Significance : Dunrossness Spilitic Group. Graphitic phyllites. Talc-magnesite schist (steatite) and komatiite-serpentinite. Viking steatite workings
This site lies just south of Cunningsburgh. The access track was cut in 1987 in order to extract a bulk talc sample with a view to opening up a commercial talc quarry in the hillside, (a remarkable proposition given its close proximity to a major archaeological site). In 1997 the mineral extraction company Talc de Luzenac (part of RTZ/CRA group) were drilling on site to remove core samples for analysis. The track does however give an excellent section through the massive steatite and serpentinite within the Dunrossness Spilitic group.
The cutting at the foot of the track is through steatite that is overlain by a thin band of black metasediments (graphitic phyllite). The track passes uphill through the light coloured steatite where it is cut by thick bands of green serpentinite. Originally these rocks were ultrabasic lavas composed mainly of olivine. Alteration of the olivine by hot hydrothermal fluids has produced serpentine and the more highly altered talc. Examination of the serpentinite shows it places to be formed as a closely packed, matrix-free breccia.
Brecciation of the rock is believed to have occurred during its formation as the original lava solidified and not by some subsequent tectonic event. In thin section the serpentine crystals (pseudomorphs of olivine) are arranged randomly through the rock and have been described as having a spinifex texture. This and major element analysis of the rock have led to the conclusion that these rocks were originally laid down as a sequence of komatiite lava flows.
Komatiite lavas were erupted at very high temperatures (near 1600ºC) so in effect represent deep and very hot mantle rocks. Since the Earth’s mantle has cooled with geological time it follows that komatiite lavas would have to be sourced deeper in the mantle and become more rare with time. Thus komatiites are extremely rare in rocks as young as these (Lower Ordovician) as most are found in much older Archean rocks, so must be due to much deeper fracturing of the Earth’s crust than is normal in crustal extension events of this age. The deep fracture that produced these komatiite lavas may been the splitting of the crust to form a constructive plate margin in an intra-continental basin related to the creation of a new ocean, the Iapetus.
Large masses of metamorphosed basic volcanic rocks also occur in the local area as do rounded masses of hornblende metagabbro which lie within the ultrabasic serpentinite-komatiite. It has been suggested that these rounded masses may be xenoliths of basic rocks brought to the surface by the komatiite lava flows. Another possibility is that these are solidified masses of more a viscous basic magma that was either entrained in the low viscosity komatiite flow or formed from the melting of basic xenoliths.
Studies of Bouguer gravity and aeromagnetic anomalies show that these volcanic rocks extend for more than 120 km to the NNE. This has important implications for the provenance of the Unst ophiolite complex.
Here the outcrops of the easily worked steatite were extensively quarried in Viking times to produce various artefacts both for local use and export. Chisel marks and hollows where bowls had been fashioned and extracted can be seen on many outcrops. The small mounds dotting the hillside are spoil heaps, much of the quarrying scars are probably now hidden under the turf, some of which has recently been cleared.
"The steatite quarries cover a very large area, almost 1km long, from the shore below and to the east of the main road, rising on either side of the Catpund Burn to its headwaters in the hills to the west. As you climb up beside the burn, you will notice traces of quarrying both in the bed of the burn and on the rock outcrops on either side. Most obvious are the projecting blanks for vessels that were never removed and the round and rectangular depressions where such blanks were removed, and the rock faces are covered with chisel marks. The method used in manufacturing steatite vessels was to carve out the rough shape of the bottom and sides of the vessel as if it were upside down on the rock, to detach this blank and to finish shaping its exterior and hollowing out the inside. The talcose rock in relatively easy to carve, being quite a soft stone that is soapy to the touch, hence its other name, soapstone, and it was used for a great number of purposes from early prehistoric times onwards. It is also known in Shetland as clebber, from the Norse term kleber.
Higher up the burn is an excellent area of quarrying, exposed by excavation and now fenced off with a stile for access and an information board.
Excavations at Jarlshof, some 24 km to the south, produced a variety of steatite vessels that can be matched by the blanks and discarded waste in the quarries, and steatite grit used to strengthen the clay for pottery vessels demonstrates that steatite was being exploited by the beginning of the second millennium BC. Steatite was being used most extensively in the Viking Age and early medieval times, when round, oval, and square vessels were in fashion consecutively from the9th to the 13th centuries, as well as steatite line-sinkers for fishing, spindle whorls, beads and lamps (small oval dishes with a hole at either end so that they could be suspended, the wick immersed in oil). At this period the industry must have been organised on a mass market, commercial scale, with its products being exported not only all over Shetland but also to Orkney and Iceland, which lack any source of steatite.