23 - 30 August 2006.
SKYE AND RAASAY
Tour:   Geosupplies.           Participants: 6
Weather:   Heavy showers and good sunny periods.

Summary of Geology:  (see also the report on my trip to Raasay in May 2006).

 

The oldest rocks of the region are the Lewisian gneisses, which were formed by metamorphism of even older, mostly igneous rocks by continental collisions in the early days of Earth's history.  They are typically hard pink and white banded gneisses, sometimes interrupted by thick darker basaltic areas.

Overlying the Lewisian are Torridonian sandstones and conglomerates - a thick sequence of virtually unmetamorphosed sedimentary rocks, formed by fluviatile erosion of remoter Lewisian areas and laid down on a buried landscape of hills and valleys carved out of the Lewisian. 


The Torridonian was tilted to the west, then eroded to a horizontal surface before being invaded by shallow seas at around 500 Ma, in the Cambrian. Pure sands were laid down, now converted with the passage of time to an orthoquartzite  - an unmetamorphosed quartzite in which the original sand grains, cemented now by silica, can be clearly seen. This is the Eriboll Quartzite. Later, and extending into Ordovician times, as the sea deepened, limestones were laid down and are seen today as the Durness Limestone.  These Cambro-Ordovician rocks contain trilobite fossils (e.g. Olenellus) quite different from those of England and Wales, which at this time were on the margins of Avalonia, up to 1500 miles away on the other side of the Iapetus Ocean!

The whole sequence was then tilted back to the east, thereby returning the Torridonian sediments to an approximately horizontal position.The Lewisian, Torridonian and Cambro-Ordovician units were essentially left unscathed by the Caledonian Orogeny and are together referred to as the Foreland.

Much later, at around 430 Ma, as part of the Caledonian Orogeny, arising from the collision of Laurentia with Avalonia, or Baltica, vast amounts of deep sediments were squeezed, metamorphosed, and thrown over the younger Cambrian and Ordovician units to the west as great overthrusts or nappes, of which the Moine Thrust is the best known.  Often, in fact, the Cambro-Ordovician units provided the slide planes on which movement took place. In north-western parts of Scotland, these overthrust rocks are generally the Moine schists, but in Skye, even deeper rocks of the Lewisian basement from east of the thrust were thrown over to the west.

Following the Caledonian Orogeny, a vast amount of time through the Devonian, Carboniferous and Permian periods is unrepresented on Skye and Raasay.  This is because these were predominantly periods of erosion in this region.
We take up the story again in the Triassic, when bright red sandstones and conglomerates were laid down in an arid climate and are exposed today in parts of Raasay.  We can distinguish these rocks from the Torridonian because the Triassic beds are less hardened and because they contain pebbles eroded from the underlying Cambro-Ordovician limestones (identified by their reaction with acid).

Rocks of Jurassic age are extensively exposed throughout the the region.  Warm shallow tropical seas laid down shales, sandstones and limestones.  The limestones are often oolitic and locally are rich in chamositic iron, as on Raasay, where they were worked for a short time at the end of the First World War. Deeper Cretaceous seas then covered the region, and indeed all but the highest mountains of Scotland.


The early Tertiary is represented on Skye by extensive sheets of basaltic lava flows, a tiny eroded outlier of which caps Dun Caan on Raasay. These lava flows protected the underlying Mesozoic rocks from erosion - for example, a fragment of Cretaceous rock can be found below the basaltic cap of Dun Caan, and provides the evidence for the former extensive cover of chalk.  Sills and dykes are common in north-east Skye and a notable sill, the granophyre sill, covers a large area of southern Raasay.  A little later in the Tertiary, huge volcanoes formed on Skye, the eroded roots of which are exposed today as the granite of the Red Hills and the gabbro of the Cuillins.

Late Tertiary erosion has exposed these coarse-grained, deep-seated rocks at the surface and has carried the vast bulk of the Mesozoic deposits into marine basins which surround the area.

This erosion continued into the Quaternary of the last two million years, whose glaciation shaped the landscape as we see it today.

