Bulletin
4, 1998
Stone for Carving: The Tarbat Geological Research Project
First Report 1999
Nigel Ruckley and Martin Carver
With the aim of discovering and distinguishing the different stones used in the Tarbat school of carving and thus their source and provenance, the geologist Nigel Ruckley kindly undertook preliminary study of the material on site and in the National Museum and has since joined the research team. The building stone used to construct the successive phases of St Colman's Church is also on the agenda for his analysis.
Aims and Methods
The project is intended to have the following elements:
- Petrological examination of the carved stones found at Tarbat, whether held by NMS or in the care of the University of York research team. Macroscopic record included Munsell colour, grain size, inclusions and sedimentary structures and a measurement of magnetic susceptibility.
- Petrological examination of the building stones in St Colman's Church.
- Collection of representative samples from quarries and exposures of freestone in the Tarbat peninsula and beyond, located by reconnaissance or from a search of the literature.
- Comparison between quarry samples and carved stones, using petrological thin sections if possible.
- Initiation of a petrological data base for the early medieval carved stones of Scotland.
Preliminary Results
Tarbat Quarries
Portmahomack lies almost at the centre of an arc of sandstones that extends from the southern shores of the Moray Firth, across the Black Isle and northwards in a thin coastal strip towards Helmsdale. The Tarbat peninsula, apart from the hill of North Sutor, is composed of Devonian sandstones belonging to the Old Red Sandstone supergroup (ORS). The coastal strip westward towards Tain is composed of Upper Old Red Sandstones (UORS) from the Balnagowan group. The base of the UORS is conjectural but is thought to run on a line from Nigg Bay to Hill of Fearn and then north-east to Pitkerrie, Meikle Tarrel and on the coast at Wilkhaven. Sandstones exposed on the south-eastern side of the peninsula from a little south of Shandwick to Balintore, Hilton of Cadboll and Wilkhaven are composed of Middle Old Red Sandstones (MORS) of the Strath Rory group (Fortey et al 1998; Johnstone and Mykura 1989).
Quarries are generally limited to coastal areas or where the drift deposits are shallow as in the Lower Pitkerrie area. Today there are no working quarries, but around a dozen quarries are known to have been in existence since the 18th century. The Old Statistical Account for 1791 records "There is a soft freestone at Pitkery, of an inferior quality, in the east of the parish, but little used; a pretty good freestone at Balintore, a good deal used for building; but at Catboll, in the rocky part of the coast there is a remarkable good freestone, little inferior to any in Scotland" (OSA 1981, 379-392)
Provenance of the Tarbat sculpture
Only one of the stones from the group so far recovered at Tarbat can be said with some degree of certainty to come from the exposures around Portmahomack; this is TR28 the "Calf" stone.
A group of stones, defined by the presence of iron blebs or of Liesegang rings, include the finest of the Tarbat sculptures (TR1, TR13 and TR20), as well as the other monumental stones sited elsewhere on the Tarbat peninsula: Nigg, Shandwick and Hilton of Cadboll (this also applies to the additional flaked fragments recovered at the Hilton chapel in 1998). On this basis, the petrological verdict is that TR13 (the inscription) and TR20 (the Dragon Stone) could have come from the same geological formation. However, TR2 and TR4 do not match TR13 and TR20 geologically and thus are unlikely to have formed part of the same monument. No rock exposures examined last August on the Tarbat peninsula showed the presence either of the iron blebs or the Liesegang rings, so the source for the major Tarbat monuments is as yet unidentified and may lie beyond the peninsula.
Other stones likely to have been brought from further afield are the bosses TR3 and TR7 and the interlace panel TR 4, which are of sandstone which might have affinities with the Triassic deposits on the south side of the Moray Firth. The grave-marker TR21 is also probably imported. It is composed of clast-free colour-laminated fine grained sandstone. The broken surfaces exhibit 5YR7/1 'light grey' and darker 2.5YR5/2 'weak red' laminated bands. The darker bands, between 5mm and 10mm thick, are rich in biotite mica. The high mafic content of this sandstone gave a comparatively high magnetic susceptibility average reading of 0.1225 (compared with, for example, the 0.015833 of TR1). Although the quarry at Shandwick has reddish laminated sandstones with mica-rich bedding planes, TR21 bore no resemblance to the range of stone in the quarry and does not seem to come from the Tarbat peninsula.
