Granite from Ailsa Craig, Scotland

BGS Image ID: P521397

Rock specimen of riebeckite granite. Ailsa Craig, Firth of Clyde, Ayrshire, Scotland. British Geological Survey Petrology Collection sample number EMC3360.

The sample is a pale-coloured coarse-grained igneous rock with a distinctive granitic texture. It is technically a microgranite dominated by pale alkali feldspar crystals embedded in dark riebeckite amphibole and grey quartz.  The distinctive granite of Ailsa Craig forms part of the vast Tertiary Igneous Province of northern Britain. These formed around 60 million years ago. Much of the distinctive scenery of areas such as Arran, Mull, Ardnamurchan, Rum and Skye, and the Antrim Mountains in Northern Ireland, were formed as a result of this huge outpouring of magma. Because these rocks are well exposed and preserved they have been studied in great detail by generations of geologists, and provide an opportunity to study the deeper levels or 'roots' of volcanoes, from which we can learn much about currently active volcanoes today. The riebeckite granite of Ailsa Craig forms a Tertiary granite boss rising out of the Clyde estuary. It is one of a series of Tertiary granites. This blue-grey microgranite is very distinctive and has been extensively used as a marker for tracing the flow directions of Quaternary ice-sheets.

BGS Image ID: P750285

Another specimen of granite from the Ailsa Craig Quarry. British Geological Survey Petrology Collection sample number Econ. 1166.

BGS IMage ID: P001614

View of the old Ailsa Craig curling stone quarry situated on the north coast, Strathclyde Region, showing drilled-out stones 28 cm. in diameter in microgranite. 

The stone has a tough fine-grained lithology and can take a very high polish. It was worked in two colours, the 'red' and 'blue' varieties, the former was rarer and was much preferred. The curling stones were the economically most significant and world-renowned resource of the island. 50 pairs were exported annually in the 1870's rising to 1000 pairs in the 1890's. The quarry restarted 1961 but becoming uneconomic closed again in 1973.

BGS Image ID: P000704

BGS Old photograph number: D02015

Oblique aerial view of the island of Ailsa Craig. Looking south towards the north end of the island. The island is a very prominent landmark in the Firth of Clyde formed of a microgranite boss. Ailsa Craig is formed of an arfvedsonite-aegirine-microgranite intruded by vertical dyke swarms of olivine to alkaline olivine-dolerite with basalt margins. Isotopic dating using the Rb/Sr isochron method dates the microgranite at 61.5 Ma. This makes it Tertiary in age. The remains of the castle can be seen on the skyline on the left. It was built on the orders of the Earl of Cassillis by Thomas Hamilton whose coat of arms may be seen on the walls.

The resistance of the microgranite to erosion compared to the response to the soft Permo-Trias sandstones into which it was intruded was of crucial importance during the phase of glacial erosion. The principal direction of the ice during the last glaciation was south. This is indicated by the slight broadening of the island from north to south, albeit that this outline has been considerably modified by later marine action. The precipitous cliffs are primarily the results of marine erosion but the initiation of a strongly positive topographic feature associated with the microgranite intrusion probably occurred during glaciation.

Posted by Bob McIntosh

Kemnay Quarries, a working granite quarry, 1939.

BGS Image ID: P000136

Kemnay Quarries. Aberdeenshire. General view of the mason's yard. Stockpiled finished stone in a variety of sizes all neatly labelled can be seen in front of the masons' sheds. A steam train is seen mounted on a light railway. In the distance is part of No. 2 Quarry with several blondins stretched across the quarry. The roofs of the sheds could open to allow the travelling steam crane to deposit stone inside for the masons to work on.

BGS Image ID: P000132

Broken granite blocks after blast. Three quarrymen working at splitting a large block, one is wielding a pneumatic drill while the other two are using large crowbars.

Date of photographs: 1939.

An earlier post on Kemnay can be viewed here

Posted by: Bob McIntosh

St. Austell Granite from Gready Quarry. 1914.

Crystals of orthoclase with inclusions of biotite

 Crystals of orthoclase with inclusions of biotite

Twinned crystal of orthoclase

 Quartz-tourmaline rock or 'stent'

Photographs are from the Geologists' Association 'Carreck Archive' held at the British Geological Survey.

Posted by Bob McIntosh

Fossil fish from the British Geological Survey collections

BGS Image ID: P549484

Thrissops, a ray-finned fish from the Jurassic. Specimen GSM85802.

