Excursion to the Isle of Eigg
A report on this
excursion has been written for the Edinburgh Geologist - read
Weekend Excursion to the Ballantrae Area
Britain is blessed with a plethora of compact areas
of great geological significance and one of these, the Ballantrae Ophiolite Complex
in the Girvan-Ballantrae area of SW Ayrshire, was the venue for the annual weekend
excursion (20-22 June 2003) of the Edinburgh Geological Society when a select
party of 12 members and guests attended. The locality’s
renown and variety of sedimentary and unusual igneous rocks with complex tectonics
attracted three farther-flung members from England. The party was based at Girvan, and the excursion
leader was Dr Phil Stone (British Geological Survey), who has an unrivalled knowledge
of the regional and local geology of the area. On Friday evening, the leader gave
an excellent slide talk on the geology with specific emphasis on localities to
Ballantrae Ophiolite Complex represents a slice through the top of the earth’s
mantle, and comprises three NE-SW volcano-sedimentary tracts (dominated by basaltic
pillow lavas) separated by a Northern and Southern Serpentinite Belt, composed
of ultramafic rocks from the mantle that are now extensively serpentinized.
was devoted to the early Ordovician Ballantrae ophiolite and its associated volcano-sedimentary
assemblage, whereas Sunday concentrated on the Ordovician and Silurian stratigraphic
succession and associated structure. In general, the
two days represented a single traverse through the Complex and cover rocks, starting
in the south and working steadily northwards.
Saturday morning began with a visit to the classic locality of Downan
Point where pillow lavas of Caradoc age, south of the Ballantrae Complex and their
slightly younger Tremadoc and Arenig pillow lavas, are superbly exposed (Photos
1-4). Cream-coloured sandy sediment infills the inter-pillow space and contrasts
sharply with the matt black basalt (Photo 2). The convex upper portions of the
pillows preferentially contain gas bubbles (now seen as zeolite-filled amygdales,
Photo 3) and identify the original orientation of the pillow (in Photo 4, the
pillow is now inverted). The first stop in the Ballantrae
Ophiolite Complex was at Bennane Lea where the Southern Serpentinite
Belt has a faulted contact with the central tract of volcano-sedimentary rocks.
On the beach here, members picked up samples of the distinctive Ailsa Craig Tertiary
microgranite (some 15km away in the Firth of Clyde), attractively speckled with
phenocrysts of blue riebeckite, and from which curling stones are fashioned.
With such an ironically macabre location nearby,
lunch simply had to be taken at Sawney Bean’s Cave at Balcreuchan
Port, where the eponymous cannibal partook of lunch (human variety) on numerous
occasions in the years preceding his demise and the cave’s demolition at
the hands of King James VI’s troops in1604 (see Phil Stone’s informative
article in the Edinburgh Geologist #30, 1997). Eighteen months later in London,
dissident English Catholics mounted a similar, but this time unsuccessful, venture
against the king himself in the Gunpowder Plot! At Balcreuchan Port, the party viewed the basalt country rocks of the
central tract (Photo 5) and, adjacent to a major fault bounding an offshore Permian
basin, were able to contrast them with the softer serpentinites at the southern
margin of the Northern Serpentinite Belt. Both fault blocks are intruded by a
Tertiary dolerite dyke with infilled vesicles adjacent to the upper contact (Photo
6). The day concluded with an examination of contacts and lithologies (gabbros
and ultramafics) from the Northern Serpentinite Belt at the southern end of Pinbain Beach.
Sunday was devoted largely to the transgressive Ordovician and Silurian
sedimentary cover, north of the Ballantrae Complex. In the morning, after a visit
to reef limestones of Caradoc age poorly exposed in the inland Craighead Inlier,
the party returned to the well exposed coastal sections where, in the northern
volcano-sedimentary tract at the northern end of Pinbain
Beach, one highlight was an unusual
porphyritic basalt with large plagioclase phenocrysts (Photo 7). Ordovician sediments
crop out directly north of the Pinbain block and, north of Kennedy’s Pass
where the siltstone turbidites of the Ardwell Formation comprise a major part
of the Caradoc succession, a series of spectacular chevron ‘boxfolds’
of probable early tectonic origin are well displayed (Photo 8).
