DEFINITION OF SCOTTISH HIGHLANDS

Located in the northern portion of the

United Kingdom, Scotland includes a variety of generally

rugged Lower Ordovician to Archean terranes, dissected by

numerous northeast trending faults that form deep valleys. The

coastline of Scotland is highly irregular and has many narrow

to wide indented arms of the sea known respectively as lochs

and firths. The Hebrides, Orkney, and Shetland Islands lie off

the coast of northern Scotland. The Southern Uplands form a

series of high rolling grassy and swampy hills known locally as

moors, underlain by a series of strongly folded and faulted

Ordovician and Silurian strata. These are separated from the

Midland Valley by the Southern Uplands fault, an early Paleozoic

strike-slip fault that was later converted to a normal fault.

The Midland Valley includes thick deposits of the Devonian-

Carboniferous Old Red Sandstone, deposited under continental

conditions. The Highland Boundary fault separates the

Midlands Valley from the Grampian Highlands, where Precambrian

to Early Paleozoic metamorphic and igneous rocks

of the Dalradian and Moine Groups are exposed in rugged

mountains. The Great Glen fault, a Late Paleozoic left-lateral

fault, separates the Grampian Highlands from the Northern

Highlands, where Grenvillian age Moine and Archean

Lewisian rocks are exposed. The tallest mountain in Scotland,

Ben Nevis (4,406 feet; 1,343 meters) is located in the highlands.

The Moine thrust forms the northwestern edge of the

Caledonian orogen, with Archean and Proterozoic rocks of the

Lewisian gneisses forming the basement to the orogen.

The oldest rocks exposed in the Scottish Highlands are

the Archean (three billion years old) through Lower Proterozoic

Lewisian gneisses formed during the Scourian tectonic

cycle, found principally in the Hebrides Islands and the

Northern Highlands. The Late Archean gneisses include

tonalitic and gabbroic types, with rare ultramafic-mafic plutonic

units, probably formed in a volcanic arc setting. Other

shallow shelf metasedimentary rocks, including quartzites,

limestones, and pelites, were metamorphosed to granulite

facies at 2.7 billion years ago.

The Invernian tectonic cycle in the Early Proterozoic

deformed large tracts of the Scourian gneisses into steep

limbed west-northwest-trending linear structures, accompanied

by retrograde amphibolite facies metamorphism. Mafic

dike swarms intruded at 2.2 billion and 1.91 billion years

ago, and are not metamorphosed. Post-1.9-billion-year-old

Laxfordian cycle events include the formation of shear zones

and intrusion of granite plutons near 1.72 billion years ago,

and a cessation of events by 1.7 billion years ago.

The Moinian Assemblage is a Middle Proterozoic (older

than 730 million years, and probably older than 1 billion

years) group of pelites and psammites that are complexly

folded into fold interference patterns and metamorphosed to

amphibolite facies. Late Proterozoic (970–790 million years

old) rocks include two sequences of red beds including the

Stoer, Sleat, and Torridon Groups. These groups include conglomerates,

siltstones, and sandstones that are more than a

mile (several kilometers) thick in most places, and up to four

miles (6 km) thick in a few places. Most of these Late Proterozoic

rocks were probably deposited in fluvial or deltaic

environments, perhaps in fault-bounded troughs along a continental

margin.

The Dalradian Supergroup is found within the Caledonian

orogen south of the Great Glen fault and north of the

Highland boundary fault. The Dalradian is more than 12

miles (20 km) thick and is divided into four groups. The lowermost

Grampian Group includes shallow to deepwater sandstones

and graywackes, overlain by shallow shelf rocks

including limestones, shales, and sandstones of the Appin

Group. The succeeding Lochaber Group includes sandstones,

siltstones, and carbonates deposited in a deltaic environment.

The top of the Dalradian consists of the Argyll Group, including

a glacial tillite, limestones, and deeper water graywackes,

interbedded with Late Proterozoic (595-million-year-old)

basalts. The Dalradian rocks were deformed into large nappe

structures in the Late Proterozoic Grampian orogeny and

metamorphosed to the amphibolite facies. The Paleozoic Era

in the Scottish Highlands is marked by a basal transgression

of the Durness sequence of shallow-marine skolithos-bearing

quartzites and limestones onto the Torridonian and Lewisian

gneisses. The basal transgressive sequence is about 1,100 feet

(350 m) thick, and is overlain by Lower Cambrian through

Ordovician shelf limestones. This sequence is correlated with

the basal Cambrian-Ordovician shelf sequence in the Appalachian

Mountains, as the Scottish Highlands was linked with

Greenland and the Laurentian margin in the Early Paleozoic.

However, there is no correlation of rocks of the Scottish Highlands

with rocks south of the Highland boundary fault, sup-

porting tectonic models that suggest that the southern British

Isles were separated from Scotland by a major ocean, known

as Iapetus. In latest Cambrian or Early Ordovician times, the

region was affected by main phases of the Caledonian orogeny,

known as the Athollian orogeny in the Scottish Highlands.

Several generations of folds and regional metamorphism are

related to the closure of the Iapetus Ocean along the Highland

boundary fault, with an oceanic assemblage of cherts, pillow

lavas, serpentinites, and Cambro-Ordovician limestones.

These events associated with the Early Paleozoic closure of the

Iapetus Ocean are correlated with the Taconic and Penobscottian

orogenies in the northern Appalachians. In the Southern

Uplands, a tectonically complex wedge of imbricated slivers of

Ordovician-Silurian deepwater turbidites, shales, and slivers

of pillow lavas may represent an oceanic accretionary wedge

associated with continued closure of additional segments of

the Iapetus Ocean.

The Moine thrust zone in the Northern Highlands formed

at the end of the Silurian, and places the Caledonian orogenic

wedge over the foreland rocks of the Lewisian and Dalradian

sequences to the northwest. The Moine thrust is one of the

world’s classic zones of imbricate thrust tectonics, clearly displaying

a sole thrust and imbricate splays, thrust-related folds,

klippen, and windows. These structures formed as a Late-Caledonian

effect of convergence and shortening between the formerly

separated margins of the Iapetus Ocean and placed the

orogen wedge allochthonously over basement rocks of the

Laurentian margin.

The Old Red Sandstone is a Silurian-Devonian sequence

of conglomerates, sandstones, siltstones, shales, and bituminous

limestones that is up to 10,000 feet (3 km) thick. These

rocks represent fluvial-delataic to lacustrian deposits eroded

from the southeast and are interpreted as a molasse sequence

representing denudation of the Caledonides. The Old Red

Sandstone is loosely correlated with the Devonian Catskill

Mountains deltaic complex in the Appalachians, representing

erosion of the Appalachian Mountains after the Devonian

Acadian orogeny.

Carboniferous deposits in Scotland include shales, coal

measures, basalts, and limestones, deposited in deltaic environments

mostly in the Midland Valley. Devonian through

Carboniferous sinistral strike-slip faults cut many parts of the

Scottish Highlands and are associated with Hercynian tectonic

events in Europe and the Acadian-Appalachian orogenies

in the Appalachians.

See also APPALACHIANS; CALEDONIDES; PALEOZOIC;

STRUCTURAL GEOLOGY.

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