Moine Thrust

Another damp day in Assynt, north-west Scotland. Went up Cul Mor in the rain, just for the exercise.  Today's blip is from a roadside geological trail at Knockan Crag, just south of Elphin. The lighter rock below is limestone that  was deposited at the bottom of a shallow sea 480 million years ago, when Scotland was near the equator; the darker rock is schist, sand and mud that was laid down between 980 and 870 million years ago and subsequently deformed and metamorphosed into schist between 800-740 million years ago.  The curious thing is that the older schist overlies the younger limestone --- the opposite way around to what you expect from a geological sequence. The bottom of the gray rock, the oldest schist, is 980 million years old, while the top of the yellow rock, the youngest limestone, is 480 million years old: you're looking at a 500 million year jump into the past at the junction between the two types of rock --- Dr Who, eat your heart out!

What happened here is that the two layers were originally deposited and then deeply buried on different continents that were later rammed together 430 million years ago, due to plate tectonics and continental drift. As a result of the collision the older rocks were pushed over and above the younger rocks, taking about 15 million years to be thrust 70km to the west, above the younger limestone.  This all happened several kilometers underground, beneath a huge mountain range (a modern equivalent is the Himalaya, where India is ramming into Asia). But the overlying rocks have since been weathered and eroded away to reveal the two layers on the surface at Knockan Crag today.