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Girvan to Ballantrae Coast
This stretch of coast was first recognised as an area of interest in the mid-1800's
by the likes of Sir
Roderick Murchison and James Nicol (1810 - 1879). The presence of mafic and ultrabasic rocks
and basaltic pillow
lavas caused great debate. Various theories on their presence were put
forward, however, it wasn't until the 1970's that the area was identified as
an obducted
ophiolite complex, emplaced during the (overall) subduction of
the Iapetus Ocean crust
during the Caledonian Orogeny.
In other words, it was a fragment of ocean crust and underlying mantle that
had become detached from the ocean floor and was stuck to the Scottish landmass
as the Iapetus closed.
The Ballantrae Complex outcrops along the coast between Girvan and Ballantrae
but is also exposed on higher ground inland. It represents rocks from various
levels in the crust, extending to perhaps 40 - 50km in depth. There are three
main components to the Ballantrae Complex:
- Obducted oceanic crust and island arc remains - Late Tremadoc to early
Arenig (approximately 490 - 480Ma)
- Pillow lavas, lava conglomerates, cherts and fossiliferous black shales,
gabbros and trondhjemites (representing the magma chambers)
- Obducted ocean mantle - Middle Arenig (approximately 480Ma)
- Ultramafic rocks (mostly dunites and harzburgites), generally serpentinised
- Uncomformably overlying Ordovician (Llanvirn)
and younger fossiliferous sediments.
- Basal conglomerates (volcanogenic), shallow-marine limestones and deep
water greywackes and shales; this represents a fault controlled fore-arc
succession to the northwest of the complex
Exposures of Ordovician age (450 million year old) deep
water sedimentary rocks at Kennedy’s Pass, Girvan.
Image provided by Colin MacFadyen.
Folded
sedimentary rock layers in the foreshore north of Kennedy’s Pass,
Girvan. The fold was formed when the sediment was still soft.
Image provided by SNH
Further reading:
Lawson, J.D. & Weedon, D.S. 1992. Geological Excursions around Glasgow & Girvan.
Geological Society of Glasgow.
Stone, P. & Smellie, J.L. 1988. The Ballantrae Area. Description of the
solid geology of parts of 1:25,000 sheets NX 08, 18, and 19. British Geological
Survey (Her Majesty's Stationery Office, London).
Stone, P (editor). 1996. Geology in south-west Scotland - An Excursion Guide.
British Geological Survey.
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