 
  Typical Lewisian gneiss, seen here in a roadside cutting in north Raasay. Finely banded gneiss with dark layers rich in biotite and hornblende and lighter layers rich in quartz and feldspar.  The larger scale horizons of dark rock are basaltic and have stretched and split - this is boudinage.
  The island of Fladday, in the background, is built on gently dipping Torridonian Sandstone. On "mainland" Raasay, in the foreground, the Torridonian has turned to dip steeply seawards (see detail in the photograph below).  Thus, these beds were formed by erosion of the underlying Lewisian, which forms higher ground just off to the right.
  Steeply dipping Torridonian conglomerate, as described above.  These conglomerates cannot contain limestone (those of the Cambro-Ordovician are younger), but they do contain clasts of the underlying Lewisian (as the geologist is indicating).
  The origin of these Torridonian conglomerates in eastern Raasay is uncertain.  They form the high ground in the foreground and close to Brochel Castle in the background. They may have been pushed up as gas vents in the Tertiary, but in that event, they would be expected to contain only angular clasts.  In fact, clasts are often rounded, and a crude sedimentary layering can be seen.
  The Moine Thrust, as seen in most of the North-West Highlands, carried deeply folded Moine Schists over the unmoved Foreland, but here, in southern Skye, it carried deeper Lewisian rocks (top) over Torridonian sandstones (the dark rocks at bottom of picture). The Lewisian is much altered.  Over a zone of about one metre above the contact, rocks are stretched and sheared by the thrusting to form a mylonite. (Allt Duisdale, Sleat Peninsula.)
  Triassic sandstones and conglomerates at Rubha na' Leac, eastern Raasay.  Clasts contain limestones of older cambro-ordovician age.  A Tertiary dyke, later faulted, can be seen cutting the beds.  Close examination also reveals light green gypsum veins.
  A typical exposure of alternating shales and limestones from the Lias (Lower Jurassic) of Raasay.  Abundant oysters (Gryphaea) can be seen, mostly but not exclusively in the shale bands. Recent erosion by acid rains is hollowing out the limestone. (close to former ironstone railway, near the ferry terminal).
  The opencast Lower Jurassic ironstone workings, as left abandoned in about 1920. Behind it can be seen topsoil taken off the top of the ironstone beds in preparation for working, which never, in fact, took place.  Beds rich in iron (up to about 15% Fe) are those with a deep brown colour. (on hillside to the right of the mine ventilation fan building, north of road to Hallaig).
  This exposure is a few yards to the left of that in the picture above.  It shows belemnites - the surviving hard parts of squid like creatures which abounded in Jurassic seas, and which sank and accumulated on the sea bottom on death.  The absence of benthos (bottom-living creatures like bivalves and brachiopods), suggests stagnant conditions).
  A detail of the Raasay ironstone beds (Lower Jurassic), as shown above.  This is a condensed sequence, which means that sedimentation was continuous, but less thick than equivalent rock units elsewhere.
  Middle Jurassic Valtos Sandstone, south of Staffin, north Skye.  They are generally white/brown, but contain concretionary darker layers.  Note the distinct cross-bedding in one layer. (roadside cutting south of Staffin).
  Middle Jurassic sandstones at Elgol in southern Skye, with a magnificent backcloth of Tertiary Cuillins.  The sandstones erode to form a typical honeycombed appearance.
  The layered Tertiary gabbro intrusion below Duntulm Castle at the north end of Skye.  About 15 layers can be detected here, each formed as heavy minerals settled to the bottom of the layer during cooling.
  Red bole weathering in Tertiary basalt lava flows.  Rapid weathering of a lower lava layer, barely visible, in the hot humid climate of the times has resulted in the formation of a  fossil lateritic soil, rich in iron, before deposition of the lava flow on top.

(The Quiraing, north Skye).
  Coarse grained gabbro, showing up as persistent white layers within a Tertiary basalt sill intruding the Jurassic of eastern Skye.

(quarry at Lealt).
  The Red Hills of Skye, a coarse-grained granite intrusion of Tertiary age, exposed today as a result of erosion of overlying sedimentary material.  The "red" colour is due to presence of pinkish orthoclase feldspar.  Glaciation of the granite results in very well rounded hills.
  The Black Cuillin Hills of Skye, a coarse grained basic intrusion of Tertiary age, exposed today as a result of erosion of overlying sedimentary material.  Glaciation of easily eroded gabbro results in hills with a jagged outline.
  The view south-eastwards from the Tertiary basaltic summit of Dun Caan on Raasay.  The nearer promontory is Rubha na' Leac, with its Triassic beds, overlain at bottom left by landslipped Jurassic.  A sliver of Cretaceous rock is known to exist below the lava flow on top of Dun Caan, and may be the light patch towards bottom right. (see photo in Raasay, May 2006 report).
  Late in Quaternary times, following the Ice Age, conditions were right in places for the formation of diatomaceous earth (Kieselguhr), a recent deposit formed when minute diatoms in lakes use fine silica to build their shells.  This long abandoned plant at Lealt used diatoms from nearby Loch Cuithir. Kieselguhr mixed with nitroglycerine was used to produce dynamite at Ardeer Factory in Ayrshire.

 
  A large area of southern Raasay is occupied by a Tertiary granophyre sill.  Here, the sill is at top, with predominantly Jurassic rocks below.  The latter are cut by a later Tertiary dyke, which does not penetrate the sill but appears to turn and to run below it on the right. The dyke is in turn cut by a thin brown semi-horizontal intrusion. (roadside exposure between ferry terminal and Inverarish).
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