The Geology of NW Scotland and the Grampians

 

by Nick Chidlaw

13th November 2013

 

This talk was given by Nick Chidlaw who has led a number of field trips for our geology group including the one to the Lizard in April 2013.

 

The first half of Nick’s talk was about the geology North West of the Scottish highlands and the second on the Grampians. Many of the photos he used were taken in the spring which he observed was the best time to visit the area.

 

North West Coastline

 

The specific area of the NW highlands described was the 20 km wide coastal strip from Cape Wrath in the North to the Kyle of Lochalsh in the South. The western fringe of this area is a plateau (~180 m above sea level) with isolated prominent peaks (e.g. Stack Polly) and a heavily indented coastline. The plateau landscape is very rugged with many small outcrops (Cnocs).The eastern area is conventionally mountainous with peaks well above 180 m.

 

There is a wide range of rock types and ages in this area dating back to the late Archean, through the Proterozoic and up to the Ordovician.

 

 

The red areas in the map, with purple inserts, are the oldest rocks known as the Lewisian Complex and are metamorphic in nature. The next youngest rocks are the Torridonian Supergroup which are sedimentary and late Proterozoic in age. These are notably unaltered since they were laid down. Further East is the Moine Supergroup which are of about the same age. The thin central white region running from top to bottom are Cambrian and Ordovician in age and separate the Torridian Supergroup from the older formations. In this white region there are a large number of thrust faults.

 

 

Looking at the upper cross section above it is apparent that the rock layers are not in age sequence. Ancient Archean rocks are lying on top of younger Cambrian and Ordovician rocks. This is the result of thrust faulting (Moine Thrust) where layers of older rocks have been slid over the top of younger rocks. The lower cross section is more in chronological order but the older Moine Supergroup is still thrust faulted over the Cambrian and Ordovician.

 

Generally speaking the Lewisian Complex rocks of NW Scotland are made up of three separate pieces (‘Terranes’) of continental crust. These terranes were formed independently and later forced together by plate tectonic action during the Achaean era. The joins are heavily thrust faulted. The current thinking is that these Terranes were originally ‘micro plates’ of continental crust. The Northern Terrane (Rhiconich) is 2800 Ma as is the Southern Terrane. The middle Terrane (Assynt) is older at 3000 Ma. The metamorphic rocks now visible throughout each terrane look ‘alien’ as they were formed at depths of several kilometres under high temperature and pressure.

 

Banded Iron Formations (BIF) are also found in these Archean rocks. This type of soluble ferrous iron is only found in rocks of this age because the atmosphere at this time had no oxygen.

 

During the early Ordovician (~470 Ma) Scotland, not yet joined to England, was on the Northern edge of the closing Iapetus Ocean under conditions of plate collision. This resulted in mountain building during the Grampian Orogeny in Scotland, Canada and the Eastern United States (Appalachians). Later in late Silurian and Devonian times (416 – 360 Ma) England and Scotland came together with another period of mountain building during the Caledonian Orogeny.

 

After this time Scotland was largely dry land and undergoing erosion. Old Red Sandstone was being laid down. Subsequently this flat layer was uplifted by plate tectonic action and then eroded to form the typical flat topped mountain landscape of NW Scotland.

 

About 2.6 Ma ice ages began, not one continuous glaciation, but several with warmer interglacial periods. The evidence for this on land has largely disappeared but the relatively undisturbed sea floor has revealed around 50 such cycles. The reasons for this are still not well understood. During these interglacial periods the British Isles supported a wide range of animals including Hippopotamus, Elephant & Hyenas.

 

Nick then showed examples of the many rock and mineral types found in NW Scotland. These included:

 

 

The Grampians and Argyll

 

 

 

This area covers the region to the South East of the Great Glen Fault and the Highland Boundary Fault from Inverness to Oban and NW of the Highland Boundary from Stonehaven to the Kintyre Peninsula. The terrain is generally lower than the highlands to the North West although it does contain many high hills and mountains.

 

The area has been extensively eroded in the glacial periods and then subsequently uplifted. The red area has had deep and wide valleys cut into it containing major rivers such as the Spey and the Dee. The wide valleys are known as Straths and the narrow valleys Glens. The lower orange area is also the remains of the eroded and uplifted plateau. It is deeply incised and contains many fresh water lochs (e.g. Loch Lomond) and several mountains such as Ben Nevis. The yellow areas to the North and South are more low lying but still the deeply glaciated and incised plateau. The Southern yellow area of Argyll has a very Fiord like coastline.

 

The Garvellachs islands near Oban show a clear shallow dipping in the strata which helps the understanding of their geology.

 

The following map shows the geological boundaries of the area and the rock types:

 

 

The Great Glen Fault marks the North West of the area and is a deep near vertical fault of Silurian age. The Highland Boundary Fault marks the South West of the area. This whole area is created by coming together of different Terranes bounded by the two faults. The central area is known as the Grampian Terrane.

 

The main rocks of the Grampian Terrane are known as the Dalradian Supergroup and are Early Orcovician in age. They were originally mostly sedimentary layered rocks which were heavily deformed and metamorphosed in the Grampian Orogeny and are up to 25km thick. The oldest rocks (shown in red) are the Central Highland Migmatite Complex. These are rocks that just got hot enough to start melting and then cooled. They compose a mixture of the melted and original rocks. There is a general pattern to rock types making up the Supergroup where each younger type is ‘wrapped around’ the older type in succession. The exception is the white areas of ‘Other Rocks’. These are mainly granite intrusions of later Devonian age.

 

The Grampian Terrane and the Dalradian Supergroup were formed during the period of the opening and closing of the Iapetus Ocean. As the Iapetus Ocean opened the crust was stretched and faulted opening up gaps which were filled by sediments. When the Iapetus Ocean closed these faults reversed under the Grampian Orogeny and the rock layers were distorted and folded. Overlaying of rock types also occurred in regions of shallow faulting. An example of this is the Tay Nappe close to the Highland Boundary Fault. In some places there is evidence of ‘Failed Rift Valleys’ where the crust thinned and fractured allowing molten mantle rocks to rise to form basalts and granites.

 

Nick then showed examples of most of the rock and mineral types in the Dalradian Supergroup including:

 

 

 

Dick Harris

December 2013