Geology of Glen Mor and the area around Loch Ness.

A brief and simplified paper on the Geology of Glen Mor and the area around Loch Ness.

Geology of Glen Mor and the area around Loch Ness.
This small area, however, is part of a much larger landform. Most of the present topography of our area is due to the effects of the recent (geologically speaking) ice age, which took place during the Pleistocene era, and began roughly around 2 million years ago continuing intermittently until the very end of what is known popularly as the Great Ice Age, around 10,000 years ago. The effects of this glaciation are seen nearly everywhere in our landscape. Glaciers, in flowing downhill under the influence of gravity are able to attack the underlying rocks in two distinct ways. First the ice has a “plucking” action, detaching blocks of rock along joints and other fractures, and then incorporating these blocks into the glacier as another addition to its load of rock fragments and other detritus. Secondly the ice has a “scouring” effect as it drags its load of rock fragments over the solid rocks forming its floor, grinding down this surface by abrasion. At a later period as the ice wanes, many of these enormous rocks are dropped and these erratics can be seen in fields around Stratherrick.

The U shaped valleys, of which obviously Loch Ness is a clear example, are a characteristic feature of the the higher ground in the Scottish Highlands. These valleys are often eroded along their lengths into a series of rock basins, now occupied by lochs, also scoured by glaciers, Lochs Garth and Farraline (now combined to form Loch Mhor) are good examples, of glaciation scouring. Interesting features along the length of Stratherrick are the deposits of boulder clay and glaciation till, left widely over the lower ground, they typically consist of scattered boulders of all shapes and sizes, lying in a gritty clay. This mostly accumulated below the ice sheet, plastering the underlying surface with detritus. Boulder clay forms a featureless landscape with slight hills and shallow valleys.

South Loch Ness
Where the boulder clay was thrown into smooth but elongate mounds, up to 100 feet in some cases, this gives rise to a landscape thought to resemble a “basket of eggs” . Drumlins are sloped facing the direction of the ice flow and steep on the far side where the glacier has plucked and pulled at the boulder clay. As the glaciers approached the sea around the Moray Firth, material was dumped into Moraines forming distinct ridges of sand and gravel. On many of the rocks quite high up along the sides of Loch Ness there appear slight grooves and scratches made by rock fragments as they were dragged along by the ice. At the view point above Inverfarigaig can be seem conglomerate rocks with possible ice scratching, which demonstrates the height of the glaciers.

Another major glaciation feature in our area are the Eskers, of which there are some fine examples. Not as well developed as the Flemington esker complex around Croy those which flank the River Ness are nevertheless excellent illustrations of the principle of sub-glacial deposition. Where the modern river has truncated these narrow sinuous ridges it is possible to see that they are composed of bedded layers of unconsolidated sand and gravel, orientated in the general direction of the former ice movement. Eskers were formed by glacial meltwaters in tunnels beneath the ice. As the ice sheet waned the tunnel infills became exposed from beneath the down wasting ice, finally to be left as steep-sided ridges running across the drift plains. Looking across to the north side of the river when approaching Inverness one can see a fine example of an esker.

A remarkable feature in this locality but not actually in Stratherrick is the phenomena known as the Parallel Roads of Glen Roy. These are strand lines of an ice dammed lake which stood at a maximum height of about 1,150 feet. As the ice front retreated the lake was progressively lowered first to approximately 1,075 feet and then to some 850 feet, as overflow cols were revealed from beneath the ice. Apart from these strand lines have been found elsewhere, and in fact Mr Hugh MacNally has identified one in our area, of which a photograph is in our possession.

No geological record would be complete without commenting on Loch Ness. This Loch at 754 feet maximum depth is Scotland’s second deepest loch, but since it remains more than 600 feet in depth for the greater part of its 23 miles, it holds the largest volume of water. Its capacity of 263,000 million cubic feet is almost three times that of Loch Lomond and more than three times that of the deepest loch, Loch Morar. Nothing in fact illustrates the efficiency of glacial scouring along a fault shatter belt better than this set of figures. The valley walls rise abruptly throughout to some 1500 feet; and it is clear that Loch Ness occupies one of the greatest glacial troughs in the British Isles. Its shores are so steep that aquatic vegetation is virtually non-existent; the absence of islands in the loch shows the power of the scouring. The intense glacial action was not confined to the trough itself, for glacial roughening is manifest on both shoulders of the Great Glen forming a plethora of lochs and lochans on the plateaux west and east of Loch Ness. Two of these rock basin lakes in Stratherrick as already mentioned Garth and Farriline now forming Loch Mhor, were linked to create the first hydro-electric scheme in Britain. The power station at Foyers, was initially required for the smelting of bauxite, but the aluminium factory here was closed down in 1967 to be replaced by a pumped water storage scheme.

The lack of islands in the lochs and the steepness of the flanking slopes of the Great Glen highlight the efficiency of the ice in clearing out not only the fault breccia but also much of the less resistant faulted outlier of Old Red Sandstone which occupies part of the floor of the G len. Ice also scoured out most of the regolith, so that the valley flanks of these glens are both too steep and too rocky for agricultural improvement. . Instead they have been given over almost entirely to forestry.

An important geological feature of our area is the Great Glen Fault. This is the best example of a wrench or tear fault in the British Isles. There is seismic activity along the fault line and minor earth tremors are still recorded.

The movement has forced the Northern Highlands block bodily southwestwards in relation to the remainder of Scotland by about 65 miles. It has been demonstrated that two of the Great Glen granites, those of Foyers and Strontium were once part of the same unit, although there have been later slight movements in the opposite direction.

The scenic difference between our own area and the east coast, apart from the hills, is due to the underlying rock and more fertile soils formed from the Old Red Sandstone deposits some 400 m. years ago. The ice sheets also dumped the products of the eroded soils from the high lands to the area around the Moray and Beauly Firths.

Stratherrick with its hard rock landforms was shaped as a result of the glaciation advances and subsequent retreats.

The area incorporates a Granite intrusion, and the Moine metamorphic rocks comprising schists and granulites, formed under intense pressure and heat towards the end of the Silurian era around 400 m. years ago. Briefly Briefly back to the massive effect that the glaciation had on this area and to give some idea of what happened here, the various centres of ice accumulation eventually coalesced to produce an ice sheet which completely buried all the Highlands and Islands. It is quite likely that even the highest peaks were covered, since it is possible that the top parts of the ice surface attained a maximum altitude of between 4 and 5,000 feet.

Geologically Stratherrick and the South Loch Ness area are part of the main section of the Highlands, with the mountains formed in a period of uplift and folding known as the Caledonian orogony some 400 m. years ago. Our present hills are really the worn out stumps of this ancient mountain chain which in its time was much higher than the present Himalayas. During this period of intense movement the granites were also laid down. During the Old Red Sandstone era the sandstone around the Moray Firth was deposited somewhere around 350-400 m . years ago.

The Moine Metamorphic Rocks which cover a major part of the Grampian and the Highlands comprising mainly schists were pre- 600 m years, apart from the very ancient foreland along the extreme North West Coast, at the limit of the Moine Thrust. These are the most ancient exposed rocks probably in the world. They are mainly gneisses dating possibly 2900/1500m. B.P. (before present). The area stretches from Durness in the north to around Torridon and Loch Carron in the south, and its geological history alone would fill a book.

Copyright © 2007 SLNHG all rights reserved

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