PRECAMBRIAN-CAMBRIAN? 550-510 Ma ( Arran at 27 degrees N) 

Metamorphosed greywackes and shales belonging to the Southern Highland Group of the Dalradian Supergroup are the oldest known rocks on Arran.

These rocks are thought to have been laid down as a series of turbidites in submarine fans at the edge of a continent named Laurentia, the forerunner to North America, nearly 600 Ma years ago (or possibly during the Early Cambrian Period...?).

Originally flat lying beds, the Dalradian rocks were metamorphosed during the Grampian Orogeny, roughly 510 Ma, to produce the striking and conspicuously folded greenschists which can be seen in the north and northwest of Arran. Being buried to depths of nearly 5 kilobars and subjected to temperatures of roughly 400 degrees C, these slates and phyllites possess fantastic cleavage and owe their grey-green colour to the growth of chlorite, a diagnostic low grade metamorphic mineral. The Dalradian outcrops along the coast from Dougarie to Lochranza. The folding and cleavage is best displayed on those outcrops freshly weathered by the sea, such as at the foot of Imachar Brae where the main road meets the shore.

Local buildings are often useful indicators of local geology. In this case, Lochanza Castle, on the north shore of Arran, is built from Dalradian rocks. In the enlarged views available below, the Daldradian rocks form the majority of the castle walls and are green or purplish in colour. The remaining rocks, pale red in colour and forming the frames to the doors and windows, are made of Permian sandstone.

You can also look at a short movie that presents an aerial fly-by of Lochranza Castle, by clicking here.