A useful overview of Raasay for the non-specialist is given in Millward & Millward (2001) which says: “Within the island’s limited space we find the richest and most varied geology of anywhere in the British Isles”.
There are two large-scale geological features of the landscape that are particularly relevant to matters botanical. Firstly, the limestone and basic sandstones that run along the east coast of Raasay from Fearns to Screapadal have a profound influence. Some plants are restricted completely to the eastern strip of limestone but as there are small pockets of basic rocks elsewhere, others reflect this in their distribution.
Secondly, Rona and the northern parts of Raasay from Holoman in the west and Screapadal in the east are made from Pre-Cambrian rocks. These rocks are amongst the oldest on the planet.
Only Ophioglossum azoricum can be said to be restricted completely to the Pre-Cambrian areas but there are many plants found elsewhere on Raasay which do not grow on the Pre-Cambrian. The Dolerite Dykes running through the Lewisian gneiss on Raasay are richer.
The underlying geology of Raasay south of the Holoman fault is complex and varied but, excluding the basic east coast, not readily associated with differing plant distributions - though local and more superficial effects are clear.