Creag na Croiche
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Condition (NatureScot)
| Unit | Feature | Condition | Area (ha) | Assessed |
|---|---|---|---|---|
| F1 | Moine | Unfavourable Declining | — | 2023-02-20 |
Geology & landscape
Rock Type: Granite
Landscape processes: Karst, Volcanic / Igneous
Description
CREAG NA CROICHE Creag na Croiche Site of Special Scientific Interest (SSSI) is located in the east of Sutherland near Rogart. This site has been notified for its ~1000 million year old ‘Moine Supergroup’ rocks which were partially melted around 425 million years ago to form a rock known as migmatite. The Moine rocks formed through the metamorphism (alteration due to heat and pressure) of rocks such as sandstones and mudstones. These rocks were partially melted 425 million years ago by contact with molten ‘tonalite’ rock (a variety of granite) that was part of the Rogart pluton. A pluton is a mass of rock that was originally magma (molten rock deep within the Earth) which solidified as it moved towards the Earth’s surface. The Rogart pluton is one of the best examples in Britain of a pluton which has ‘migmatised’ (partially melted) the surrounding rock. Creag na Croiche SSSI contains areas of both migmatite (Moine rock) and tonalite (pluton rock). At the contact between the migmatite and tonalite, one rock type grades into the other over a few tens of meters. Evidence from the rocks indicates a two-stage formation process for the migmatite on this site. The migmatites originally formed as an ‘envelope’ around the rising tonalite pluton, due to heating and partial melting of the Moine rocks. However, after migmatite formation, the pluton continued (or resumed) its ascent, deforming the original migmatite envelope and causing parallel folds and layers in the migmatite, and alignment of minerals in the tonalite. Good evidence that the tonalite formed by cooling and solidification of molten rock which pushed its way (intruded) into the Moine rocks, is also visible where small off-shoots of the main body of tonalite rock can be seen to cut through the migmatites. There is much potential at this site for further study of the detailed processes of the formation of migmatites by intrusion of a pluton.