The Building Stone from St Colman's Church
Examination of the stones from exposed wall-fabric, allowed samples of the different phases of the church construction (Bull 3, 3-13) to be distinguished and provisionally assigned to source. The east wall of the crypt (Phase 1, probably 9th century) contained stones likely to belong to the Middle Old Red Sandstone; while building stone from the foundations of the south wall of the Phase 2 church (11-12th century) appear to be from the Upper Old Red Sandstone. Some of the blocks from this wall are fine to medium grained, others are of coarser texture. There are mudstone inclusions (or voids where mudstone has weathered out) up to 10mm long by 2mm thick, and weathering has reduced the already low mafic and mica content. The colour varies from 2.5YR6/2 "pale red" to 2.5YR5/2 "weak red". These stones were probably extracted from the outcrops on the north-west side of Tarbat Ness as at Portmahomack itself. The Phase 2 threshold stone in the same wall, however, is formed from an extremely fine-grained sandstone not from the immediate area and having affinities with the MORS on the east side of the peninsula. It may of course be a re-used piece of sculpture. The west end of the crypt, belonging to the Phase 3 (13th century ) church seems also to have been constructed from the finer MORS.
The west wall was thickened with a relieving arch in the 16-17th century (Phase 5) using very fine to fine current-bedded sandstone of 5YR6/3 "pale olive" colour. Mudstone clasts of 5Y4/1 "dark grey" to 5GY4/1 "dark greenish grey" colour can be seen and the bedding is picked out by grey 1mm thick laminae. Some iron blebs are present with iron staining on the surface. The stone is not from the immediate area of the church and sandstone with iron blebs has not been recorded in the peninsula. The pillars used to help support the arch were also imported. The grain is very fine and compact with a high quartz content. No lithic or mudstone clasts were noted and the 5Y5/3 "olive" colour was noted to change to a 5YR5/3 `reddish brown' on a weathered surface. In Phase 6 (mid 18th century) the present belfry was constructed from well-sorted very fine- to fine- grained sandstone with numerous sedimentary structures, including current and planar bedding. Its colour varies from 2.5Y7/2 "light grey" to 2.5Y7/4 "pale yellow". There is a resemblance to the MORS seen at the former quarry at Cadboll. Most of the fabric of the present church dates from the re-building in the later 18th century (Phase 7). The main fabric of the lower south wall is comprised of a 10YR6/6 "brownish yellow" well-sorted medium sandstone, probably from the MORS beds on the peninsula. Another possible source for a similar stone is to be found on the Portmahomack coast at Port a' Chaisteil opposite Wilkhaven. Here a medium-grained, moderately to poorly cemented, clast-free, 10YR6/8 "brownish yellow" sandstone belonging to UORS supergroup was formerly quarried.
The upper course of the same (south) wall immediately below the roof line are made from 10YR5/4 'weak red' coloured fine to medium grained laminated sandstone. Mica was evident on the bedding planes and mudstone clasts varied from pea-sized to 80x35mm. Some clasts have green-coloured reduction spots. This is probably ORS from the Black Isle or Cromarty Firth area.
Conclusion
From this preliminary work it is evident that a wide variety of sandstone has been brought to the Tarbat site from several locations to manufacture sculptural monuments and to construct St Colman's Church in its different phases. In the 8-9th century, the period of the earliest church and finest sculpture, the good freestone from the MORS at the Hilton quarry was employed for the church, while even finer stone from a source yet to be located was imported to make the major Tarbat monuments (TR1, TR13, TR20, Hilton, Shandwick and Nigg). Stone to make at least one grave-marker, or perhaps the grave-marker itself was also imported from a source which probably lies beyond the peninsula. However, one piece, TR28, the "Calf stone", was made from the local ORS near Portmahomack beach.
Where they did not re-use the now redundant sculpture, the church-builders of the 12th century quarried from the UORS towards Tain. In the 13th century, the builders again quarried from the MORS in the Hilton area. But the church-builders at the Reformation again used stone imported from unidentified sources beyond the peninsula. The local Hilton quarries were seemingly favoured again in the 18th century to construct the belfry and the greater part of the church following its refurbishment after 1756.
The next goals of the project will be:
- To group the remaining specimens of the Tarbat collection and if possible assign them to quarry.
- To locate the sources for the major Tarbat monuments and grave-marker TR21.
- To thin-section the rock cores extracted from the sculptures in their care by NMS, in order to build up a petrological reference collection. Loose fragments from the Tarbat group will also be thin-sectioned as they become available from the Tarbat Discovery Programme.
- The results of the investigation are to be incorporated into the planned publications of the church and of the Tarbat School of carving.
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Last updated 10 October, 2003.
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