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Caithness Field Club Bulletin
April 1985

Donald Omand

In their "Geology of Caithness" published in 1914 Crornpton and Carruthers, Officers of the Geological Survey discussed volcanic vents (one at Dunnet Head and two at Duncansby) and monchiquite-comptonite dykes at a number of sites. 

At least five highly decomposed dykes of basic rock were observed to cut the sandstones of Dunnet Head.  The most accessible dykes, some 3 feet wide, was discovered exposed at low tide beneath the cliff near the south-east angle of Dunnet Bay.  Other decomposing dykes have determined the inlet known as Chapel Goe to the east of the Loch of Bushta.  Still farther north is Red Goe and an impressively eroded hollow called the Long Byre, both excavated along these ancient dykes of intruded rock. 

Dykes were also noted on the opposite side of the Bay at Castlehill.  One was observed in a quarry between Barrack Mill and Reaster House and another at Toftcarl near Thrumster. 

In 1983 N.  Rock published a paper considering the distribution of these dykes suites in the Highlands and Islands.1  The dykes belong to a widespread magmatic episode represented in other areas of Britain and Scandinavia.  The vents appear to represent "exploded" fissures filled with gaseous magma. 

In 1984 A.  Baxter and J.  Mitchell produced a series of radiometric dates for the comptonite-monchiquite dykes including three samples taken from the Thurso area. 2

The age of such dykes, the subject of considerable debate for many decades, appears to have been solved!  The three Thurso dykes, which are likely to belong to the broadly SW-NE trending swarm in Orkney, gave age ranges from 249-268 million years ago, i.e.  they were formed during the period known to geologists as the Permian.  The overall strike of these dykes in Caithness and Orkney parallels the Caledonian trend of the Highlands. 

It is also of interest to note that sediments of Permian age hove been proved by shallow boring to lie between Caithness and Orkney. 3


1 .  ROCK, N.  M.  1983.  The Permo-Carboniferous comptonite-monchiquite dyke suite of the Scottish Highlands and Islands: distribution, field and petrologic aspects.  Rep.  Inst.  Geol.  Sci., No.  82/14. 

2 .  BAXTER, A.  N.  and MITCHELL, J.  G.  1984.  Comptonite-Monchiquite dyke swarms of Northern Scotland: Age relationships and their implications.  Scott.  J.  Geol.  20, (3) 297-308. 

3 .  BREWER, J.  A.  and SMYTHE,  D.  K.  1984.  MOIST and the continuity of crustal reflector geometry along the Caledonian-Appalachian orogen.  J.  Geol.  Soc.  141, 105-20.