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By July 6, we were ready to make our way to the northernmost point of our journey, crossing the Northern Highlands on our way to the coast near Inverewe Gardens. Ironically, even though we were as far north as Moscow, the warm Atlantic Current keeps conditions in the coastal area moderate enough that semi-tropical vegetation can be grown. We crossed two major boundaries during the journey, the Great Glen Fault, which separates the Dalradian Supergroup rocks from the Moine Schists, which are Proterozoic metasedimentary rocks that formed along the coast of North America and Greenland. During the Caledonian Orogeny, caused by the collision of the continents, the rocks were contorted and baked into their present form. Later in the day, we crossed the Moine Thrust, where Moine rocks have been pushed up and over extremely old rocks that once formed the core of the North American/Greenland continent. The so-called Lewis Gneiss Complex may be as much as 3 BILLION years old, and may be the oldest rocks in all of Europe. They are overlain by sandstones and other sedimentary rocks of the Torridonian Group.
Our first stop of the day was the Corrieshalloch Gorge, a narrow cleft 200 feet deep that had been carved through Moine Schist. At the upper end of the gorge we could see the beautiful Measach Falls.
It was several minutes of crowding onto the viewing platform that we noticed that the sign said the safe capacity was two people....
The falls were quite beautiful. See a high resolution image here.
The suspension bridge sits over the top of the waterfall. And it wavers around a bit....
Despite all the beauty and the greenery at Corrieshalloch, they also had to carve into the slopes to build the highway. And that meant rock exposures, in this case of Moine Schist!
As they have in all of Scotland, Ice Age glaciers have had a profound effect on the landscape. Just beyond the gorge, the fjords of the west coast come into view. Loch Broom was one of the most striking...
For a higher resolution view, click here.
By this time we had crossed the Moine Thrust and were now driving over Torridonian Sandstones. We were about to encounter the Lewis Gneiss. At the very first roadcut, we begged Frank the bus driver to stop and let us collect pieces of the oldest rocks most of us had ever seen.
The Lewis Gneiss had never been eroded to a flat surface before being buried by Torridonian Sandstone. So the hills and mountains that we see today have existed for billions of years, even though they have been exhumed only fairly recently, by geological time standards.
For a nice high resolution pic of a typical Torridonian landscape, click here.
Our next stop was the Gardens of Inverewe. While the gardens are not a particularly geological location, they illustrated some important principes about environmental geology, i.e., the importance of soil as the basis of terrestrial life. As mentioned earlier, the climate is very mild due to warm Atlantic currents just offshore. When the garden's designer arrived in the middle 1800's, he described finding a single willow growing on the property, and lots of peat. He could grow tropical plants, but not without soil. So they hauled in the soils of the gardens from elsewhere! Tons of it. And the results are rather spectacular...
And some of the biggest leaves I have ever seen. If it weren't for the thorns, what a salad!
The last stop of the day was the legendary Loch Ness. The importance of the loch is NOT the supposed Plesiosaur that lurks in the murky water, but the location of the lake on top of the Great Glen Fault, a major boundary that has shifted a minimum of 40 miles horizontally. Before the famous loch, we passed beautiful Loch Maree...
See a high resolution image here.
We stood around for awhile at Loch Ness waiting to see the monster, but didn't ...wait...isn't the water rippling over there?
Continuing down the Great Glen, we saw some spectacular clouds pouring over the ridges near Loch Lochy...
and a very momentary view of Great Britain's highest point, Ben Nevis, just over 4,000 feet high. We stayed the night in Fort William.
Geological Notes:
Three major rock units dominate the scenery of the Northern Highlands: the Lewisian Gneiss, the Torridonian Sandstone, and the Moine Schist. The dominant structures are the Great Glen Fault and the Moine Thrust. The story is complex, so hang in there!
Leaving Inverness, we cross the trace of the Great Glen Fault, a major strike-slip (transcurrent) fault that separates the Dalradian Supergroup to the south from the Moine Schists to the north. The fault has moved at least 60 miles in a left-lateral sense, dividing the two very different terranes.
As we pass the vicinity of the Corrieshalloch Gorge and Inverewe Gardens, we cross the Moine Thrust, where Moine Schists have been pushed several tens of miles northwest over the Lewis Gneiss, Torridonian Sandstone, and overlying Cambrian-Ordovician Sediments. The fault’s location is often indicated by a dense dark rock called mylonite.
The story of the rocks encompasses a span of time as much as 2.9 billion years; nearly five times the time that complex life forms have existed on this planet!
The Lewis Gneiss is a complex terrane that originated as part of the Laurentian continent, which also includes Greenland, and North America. The original nature of the rocks was obscured by intense metamorphism between 2.9 and 2.7 billion years ago, and intruded by basaltic dikes (the Scourie Dikes) around 2.4 billion years ago. The deformations continued until about 1.4 billion years ago, and then the gneisses were deeply eroded, forming a landscape of low hills and stream valleys. It would have been an alien landscape to see, with no plants, no animals, only barren rock.
The Laurentian continent stabilized, and subsided, and thick sediments were deposited on the rough, uneven landscape of Lewisian Gneiss by rivers and streams. The sediments, deposited in two periods about 995 mya and 810 mya, are called the Torridonian Sandstone.
Continental shelf deposits of Cambrian to Ordovician age cover the Torridonian Sandstones. These sediments include sandstone, shale and limestone. Those who have visited the Grand Canyon in Arizona may notice a similarity in sequence and age between the Lewis-Torridonian-Cambrian rocks of Scotland and the Vishnu Schist-Grand Canyon Supergroup-Tonto Group of Arizona. These regions lay on opposite ends of the North American continent, but experienced many of the same events.
When the Torridonian Sandstones were laid down, sandstone, shales and mudstones were deposited in a shallow sea to the southwest. These were later metamorphosed to form the Moine Schists.