Age of the Torridan Mountains?

Question

Would like to know how old the Torridon Mountains are.

Answer 1

Dating sediments is particularly difficult - especially when they are essentially unfossiliferous. We know that the Stoer is younger than the basement that they overlie - which makes it younger than about 1400 million years old. Thin limestones within the Stoer Group have yielded radiometric ages of about 1200 Ma (based on Pb-Pb geochronology) - thought to give the time of diagenesis (alteration soon after deposition).

The Torridon Group sediments are younger than the Stoer Group and older than the oldest Cambrian strata, placing them in the interval of 1200 to about 520 million years old. This rather wide age range can be narrowed if we consider the ages of some of the detritus within the Torridon Group sediments. Some robust minerals called zircons have yielded an age of about 1050 million years old. At this time the particular zircon must have been deeply buried within some basement unit - so the sedimentary rocks that now contain the zircon must be significantly younger. The best guess is that the Torridon Group was deposited about 1000 million years ago.

In summary, the Torridon Rocks are actually two distinct sequences separated by an unconformity representative of a significant time gap - perhaps as much as 200 million years. Support for this notion comes from palaeomagnetic data. This method can determine the palaeolatitude of the sediments at the time of deposition. For the Stoer Group the palaeolatitude is estimated as having been about 15 degrees from the equator. The Torridon Group on the other hand records palaeolatitudes of about 42 degrees from the equator. A significant time period is needed to account for the plate motion to take what was eventually to become NW Scotland through this distance.

Answer 2

The geology of north-west Sutherland is world-renowned and recognised as a special 'Geopark'. The three thousand million year old metamorphic Lewisian gneiss rocks, which outcrop across the landscape, are amongst the oldest in the world and were once part of an ancient mountain range as high as the Himalayas. You are unlikely to see or touch anything older in your life! Lewisan gneiss is multicoloured with stripes, swirls and bubbles.
The area's other major rocks are Torridon sandstone, Cambrian quartzite and Durness limestone. Torridon sandstone is a sandy, layered, sedimentary rock sometimes looking like concrete. The distinctive mountains of Cul Mor, Suilven, Canisp and Quinag are huge masses of Torridon sandstone capped with quartzite. Arkle is largely composed of Cambrian quartzite. Limestone forms the steep slopes of Knockan as well as the limestone pavements, underground water systems and caves at Inchnadamph. These rocks date from a time when this part of Sutherland was attached to North America and the rest of Britain was yet to join it. The world famous and much studied Moine Thrust Fault is visible here. More 'recently', the effects of the Ice Age have left a spectacular scraped landscape of deeply-gouged glens, myriad lochans, dramatic coastal fjords (such as Loch Inchard) and bold mountains (such as Foinaven and Arkle).

Between Laxford and Rhiconich, the building of the new road has exposed folded patterns in the Lewisian gneiss rocks. The original horizontal layers have become twisted, broken and folded through the intense pressure and movements of the earth's crust.

Knockan Crag, situated just south of Elphin near the border of Sutherland with Ross-shire, is one of the most important sites in the Scotland for understanding how the landscape of northern Britain was formed. The crag shows an unusual situation where older Moine rocks have come to lie over much younger Durness limestone. Situated there is an exhibition about the landscape and geology of the area, and two circular trails for walking. It was at Knockan that the prominent Scottish geologists Ben Peach and John Horne first identified fault lines, and a memorial beside Loch Assynt recognises their pioneering work.

Answer 3

Really 2 questions here - the age of the Torridonian rocks and the age of the mountains. The rocks are very old, but the mountains in their present form are the result of ice-age glaciation, possibly as recent as 20,000 years ago