Talaidh at 761m and it was relegated to Graham status. However, the latest OS 1:50,000 map shows Beinn Talaidh as a confusing 761(763)m. The OS seem to be doing this when the trig point is not the highest point on a summit. John Barnard and Graham Jackson in 2009, using sophisticated GPS equipment, measured the highest point as 761.7 ±0.1m and this height has now been accepted, missing the requirement for a Corbett by about 30cm (1ft). Barnard and Jackson believe the 763m may be a mistake by the OS measuring to the top of the large ‘prehistoric tumulus’ which is nearby. Incidentally, the author’s measurements gave a reading of 766m which is the same reading as he got for nearby Dun da Ghaoithe which is listed at 766m!
The OS only claim an accuracy of ±3.3m for spot heights on their maps derived from aerial photography.
Geology
An Teallach from Sail Mhor, Fisherfield Forest (Route 74)
Much of the early pioneering work in geological theory was based on investigations on the rock formations of NW Scotland. This is recognised in the UNESCO-endorsed award of Geopark status to the North-West Highlands. Geopark designation is intended to encourage geotourism, and a number of excellent visitor centres as well as roadside information boards have been developed to encourage this. This Geopark area is undoubtedly the most scenically attractive area in the UK.
The North-West Highlands contain rock formations which span over 3000 million years of earth history and include some of the oldest rocks in the world. Along the west coast, the oldest rock in the region, Lewisian gneiss, creates a landscape of low hills and scattered lochans. Rising from this gneiss landscape are huge masses of Torridonian sandstone, capped by quartzite, which form the distinctive mountains Cul Mor, Suilven, Canisp and Quinag within the Geopark as well as the mountains of Fisherfield, Torridon and Applecross further south-west.
Lewisian gneiss 2900–1100 million years old
Lewisian gneiss: Most of the Outer Hebrides and the North West Highlands Geopark have a bedrock formed from Lewisian gneiss. These are among the oldest rocks in the world, having been formed up to 3 billion years ago. About 2900–2700 million years ago north-west Scotland, together with parts of Greenland and North America, made up the ancient continent of Laurentia, which was being built up as igneous rock deep in the earth’s crust and then metamorphosed at very high temperatures. These rocks, with irregular light and dark layers, were intruded by later basaltic dykes and granite magma.
Torridonian sandstone 1000–750 million years old
Torridonian sandstone: Many of the most spectacular mountains in north-west Scotland are composed of Torridonian sandstone, a coarse-grained purplish-red sandstone. Sediments were laid down upon the gneiss by broad, shallow rivers, where the water flowed in many small channels separated by sand-bars. These sand grains and pebbles would have come from an eroding mountain range whose roots are now on the other side of the Atlantic. Deposition over 200 million years was followed by uplift and tilting downwards to the west. By 540 million years ago, erosion had formed a near-horizontal surface.
Moine schists: At the same time as the Torridonian sandstone was being laid down in the west, silt was being laid down in the east. The silt was pushed down into the earth’s crust and passed through a series of metamorphic processes, with the individual mineral grains in the schist being drawn out into flaky scales by heat and pressure, with the result that the schist splits easily into flakes or slabs. The gentler mountains in the north-east of Scotland are mainly composed of these Moine schists.
Basal quartzite 570–550 million years old
Basal quartzite, pipe rock, fucoid beds, Salterella grit and Durness limestone: Eventually, after a long period of erosion, the remains of the Torridonian sandstone in this part of Laurentia sank beneath a shallow sea. A thin sequence of sandstones, siltstones and limestones were formed in the coastal sand-bars, tidal flats and in the sea. Clean quartz sand was deposited on top of the Torridonian sandstone and a pure sandstone composed of rounded grains of quartz, cemented by further quartz between the grains was formed. This sandstone had completely different properties and is the rock we now know as basal quartzite, a white rock which is resistant to weathering and erosion. Life was developing in these waters and there is a layer of quartzite called ‘pipe rock’ where there are pipe-like burrows of fossil worms. As animals with shells developed, their fossilised remains resulted in the sediments becoming richer in ‘lime’ and in the formation of the Durness limestone.
The remarkable thing about these rocks is that the fossils are identical to those found in the Appalachian Mountains in the US but distinct from those of the same age in the rest of Britain. It is now clear that 500 million years ago Scotland, Scandinavia, Greenland and north-east America were one continent and that they were on the opposite sides of a now vanished Lapetus Ocean to the rest of Europe. This was the first conclusive evidence for continental drift.
About 430 million years ago there were intrusions of magma creating sills of crystalline rock parallel to the bedding plane, dykes cutting the bedding plane as well as plutons (irregular masses).
Between 430 and 420 million years ago a vast mountain range was being built up to the south-east as the Lapetus Ocean had closed and the ‘European plate’ was moving north-west and colliding with Laurentia. The most dramatic effect on Scotland was the pushing of a large slab of the old Moine schists over the younger rocks of north-west Scotland. This is known as the Moine Thrust.
Eventually, Scotland collided with England and fused to form one land mass before quiet conditions returned. There is no evidence in the rock record of any activity in north-west Scotland but this was a period of the build-up of sediment forming sandstone, coal, limestone, chalk and other sedimentary rocks in the remainder of Britain.
Around 60 million years ago the continent split apart, with Europe and Africa separating from America, as the Atlantic Ocean began to develop and there was volcanic activity along Scotland’s western edge.
Glacial moraine hummocks in Choire a’ Cheud-Chnoic, Torridon (Route 59)
During this time the climate cooled and by 2 million years ago the Ice Age began to affect the whole planet. Scotland would have been buried under ice and would have looked like Greenland today. It was during this time that today’s landscape developed, with massive glaciers carving out the U-shaped valleys which are such a feature of the Highlands today. Glaciers would also have carved out the corries and transported boulders long distances to scatter them over the landscape.
As in Greenland, the highest peaks would have stuck out of the ice and so wouldn’t have been smoothed by the ice, but left jagged and angular. The white cap of resistant quartzite on many of the Torridonian sandstone peaks would have helped protect them from erosion, leaving the spectacular peaks we see today. As the glaciers and icefields melted there would have been enormous flows of meltwater flowing beneath and out of the glaciers, cutting out gullies and gorges.
It wasn’t until about 11,000 years ago that the last of the glaciers finally left Scotland and peat bogs began to build up in the warmer wet conditions. Peat forms when plant material is inhibited from decaying fully by acidic and anaerobic
