Quaternary and Recent geology of the Isle of Skye
|From: Bell, B.R. and Harris, J.W. An excursion guide to the geology of the Isle of Skye : Geological Society of Glasgow, 1986. © 1986 B.R. Bell & J.W. Harris. All rights reserved.|
Chapter 11 Quaternary and Recent geology
Volcanic activity ceased on Skye approximately 52 m.y. ago. A protracted period of intense weathering and erosion then followed, prior to the more recent glacial activity of the Pleistocene Epoch. Examples of this weathering include: deeply-inweathered dolerite dykes; and, the preservation of deeply-weathered gabbro in Coire na Banachdich in the Western Cuillin Hills. It is likely that the large landslipped masses at The Storr and The Quirang started to develop during this period.
The Pleistocene glacial activity played a significant role in sculpting the various rock-types to their present appearance. Striation marks and roche moutonnees are readily identified and deposits of boulder clay and moraine are found throughout Skye, particularly in the central mountainous belt of the Cuillin and the Red Hills. Three raised beach levels can be identified. Other features include: screes and rockfalls; diatomite deposits; alluvium; peat; and, blown sand.
The account presented below is mainly taken from Harker (1904), Peach et al. (1910) and Anderson and Dunham (1966).
The following table indicates the relative ages of the deposits:
|Screes and rockfalls|
|Raised beaches||Period of most active landslipping|
|Upper Tertiary weathering|
(B) Glaciation and glacial deposits
Two phases of Pleistocene glacial activity have been identified: (a) The main ice-sheet(s) which covered a significant portion of the island (1, 2 and 3, below); and, (b) Later valley glaciers (4, below). Separating these two periods of glacial activity was a period of milder climatic conditions.
The main glacial events were:
- An early ice-cap in the Cuillin and Red Hills district which fed valley glaciers. This accumulation of ice completely enveloped even the highest summits of the district. The general ice-flow was radiate and formed features such as Coire na Banachdich, Coire Lagan and Coir'-uisg (the rock basin of Loch Coruisk). Modifications of this simple pattern include the crossing of ridges by individual valley glaciers. This occurred, for example, with the ice in Lota Corrie and the upper part of Harta Corrie, which moved obliquely over the Druim na Ramh ridge, instead of passing into Glen Sligachan.
- Early valley glaciers in the Trotternish area of north Skye.
- The Loch Alsh–Glenelg–Loch Hourn group of ice-sheets, which migrated west from the mainland. Members of this group locally diverted the Cuillin valley glaciers (1, above). In addition, summits such as Beinn na Caillich, Ben Aslak and Beinn na Seamraig caused this mainland group to deviate slightly in direction. The more southerly glaciers (Loch Hourn) moved south of the Cuillin Hills over the district of Strath, whilst further north the Loch Alsh glaciers flowed northwards over Raasay and the eastern side of north Skye. This northerly flow of ice, to a large extent, was limited to the area east of the Storr-Quirang ridge, which acted as a 700m-high (or more) ice barrier. It is probable that small piedmont glaciers breached this barrier in places and flowed westwards over the northern part of the island. Further north, towards Kilmaluag, the ice was able to flow, without hindrance, westwards.
- A late-glacial readvance (10,800–10,300 years b.p.), which established a small ice-cap on the Cuillin and the Red Hills, again with a radial suite of glaciers. Important directions of ice-flow include: southwards into Camasunary Bay from both sides of Blaven; NW into Glen Sligachan from Mam a' Phobuill and Coire na Sgairde (north of Marsco); NE into Loch Ainort from Coire nam Bruadaran and Coire na Seilg; and, radially outwards from the Beinn na Caillich–Beinn Dearg Mhor–Beinn Dearg Bheag group of summits in the district of Strath. A small group of glaciers probably redeveloped in the Trotternish area.
The commonest glacial deposit on Skye is a thin, discontinuous covering of boulder clay. Such deposits are typically restricted to ground below 500m O.D. Different generations of boulder clay probably exist, reflecting the different stages in glacial activity, but they have not, as yet, been differentiated. Other glacial products include "hummocky drift" or "kettle moraine", consisting of circular or oval mounds of often grass-covered ground, up to 25m in height, which formed during periods of ice-flow stagnation. Examples can be found in the area around Strollamus and Luib, in Harta Corrie and Glen Sligachan, and in Glen Varragill, Glen Drynoch and Glenmore in the plateau lava field of north Skye. Lateral moraines have been identified in valleys east of the Trotternish ridge, in the upper reaches of the Lealt River, and terminal moraines have been recorded from the upper reaches of the Lon Druiseach, north of Portree.
(C) Raised beaches
Since the end of the volcanic activity the coast of Skye has been dominated by cliffs, or at least steep slopes. As a result of isostatic recovery and subsequent erosion, at least three raised beach levels have developed: (1) "100-Foot" (marked by platforms between 25 and 30m above sea level); (2) "50-Foot" (marked by features 10–15m above sea level); and, (3) "25-Foot" (marked by features 3–6m above sea level). Where storm beaches exist, there is a gradation of deposits from the "25-Foot" level towards those of the present-day beaches. Examples of each of the three categories defined above are: (1) "100-Foot", between Kyleakin and Broadford, and along the coast of the Sleat Peninsula, and at Lub Score at the northern end of the Trotternish Peninsula; (2) "50-Foot", at Tokavaig on the Sleat Peninsula, and at Eilean Heast in south Strath; and, (3) "25-Foot", along much of the coastline of Skye, especially in the southern part of the island, for example, Camasunary Bay, Camas Malag (on the east side of Loch Slapin), and at the heads of Loch Slapin and Loch Ainort.
