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The following article was published in the Arran Banner on 14 October 2005.

Arran's Unique Rocks

You may have wondered about, perhaps even cursed, the legions of geology students who descend on Arran. Why here? Well, because Arran is unique in the range of rocks contained in a small area, allowing the exploration of lots of different rocks and features. This diversity is due to Arran's position at the crossing of two of Scotland's important geological zones, where the Highland Boundary Fault and the Tertiary Volcanic Zone meet. The island has rocks from the Highlands and the Lowlands, and also rocks formed by the volcanoes which marked the birth of the north Atlantic Ocean around 60 million years ago.

Arran's story started much further back in time, around 600 million years ago, which is pretty ancient stuff. Back then, the geography of our region was completely unrecognisable, and the rocks now found in the north of Arran were forming on the floor of a long-lost sea. These Highland schists are metamorphic rocks, and Arran's schists are just a fragment of the rocks that were created and then deformed on the northern shores of a once-great ocean lying between Scotland and England. In these rocks we see the result of the dance of the continents, a very slow dance at that, as over hundreds of millions of years bits of continental crust on the surface of the earth have pulled apart, then come together to pull apart again, carrying a cargo of slowly evolving rocks and life. Around 400 million years ago the rocks of this area were caught up in continental collision, forming the Caledonian mountain range and turning sedimentary rocks into schists.

This set the stage for the formation of the next group of rocks, the sandstones, lavas and limestone that form the bulk of Arran. The Highland Boundary Fault was active then, allowing a great wedge of land to sink downwards and giving ideal conditions for the accumulation and preservation of sedimentary rocks, such as sandstone and limestone. The junction between the older metamorphic rocks and the overlying sandstones at North Newton was discovered by James Hutton in 1787, and used as evidence to support his theory of an ancient earth, giving Arran a place in the development of one of science's most important ideas.

The sedimentary rocks record the changing climate as this area drifted slowly north over the equator, from desert to tropical seas to desert again. The rocks of south Arran are amongst the youngest sandstones in Scotland, formed around 250 million years ago. There were volcanoes too, although only a small area of lava is now preserved to the north of Corrie.

The final episode in the formation of Arran's rocks began a mere 60 million years ago, with volcanic eruptions on a vast scale creating the granites of Arran, Ailsa Craig and many other west coast islands. The continent formed by long-ago collisions finally wrenched apart, creating the north Atlantic and separating Scotland from North America and Greenland. The stretching and thinning of the crust gave us volcanoes, and also the walls of basalt rock (dykes) seen along the south coast of Arran.

When you consider how quiet and peaceful this area is now, far removed from the world's active volcanoes and earthquakes, it is pretty mind-blowing to imagine Arran's vigorous past. However, its position at the cross-roads of two major zones of past activity has left a rich legacy of rocks and scenery. To be enjoyed for now, always bearing in mind that on a geologic timescale change is inevitable, and Arran's future will be just as exciting as its past!

 


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