After lunch, the
youngest beds of the area were examined from north to south in a traverse descending
the stratigraphical succession. At Cow Rock directly
south of Girvan (Photo 9), the channelled contact of the mid-Llandovery Quartz
Conglomerate was seen above the slumped and folded turbidites of the Woodland
Formation (early Llandovery). At the nearby Horse Rock, a slightly older coarse
debris flow conglomerate, the Craigskelly Conglomerate of basal Llandovery age
(Photo 10), rests with slight unconformity on the siltstone turbidites of the
Shalloch Formation of Ashgill age (Photo 11). The
final locality (optional) was a roadside farm visited by some members who purchased
the local Ayrshire tatties howked on the adjacent land.
1. Cliff (4m) in basalt pillow lavas of Caradoc age
at Downan Point (scale from tufts of sea-pink at top right). Face represents a
cross-section cutting through the base of the pillows and dip is steeply away
from the observer.
2. Matt black basaltic pillow lavas (Caradoc age)
of pillows of various size with the inter-pillow space infilled by creamy to ochreous
sandy sediment. Downan Point.
3. Clean, wave-washed surface of pillow lavas showing
concentric trains of zeolite-filled vesicles in the upper zones of individual
pillows. Creamy white sandy sediment, infilling the space between the pillows,
is prominent at the bottom right of the photo, though the actual margin of some
of the pillows is rimmed by a narrow zone of greyish white-weathering crystalline
calcite. Downan Point.
4. Close up of the basal contact of an individual pillow that, soon after
extrusion, inverted and toppled onto its convex upper surface, as confirmed by
the underlying trains of concentric vesicles in the original uppermost zone of
the pillow. Downan Point.
The dynamited remains of Sawney Bean’s Cave at Balcreuchan
Port, developed along a minor NE-SW
fault in basalt pillow lavas of Arenig age at the northern margin of the central
6. Sub-vertical (85º westward seaward dip) Tertiary dolerite dyke (with
hammer) from the Arran swarm intruding piebald, serpentinised
ultramafic rocks, Balcreuchan Port.
Trains of zeolite-filled vesicles are slightly oblique to the chilled upper contact
of the dyke (left hand margin). Sawney Bean’s Cave lies just to the right
of the photo across a major NNW-SSE Permian fault separating the sheared and softer
serpentinites from the more resistant basalts forming the cliff at top right.
7. Porphyritic basalt lava with large plagioclase
laths displaying swirling texture and local flow-banding. Slockenray headland
near Pinbain Beach
in the northern volcano-sedimentary tract (Pinbain block). Nearby, this lava is
mixed with a fine-grained aphyric basalt.
Photo 8. Chevron ‘box-fold’ pair (anticline
on the left), north of Kennedy’s Pass in Ardwell Formation turbidites of
9. Very steep-sided channel infilled with pebbly Quartz
Conglomerate (a mid-Llandovery matrix-supported debris flow) at Cow Rock south
of Girvan. The conglomerate truncates the slumped and locally strongly folded,
orange-weathering siltstone turbidites of the Woodland Formation (early Llandovery),
left of the hammer.
10. Thicker beds of coarse cobble conglomerate containing
prominent pink granite clasts interbedded with, and generally softer weathering
than, thinner, slightly more resistant turbiditic sandstones dipping steeply northwards
at 80º (three sandstone beds visible). Craigskelly Conglomerate, a basal Llandovery proximal
debris flow; Horse Rock, south of Girvan.
Photo 11. Very tight synform in thin-bedded siltstone
turbidites (Shalloch Formation) of Lower Ashgill age, at Horse Rock.