1.2 billion years ago, the far north of Sutherland was struck by a meteorite. The wreckage of this cataclysmic event has been almost completely worn away by time, but the rocks in the landscape still bear some traces, which Paul unpicks as he embarks on a grand tour from Lairg on Loch Shin to Lochinver and, finally, to the summit of Suilven - the sugarloaf mountain.