Rock Facts: The Mystery of Knockan Crag
In the late 19th century, a discussion arose in the geology community. It had been observed at Knockan Crag - a cliff in the Northwest Highlands, beside the road between Ullapool and Lochinver - that the rocks at the top seemed to be older than those below, with the Neoproterozoic Moine Schists lying above the Ordovician Durness Limestone. This turned out to be a topic of much controversy; while most people had long since moved on from the diluvial hypothesis, accepting that no, the rocks we see today were not all laid down in the single Biblical flood, the (fairly logical) idea that Younger Rocks Are Always Above gave rise to a couple of different schools of thought. Firstly: the Moine Schists were younger, they’d just been misidentified. Secondly: they actually were older than the rocks below and summin’ weird was afoot.
Enter British Geological Surveyors Ben Peach and John Horne. That’s them in the statue, though I’m not sure which is which. They carried out a long-term detailed mapping project of the area and, eventually, proved conclusively that the difference in age was due to tectonic activity: the older rocks of the Moine Schists had, over millions of years, been shoved up and over the younger rocks of the Cambrian/Ordovician succession in what’s now called the Moine Thrust, one of the first thrust faults to be identified.
Surveying techniques have come a long way since Peach and Horne and we now know that the Moine Thrust itself is just one part of a series of thrust faults running right through the Northwest Highlands from Loch Eriboll in the north to the eastern tip of the Isle of Skye. But in honour of that first identification, the whole complex is referred to as the Moine Thrust Belt.
I spent two weeks on a geological mapping trip based out of the Inchnadamph Field Centre at the end of my second year of university. I am extremely familiar with the rocks of the area.
