsula in north Mayo last Saturday, three young fishermen were carrying their rods back to the car. "What are you trying to catch ?", I asked as they filed through the stile separating the road from the coastal path. "Nothing - it was too rough", came the reply. (Notice how they didn't answer the question I asked - some fishermen are more secretive than gold prospectors). They were right about the sea - huge waves were breaking against the rock. It was 4.30 in the evening, the light was fading and the haze would soon turn to drizzle that would in turn to rain. Nevertheless, it is a spectacular scene. In the glow of twilight, the pink, oranges and other colours in the rocks seem to have an iridescent glow. And what rocks. There is no beach as such, just a huge jumble of rocks, some as big as small cars, strewn along the coastline. Welcome to pre-Cambrian Ireland.
If you look at the colour-coded geological map of Ireland designed by the Geological Survey, you'll see that most of the country is shaded blue, for limestone. You'll also see two stripes of pink, at opposite ends of the country - at the south-east tip in Leinster and the north west, covering Donegal and north Mayo. Pink is an appropriate colour, since it stands for a pink-tinged rock called gneiss. The longest, and oldest geological period, lasting from about four billion years ago to about 570 million years ago, is the pre Cambrian period. Gneiss is a metamorphic rock, which means that it was formed by a different rock being subjected to huge amounts of heat and pressure. The gneiss formed on the Mullet peninsula was probably formed from igneous and sedimentary rock, and you can see a small red patch on the Geological Survey that indicates a seam of granite (the Black Sod lighthouse is made of local granite).
If you played a tape back of the Earth say, about a billion years, you'd be about half way through when the solid blob of land that comprised all the landmass of the Earth began to separate and form continents (well, it you were playing the tape backwards, the continents would actually coalesce).* One particular patch of gneiss would drift off and bump into a bigger lump of rock called America. It's Celtic siblings, separated by what would become the North Atlantic, would help form the western isles of Scotland, part of Donegal and an island off the Mayo coast that has, much more recently, become a peninsula - the Mullet.
The gneiss of the north west is the oldest type of rock that you can find in Ireland. The rock in Annagh Head is 1,753 million years old, a shade younger than the gneiss at Inishtrahull, Donegal, that is 1,778 million year old.** I haven't been to the Donegal site yet, but it will be doing well to be as beautiful as Annagh Head.
* This is the theory of Continental Drift, proposed by Arthur Wegener
** Source: Mary Mulvihill - Ingenious Ireland