Large masses of rock which have landslipped downwards and outwards along rotational glide planes are most spectacularly developed on the Trotternish Peninsula, north of Portree. Movement has taken place by shearing within horizons which behaved in a plastic or semi-plastic manner. Details of the mechanics of these landslips are presented by Anderson and Dunham (1966).
On the Trotternish Peninsula, plateau lavas cap Jurassic strata intruded by thick dolerite sills (see Chapter 1). All of these units dip to the west at a shallow angle and are cut by numerous N-S -trending faults. Because of these faults, large cliffs developed along the eastern margin of the lava field, with heights in excess of 600m. Such an arrangement of faulted rock proved to be unstable and led to shearing within the Jurassic strata and slipping of large masses of lava and sedimentary rock towards the east. Worthy of brief consideration are the landslips at The Storr, The Quirang and Ben Tianavaig.
Between The Storr and the coast, 2km to the east, are mature, stable landslips which are entirely post-glacial. Prior to the landslip events the main escarpment was located 600m east of the present cliff face of The Storr.
The Quirang Landslip lies to the west of Staffin Bay and is essentially a continuation of the landslipped escarpment of The Storr. Movement, however, has been much greater, extending to the coast—a distance in excess of 2000m. At Flodigarry, material is constantly being removed by the sea and, therefore, movement still takes place, although on a very small scale. Essentially, the Quirang Landslip is stable.
The landslip east of Ben Tianavaig, on the south side of Loch Portree, is immature and unstable. It is best observed from the north side of the loch or the west side of the island of Raasay. The toe of the landslip is being removed by the sea and it is suggested that movement will not cease until the bulk of the summit has been removed (Anderson and Dunham 1966).
(E) Screes and rockfalls
Screes and rockfalls have developed in many parts of the island. Screes are common along the bases of scarp lines in the plateau lavas of north Skye and Strathaird. Large quantites of scree are also found in the steep-sided corries of the Cuillin and the Red Hills. Within the Cuillin Hills such screes are commonly referred to as "stone chutes" and are composed of angular fragments derived from the rock-units which crop out locally. The most famous of the stone chutes are: the Great Stone Chute; and, the Sgurr Dearg Stone Chute, both in the upper part of Coire Lagan.
Major rockfalls, as opposed to landslips (see Section (11D), above), are not a common feature on Skye. They have developed principally where massive columns, from sills or lava flows, have toppled from precipitous heights. Prominent examples include: 1km south of Rubha Garbhaig, Staffin; at Lub Score, south of Duntulm; and, at Cam Mor, north of Elgol on the west side of the Strathaird Peninsula.
(F) Diatomite deposits
Diatomite deposits occur throughout north Skye (Wilson and Macadam 1886; Wilson 1888; Anderson and Dunham 1966). They are younger than the glacial moraines, but older than the peat deposits (see table in Section (11A), above). They developed during a period of warmer conditions following the Quaternary glacial events. Cessation of diatom growth occurred when cooler and wetter conditions favoured peat accumulation.
In detail, these fresh-water lake deposits consist of the remains of minute, siliceous, unicellular, aquatic plants. Species and genera are documented by Wilson and Macadam (1886), with almost half the known Quaternary genera recorded.
The following occurrences are listed by Anderson and Dunham (1966):
(1) Sartle. On the north side of the Staffin–Uig road (via The Quirang), 2km west of Staffin. (2) Loch Vallerain. NW of Digg on the north side of Staffin Bay. (3) Glashvin. 800m north of the Quirang turn-off at Staffin, on the main coastal (A855) road (immediately to the west of the road in a small depression). (4) Loch Cleat. Immediately east of Duntulm. (5) Loch Sneosdal. 3km ESE of Kilmuir on the west side of the Trotternish Peninsula. (6) Loch Chaluim Chille. 2km south of Kilmuir on the west side of the Trotternish Peninsula. An old lake basin, now drained, with an area in excess of 2km2. Originally, in the centre of the loch was a small island (Eilean Chaluim Chille), upon which a monastery stood (now in ruins). North of the 'island' the diatomite varies in thickness between 30cm and 1.5m. It is interlayered with mud and silt and overlain by peat. (7) Loch Mealt. West of the loch, south of the settlement of Elishader. (8) Loch Beinnichte. On the east side of the Waternish Peninsula. Includes a small, dry lochan at Score Horan. (9) Loch Cuithir. 5km west of Lealt, on the east side of the Trotternish Peninsula.
These deposits were worked during the first two decades of the this century: first by the British Diatomite Company, and subsequently by the Skye Mineral Syndicate Company. The main uses of the extracted material were: an absorptive for nitro-glycerine; a filler; an insulator; a component in polishes; and, as a filter in paint manufacture (Anderson and Dunham 1966).
Freshwater alluvium occurs in narrow tracts of ground marked out by stream courses and in small lochans. Examples include: in Srath na Creitheach (peat-covered in part); around Loch na Sguabaidh, at the head of Loch Slapin; and, around Loch Cill Chriosd, in Strath.
In places, thick accumulations of peat overlie glacial deposits, diatomite, or bed-rock. Thicknesses of up to 3m have been recorded, for example, from the Loch Cuithir and Loch Mealt areas (Anderson and Dunham 1966). Peat formation at present appears to be minimal. Exploitation of the peat as a fuel has been limited to the local population and in recent years has gradually declined.
(I) Blown Sand
Deposits of blown sand are found at An Corran, on the east side of Staffin Bay. These accumulations are the result of south-easterly winds which have caused the sand to bank up against the dolerite of the cliff. Pebbles, which resemble the classic dreikanter shape, have been recorded from this locality (Anderson and Dunham 1966).
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