Portsoy Central – Links Bay – East Head. Day 4. Excursion to the Banffshire Coast
Unpublished report prepared for the Edinburgh Geological Society Excursion to the Banffshire Coast. 14th – 21st May, 2005. Leaders: John Mendum, Douglas Fettes, David Stephenson and David Gould (British Geological Survey)
Portsoy Central (1) [NJ 585 665]
(Figures 27, 28, 29; plates 12, 13)
Drive down to swimming pool and park vehicles. Inspect western part of section; the more easterly bays may involve some up and down traversing (small paths).
The varied lithologies of the Portsoy Limestone Formation and its contact with the underlying Castle Point Pelite Formation (both Easdale Subgroup) will be examined. Metalimestones, psammites, quartzites (locally pebbly), pelites, semipelites, schistose graphitic pelites, some with kyanite after andalusite (chiastolite) are all seen. These rocks are highly folded, strongly strained, steeply dipping, rodded and lineated. They lie within the Portsoy Shear Zone. Metagabbros, meta-anorthosite and serpentinised ultramafic bodies (‘Portsoy Marble’) are intercalated with the metasedimentary rocks. Finish at the rubbish shoot by the ‘Marble’ workshop where there are good sheared gabbros and pegmatites.
Portsoy Central (2) [NJ 593 666]
(Figures 27, 28, 29)
Drive to the east side of the New Harbour and briefly look at partly deformed pegmatitic granites in gabbros at the west side of Links Bay. Recent U-Pb age dates show that these bodies were also intruded at c. 470 Ma.
East side of Links Bay and East Head, Portsoy [NJ 596 662] (Figures 27, 28, 30)
Drive to east end of bay and park. Examine section along the east side of Links Bay and follow path out to East Head. The section crosses the eastern edge of the Portsoy Shear Zone and passes from very highly deformed metalimestones, calc-silicate rocks, semipelites and quartzites of the Portsoy Limestone Formation (Easdale Subgroup) into the gneissose Cowhythe Psammite Formation (Crinan Subgroup). Prominent tourmaline pegmatites are seen as discordant veins. The Rosehall Croft Limestone Member shows complex fold patterns. Near East Head gabbro and ultramafic pods lie in the gneissose psammites and semipelites.
Old Hythe Bay [NJ 615 663]
(Figures 27, 28; Plate 14)
Drive down the road to the Boyne Limestone Quarry and park beside the portacabin in the quarry. Do not enter the working area. Leave quarry and walk down to the bay. Cross the Burn of Boyne and examine the immediate outcrops. Climb the grassy slope on west side and then go down the grassy slot to Old Hythe Bay. Examine the Boyne Castle Limestone and Old Hythe Semipelite members of the Boyne Limestone Formation (Tayvallich Subgroup). Superb clean exposures of folded and refolded metalimestone with calc-silicate layers and lenses are now seen beneath the spoil of till tipped from the quarry. Walk west to the ‘Boyne Line’ and the gneissose Cowhythe Psammite Formation (Crinan Subgroup?).
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