NW Highlands - Deep Time

In a human lifetime the North Atlantic will have widened by about 2 metres, the ongoing collision of the Indian plate with the the Eurasian plate will have increased the height of Nanga Parbat by 60 - 80 cm and a similar rise in land surface will occur near Hudson Bay, as the Earth's crust is still rebounding from the melting of the North American Ice Sheet. With a few interesting exceptions, geological processes in terms of human perception are generally slow paced affairs and a fundamental concept in geology is geological time or Deep Time. The first scientific attempts at dating the earth, involved cooling rates, seawater salinity and sedimentation rates, that resulted in calculated ages ranging from 24 to 400 million years. Whereas there is some familiarity with figures of millions and billions expressed in monetary and financial terms, many people can picture a 100 pounds/euros/dollars be it coinage or notes, few people will ever become centenarians and experience 100 years of time. To most people a million years is not an intuitive length of time. Even when reduced to seconds, a million seconds equates to just under 12 days. The Earth's age as quoted in textbooks towards the end of the 19th C was 100 million years, which seems a great length of time, but some geologists and naturalists thought it too short.

View of the Billion year old Torridonian / Lewisian nonconformity on the NE spur of A' Mhaighdean

In 1891 the discovery in the NW Highlands of Olenellus fossils in the Fucoid beds of the Saltera Grits formation, finally determined their position in the stratigraphic column as lower Cambrian in age. The fossil discovery again changed the geological history of the rocks in the NW Highlands, as the underlying Torridonian sedimentary rocks were relegated from their presumed Cambrian age and down into the Precambrian. The angular unconformity between the newly assigned Cambrian rocks with the underlying Torridonian, showed compelling evidence that the Torridonian were some way down into the Precambrian too. The Cambrian sedimentary rocks were deposited on a planation surface that had eroded away the Torridonian rock and also an unknown thickness of the underlying rocks of the Lewisian complex. Which implies there had been a long period of erosion and before that a long period of time to deposit the thick pile of Torridonian sediments. The Torridonian sediments are deposited on rocks of the Lewisian Complex and on a palaeo land surface with over 600m of relief between hill summits, valleys, cliffs and a surface covered with weathering debris. This presented another long period of time. Then there were the rocks of the Lewisian Complex, whose very nature implied a long and rich geological history.

The Torridonian inselberg Suilven rising above the Lewisian.

Amongst the many achievements of the Geological Surveys work in the North West Highlands in the closing decades of the 19th C, a notable one was the elucidation that the rocks outcropping in the NW Highlands were a window into the depths of Deep Time and a point emphasised on more than one occasion in the Geology Memoir 'The Geological Structure of the North West Highlands of Scotland" published 1907.

Coincidentally in 1907 the chemist Bertram Boltwood in a pioneering example of geophysics used the then estimate of the radioactive decay rate of uranium into lead, to measure the ratio of lead and uranium from some rocks and determined ages of 400 million - 2,200 million years. It is reasonable to suggest the ages were the subject of some controversy with some prominent geologists and Boltwood's interest in geological dating then waned. The challenge of dating rocks through radioactive decay was taken up by Arthur Holmes, it is difficult to challenge the hard science of physics and the geological community then set about ordering the rocks into the timeline of Earth's history. Dating of rocks has become ever more sophisticated and innovative, currently the Torridonian rocks have been dated to between 1.2 and 0.95 billion years old. It is worth stating that a billion seconds equates to a few months short of 32 years, a billion years is a vast stretch of time.

The Lewisian complex comprises rocks with an age spread from 1.6 to 3.0 Billion years, confirming their place in the depths of Deep Time.