Summary of Geology: (see also my field trips on Skye
and Raasay of 23-30 August 2006)
A wide variety of rocks of many
geological ages can be found on Raasay including:
Lewisian Gneiss: - the oldest rocks
found in the British Isles, ~ 3000-2000 Ma; extensive exposures in the far north of the
island.
Torridonian
Sandstone: - the
oldest sedimentary rocks in the British Isles, ~1000-800 Ma,
principally found in north-western parts of the island. Red-brown
fluvial feldspathic sandstones (arkoses), and conglomerates with frequent clasts
eroded from the Lewisian. They were very slightly metamorphosed
during the Caledonian Orogeny, but retain fully their sedimentary
structures.
Triassic
sediments: -
~220 Ma, found in small areas
in the south of the island, at the Rhuba na' Leac, and also in small patches
which appear to overlie the dolerite sill at Oskaig. They consist
of bright red sandstones and conglomerates, distinguishable from the
older Torridonian in that the Triassic beds are less hard and that they
can, and do, contain frequent clasts of Cambro-Ordovician limestone (reacts with acid).
Jurassic
sediments: - fossiliferous
sandstones, limestones and shales, of lower and middle jurassic age, ~200 Ma, found widely, mostly in the
south-east. Also found are ironstones in chamositic oolitic
limestone, which were mined briefly until just after the First World War.
Tertiary
rocks: -
~60-50Ma, an extensive granophyre
sill covers a wide area in the south of the island. Granophyre is a
fine-grained porphyritic granite with intergrowths of quartz and
feldspar. The granophyre sill is displaced by a fault which traverses
the island from a point half-a-mile west of Eyre Point north-westwards to near Oskaig.
The flat cap of Dun Caan, composed of basalt, is a detached part of the Skye lava field. |
