The Grampian Highlands – Argyll region, is underlain by deformed and metamorphosed rocks. Known as the Dalradian Supergroup, these rocks represent what was originally a very thick pile of marine sedimentary and volcanic rocks.
Underlying more of the Scottish landscape than any other group of rocks, the Dalradian Supergoup is long established as classic area for the study of rock metamorphism (alteration by intense heat and pressure) and deformation arising from ‘mountain building’, caused by continental collision.
Confined to the area defined by the Great Glen Fault to the NW and by the Highland Boundary Fault to the SE, the Dalradian sediments and volcanics were originally laid down, or in the case of the volcanics erupted onto, the southern margin of a continent known by geologists as Laurasia. This happened during the Precambrian and Cambrian periods between 700 and 600 million (and very possibly as recent as 500 million) years ago. In late Cambrian and through Ordovician times, the Dalradian Supergroup underwent deformation and metamorphism as Laurentia and a northern European continental landmass comprising England and Scandinavia collided, with the closure of an intervening ocean geologists call Iapetus.
The Paps of Jura from Caol lla, Islay – A view illustrating 610 million year Dalradian geology. The rocks in the foreground were deposited in an estuarine environment. The quartzite forming the Paps of Jura originated as quartz sands deposited in a shallow sea © Scottish Natural Heritage.
This continental collision, known as the Caledonian Orogeny, deformed and folded the various sedimentary rocks, which were also metamorphosed, with the recrystallisation of sandstones to quartzites and mudstones to slates. There was also the intrusion of granite magma, derived from the actual partial melting of rocks lower within the crust, where the heat and deformation caused by the continental collision, was most intense.
The Highland Boundary Fault represents a very major break in the crust of Scotland that separates the Midland Valley from the Highlands, the fault formed as the crustal fragments that underlie Scotland and the rest of Britain came together during the Caledonian Orogeny.
At the northern margin of the region, along the southern margin the Moray Firth, there occur Devonian age sedimentary rocks that formed from sediment that accumulated in the low-lying Orcadian Basin. The sediment was derived from the weathering and erosion of the Caledonian Mountains to the south and west. Within the basin there was a large freshwater lake that was home to rich diversity of fish, the fossil remains of which can be found in the Devonian rock layer sequence.
At the margins of the Moray Firth, in the vicinity of Elgin, areas of Permian and Triassic rocks occur. These have yielded the fossil remains of mammal-like reptiles and other animals which inhabited a desert environment around 250 millions years ago. The footprints, trackways and traces of these animals have also been spectacularly preserved. The youngest rocks in the Region also occur in the Elgin area and are Jurassic in age.
The region contains an exceptionally diverse range of pre-glacial, glacial, periglacial, coastal and river landforms.
The Cairngorms represent the largest area of highest ground in Britain, containing an outstanding array of mountain glacial landforms. The adjacent glens and straths contain a variety of glacial meltwater features and glacial deposits, notably meltwater channels, eskers, kames, kettle holes, terraces and moraines.
To the west, the mountains of Lochaber have been heavily dissected by glacial erosion with the formation of corries and over-deepened valleys. Much of the area to the south-west comprises ice-scoured plateau landscapes, with the grain of the topography strongly controlled by the underlying geology. Glacial erosion has also shaped the main valleys which are deeply incised. The lower parts of many of these valleys have been ‘drowned’ by the sea, including the classic fjord landscape between Loch Fyne and the Clyde.
To the east, the inland area of Buchan is the best preserved pre-glacial land surface in Scotland. Although the area largely escaped significant glacial erosion, glacial deposits and meltwater channels are widespread. Raised marine deposits and landforms, including ancient caves, stacks and arches on Islay and Jura, are particularly well represented and provide a record of sea-level fluctuations dating back to before the last glaciation. Many of the lochs and peat bogs of the area contain valuable archives of past environmental changes and vegetation history.