The highest peaks in each of the five countries of the British Isles are all within around 15 kilometres of the coast. Is there any geological reason why they are all so close to the sea?
Hillary Shaw
Newport, Shropshire, UK
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Two factors may apply here: geometry and geology.
Geometrically, a significant fraction of Britain is within 15 kilometres of the sea. England, its biggest country, approximates to a 500-kilometre x 200-kilometre rectangle (minus Devon and Cornwall). This means that 485km x 170km of it (allowing for the landlocked north border), or around 17.5 per cent of the land, is within 15km of the sea. Similar calculations show that around 23.5 per cent of Ireland is less than 15km from the coast.
Indented coastlines, common in the British Isles, increase this percentage, so a random location in England has a better than 18 per cent chance of being within 15km of seawater, and one in Ireland has a better than 24 per cent chance.
Then we have geology. Peaks that are higher than everything in the landscape have probably been geologically uplifted recently. Uplifts are often associated with a lowered 鈥渟yncline鈥� region close by, where Earth鈥檚 crust has been forced downwards. In an island country, that syncline may well be under the sea.
David Jarman
Ross-shire, UK
Although four of the five tallest mountains of the British Isles are formed primarily of igneous rock, it isn鈥檛 the rock type but tectonics that makes them all close to the coast. The coastline has fluctuated wildly over time and these mountains were once located much further from the sea.
Tectonic events created the bodies of water that are now close to them. The fact that the highest mountains of Scotland, Wales and England are all near the west coast is probably due to a continental collision that took place around 300 million years ago called the Variscan Orogeny. This directly affected only southern areas of the landmass of Britain-Ireland, giving us the MacGillycuddy鈥檚 Reeks mountain range in Ireland and the Brecon Beacons in Wales.
However, the aftermath of that collision provoked a rift zigzagging up through the landmass of Britain-Ireland, opening basins such as the Irish Sea, plus the Firth of Clyde and the Sea of the Hebrides on the west coast of Scotland. This meant that the spine of mountains and hills running down the island of Britain, which had been fairly central when Britain-Ireland was created, now became west-centred.
Thus, Ben Nevis, Scotland鈥檚 tallest mountain, is now a stone鈥檚 throw from seawater in Loch Linnhe. England鈥檚 highest peak, Scafell Pike, and Snowdon in Wales are both close to the Irish Sea.
Also, bear in mind that around 60 million years ago, a chain of volcanoes that we now know as the islands of Skye, Mull and Arran, off the west of Scotland, would briefly have dwarfed today鈥檚 highest peaks.
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