• The oldest rocks on Arran are thought to have been  laid down during the Lower Cambrian, they belong to the Southern Highland Group, in particular the Dalradian Subgroup. Similar Southern Highland Group rocks found at Callendar in Central Scotland, contained a fossil trilobite definitively age dating them to the early Cambrian. No such evidence has been found in the Dalradian rocks on Arran, but, it is likely that they were initially laid down during the Cambrian. The Dalradian rocks found on Arran were deposited on the bottom of a deep Ocean as sedimentary rocks such as greywackes , black muds and shales. They were laid down when the land mass that is now Arran was at a latitude of 27^oN. These marine sediments have been classified as part of the Dalradian Subgroup which can be be seen outcropping in other parts of Scotland between the Highland Boundary Fault and the Great Glen Fault (Harris et. al., 1978). If we look at the simplified map of the solid geology of Arran below, we can see that the Dalradian deep marine sediments occur on Arran in the northern part of the Island. After their deposition as marine turbiditic rocks,  the Dalradian Rocks found on Arran have undergone metamorphism to produce metamorphosed greywackes and schists. This metamorphism was due to major geological events that were occurring during the evolution of the earth, in particular the Caledonian Orogeny, and took the Dalradian marine sediments to Chlorite grade metamorphism (that is, that they were cooked and compressed up to ca. 400^oC temperature and 500 kilobars pressure), the Dalradian rocks on Arran were also folded when the Tertiary granite that forms the northern granite mountians of the island was emplaced .

• Although the Dalradian metasediments on Arran have undergone a complicated geological history of metamorphism and deformations, it is still possible to see graded bedding in them (Image 1.) , this fining upwards of the sediment is typical of deep marine turbiditic sequences. Image 1, is a photograph of the Dalradian metamorphosed sediments found in the North Sannox burn just beside the road bridge and car park, where you can still see the coarser grains at the base of each depositional unit fining upwards. In other locations such as Imachar the original Dalradian sediments were more muddy and when they were metamorphosed a schistose cleavage  was produced Image 2. The cleavage plains contain micaceous type minerals of muscovite and chlorite aligned in the same direction, this can easily be seen, by studying the rock down a petrological microscope Image 3.

Image 1. Graded bedding in a Dalradian greywacke.

Image 2. Dalradian schists near Imachar, the original muddy deep marine sediments have been metamorphosed to produce shists with obvious cleavage planes.

Image 3. A view of some Dalradian Schist, observed down a petrological microscope.

• In Glen Rosa Dalradian rocks can be observed in contact with Devonian rocks (Gavrock member of the Lower Old Red Sandstone) Image  4. This contact is thought by some to represent a possible  extension of the Highland Boundary Fault, others have described the contact as an unconformity. You may want to go and have a look for yourself! There are numerous outcrops of the contact between the Dalradian rocks and the Tertiary Granite that forms the north Arran mountains. An example of the contact between Tertiary Granite and Dalradian can be seen in the North Sannox Burn Image 5. In other outcrops on Arran the Dalradian rocks can be seen to have been locally metamorphosed by the intruded granite, this is in addition to the general metamorphism of the Dalradian rocks during the Ordovician.

Image 4. The contact between the Devonian Lower Old Red Sandstone and Dalradian in Glen Rosa.

Image 5. A contact between Tertiary Granite and Dalradian rocks, in the North Sannox burn.