• The Ordovician rocks on Arran were a subject of some controversy for quite some time. However, recently fossil, geological and geochemical evidence has been found to strongly suggest that a sequence of rocks are  indeed of Ordovician age and belong to the Arenig sub unit of the Ordovician (488-478 Ma), and part of the Highland Boundary Complex . They can best be observed in outcrop in the North Sannox Burn, just below the road bridge. At this point Dalradian (Cambrian) deepwater marine sediments, progress  into a suite of Ordovician rocks consisting of black phyllites, brecciated volcanic agglomerates, black slaty shists and pillow lavas. These rocks were formed  in a deep marine environment with the pillow lavas issuing onto the sea floor and cooling in pillow shapes. In Image 1., we can see the contact between the Dalradian and the Ordovician rocks. In Image 2, we can see the beautiful pillow lavas that were formed as basaltic lava issued onto the sea bed and cooled rapidly to form pillows.
• During the early Ordovician (Tremadoc age, 505-488 Ma) The Grampian Orogeny took place and the Dalradian sediments found on Arran underwent folding and metamorphism Image 3. The Grampian Orogeny was related to large scale plate tectonic events and readers should refer to M^ckerrow et. al. 2000.

Image 1. A contact in the North Sannox Burn, between Dalradian rocks and Ordovician rocks. The hammer head rests on the Dalradian rocks and the shaft on the Ordovician black pyllites.

Image 2. Ordovician (Arenig age) pillow lavas exposed in the North Sannox Burn.

Image 3. Folding in Dalradian Rocks due to the Grampian Orogeny that occurred during the Ordovician Tremadoc period.