Summary of
Geology:
Sedimentary
rocks on Great Cumbrae are predominantly of Upper Old Red
sandstone age, overlain in the south west corner of the island
by Calciferous Sandstone (lowermost Carboniferous) age rocks.
The Old Red Sandstone rocks are mainly typical
red sandstones with some conglomerates bearing small to moderate
sized clasts. The Calciferous Sandstone age rocks are
lighter coloured sandstones and marls, and are followed upwards
in the succession by lavas exposed on Little Cumbrae.
The island is perhaps best
known for its suite of dykes, which are of three ages:
(a) a suite of Calciferous Sandstone age (i.e. early Carboniferous Limestone age,~360 Ma), striking
NE-SW. They are bostonites and felsites and are well
exposed north of the ferry terminal. Plugs and bosses
filled with basalts and representing old volcanic centres can
also be found on the island.
(b) a suite of Permo-Carboniferous quartz dolerite dykes,
trending E-W, (~290Ma), notably the 75ft thick Downcraig Ferry
dyke, which can also be found on the west coast south of Bell
Bay. Dykes of this suite are common in the Midland Valley
and can be explained by crustal extension following the Variscan
movements in southern Britain.
(c) a suite of Tertiary age (~60 Ma), spectacularly exposed in
the south east of the island at the Lion Rock and Deil's Dyke.
They trend NW-SE and can therefore be traced towards the Mull
volcanic centre. They are composed of cumbraite, a
porphyritic rock with large white phenocrysts of feldspar in a
black vitreous groundmass.
Structurally, the island is cut
by a significant north-south trending fault, the Great Cumbrae
Fault, which runs from parallel to the east coast of
Millport Bay, just offshore, to the north coast at White Bay.
A marked break of slope is associated with it in the south.
Movements on the fault have variously been sinistral and dextral
throughout its history as indicated by the variable displacement
of dykes. All sedimentary rocks to the east of the fault
are of Upper Old Red Sandstone age. |
