Classic localities explained 25

Abstract

Britain's geology is perhaps more diverse than any equivalent area in the world, spans almost 3 billion years, and has been studied for more than two centuries yet, for too long, it seemed that we could find no evidence here for one of the most spectacular events on the Earth—a giant meteorite impact. Perhaps, the only evidence might be localized and easily overlooked, like the thin layer of millimetre-scale microtektites, once molten beads of rock blasted out by an impact, found near Bristol in 2001. Alas, these proved actually to have originated more than a thousand kilometres from Britain, in the 100 km Manicouagan Crater in eastern Canada. However, just a few years later, a spectacular discovery revealed that a world-class impact deposit, metres thick and extending for tens of kilometres, had been hiding in plain sight at a location visited by countless geology students and their teachers. For decades, the Assynt region in northwest Scotland has been a training ground for geologists, drawn by the immensely old Lewisian Gneiss, the spectacular hills of Torridon sandstone that overlie it, and the structural complexity of the Moine Thrust Zone. How could this remarkable impact deposit have gone unnoticed for so long?