Thrissops was a member of the more advanced ray-finned fish, an early teleost. It lived in the seas of many places in Europe and fossils have been found in Jurassic and Cretaceous sediments of England, France and Germany. There may be one group of living fish related to Thrissops, the freshwater 'bony tongue' such as the pirarucu which lives in the rivers of South America. Thrissops grew up to about 60 cm. in length and once swam in the late Jurassic seas of southern England, about 145 million years ago. Although not seen in the specimen illustrated, Thrissops had a large symmetrical tail which enabled the fish to swim rapidly. It had small pelvic fins, but long anal fins, which were used to stabilise it. The pectoral fins were large as can be seen in the photograph. Fish are cold-blooded vertebrates with a braincase, fins for swimming and gills to take oxygen from the water (although some also have lungs). They therefore differ from other aquatic creatures like invertebrate molluscs or crabs; amphibians and reptiles, which have lungs and limbs rather than gills and fins; and whales and dolphins which are warm-blooded mammals. Fish are the first vertebrates, having evolved during the early Cambrian over 500 million years ago.

BGS Image ID: P549485

Macropoma, a coelocanth from the late Cretaceous. Specimen GSM109036.

Lobe-finned fish like coelocanths are occasionally found in the late Cretaceous (about 70 million years old) Chalk of Britain and in eastern Europe. However, coelocanths then seemed to disappear from the fossil record, and it was assumed that they had become extinct. However, in 1938 a living representative was found and they have since been found living off the Comoro Islands, Indian Ocean, and off Indonesia. Macropoma grew to about half a metre long. It had a deep body and a bulbous, 3-lobed tail. Its teeth are confined to the very front of the jaw. It had two dorsal fins, a pair of pectoral fins on its side and pelvic fins mid-way along on the underside of the body. Macropoma scales are very thin, ovoid plates of fibrous bone, enamelled on the outer surface. Fish are cold-blooded vertebrates with a braincase, fins for swimming and gills to take oxygen from the water (although some also have lungs). They therefore differ from other aquatic creatures like invertebrate molluscs or crabs; amphibians and reptiles, which have lungs and limbs rather than gills and fins; and whales and dolphins which are warm-blooded mammals. Fish are the first vertebrates, having evolved during the early Cambrian over 500 million years ago.

BGS Image ID: P521130

A fossil specimen of Cephalaspis lyelli Agassiz. A fossil fish. (Vertebrata, Pisces.) Glammis, Perthshire, Scotland.

 Cephalaspis lyelli is a jawless fish; a member of the Ostracoderms which were armoured with bony plates and scales. British Geological Survey Biostratigraphy Collection number GSM 5107. Cast of Lectotype. They occurred mainly in fresh and brackish water and were most abundant in the Lower Old Red Sandstone Devonian. Cephalaspis is one of the best known ostracoderms with the head covered by a rigid bony shield and the rest of the body by elongated scales. A pair of fins lie just behind the head shield which has the depressed shape of a bottom dweller. Cephalaspis is thought to have lived in fresh-water pools or streams feeding on organic material filtered in the gill pouches from bottom sediments. Figd. Agassig, Rech, Poiss. Foss. 2. 1838, pl. 1, fig.2, and Lankester, Mon. Pal. Soc. 1868, O.R.S. Fishes. Pl. VIII, fig. 1, & Stensio, Ceph. Great Britain 1932 B.M.(N.H.) pub. Text fig. 40, p.121. Cited Lankester, as before, p 44, and Stensio as before .

BGS Image ID: P521200

A fossil specimen of Osteolepis macrolepidotus Agassiz. A fossil fish. (Vertebrata, Pisces.) Quarry below High Water Mark near Verron, Bay of Skaill, Orkney, Scotland.

Osteolepis macrolepidotus is a crossopterygian fish from the Old Red Sandstone (Devonian). British Geological Survey Biostratigraphy Collection number GSE 12832. The lobe-finned fish distinguised by paired fins supported by scale-covered fleshy lobes and axial bony skeleton, and nostrils in the upper mouth include two groups, the lungfish such as Dipterus and the crossopterygians such as Osteolepis and Holoptychius. Crossopterygians were noted for their sharp pointed grasping teeth with vertically-grooved surface giving a labyrinthodont form well suited to a voracious predator such as Osteolepis. The evolution of the lobe-finned fish was one of the most important steps in the evolution of the vertebrates. Some features of Osteolepis are found in the labyrinthodont amphibians such as the ichthyostegids.

Posted by Bob McIntosh.