OR/15/053 Castle Sween

Everett, P A, Gillespie, M R and Tracey, E A. 2015. Provenance of building stones in four 'galley castles' in Argyll. British Geological Survey Internal Report, OR/15/053.

Castle Sween is believed to be the oldest castle still standing in Scotland. It is located on a low rocky ridge on the east shore of Loch Sween, in Knapdale. The site would have provided the occupiers with an extensive view of Loch Sween, and as far as Jura.

Layout and building chronology

The castle is comprised of a massive Curtain Wall with two towers: the West Wing Garderobe Tower, and the Northeast Tower. The entrance to the Castle is in the middle of the South Curtain Wall. The inner courtyard is comprised of ruinous East and West Ranges, and a Well. A timeline of construction and alterations for Castle Sween is presented in Table 2.

Table 2 Timeline of construction and alterations in Castle Sween
Information from RCAHMS, (1992) 

Date	Action	Location or masonry element
c.1200	Construction	Small courtyard flanked by ranges
Early 13th C	Construction	Upper parts of Curtain Walls
13th C	Construction	West Wing (single-storey, outside West Curtain Wall)
13th C	Construction	Northeast Range
Early 14th C	Construction	West Wing Garderobe Tower (3-storeys)
15th C	Construction	Northeast Tower with kitchen and chambers (outside North Curtain Wall)
15th C	Construction	East Range with Great Hall (2 to 3 storeys)
15th C	Construction	West Range
16th C	Construction	Vaults in Northeast Tower and East Range
Mid- to Late- 17th C	Abandoned	All
1985–9	Repair	All, major consolidation works

Building stones

Walling stone

Several different types of metamorphosed rock have been used as walling stone in Castle Sween, including metasedimentary rocks (metamorphosed sandstone and mudstone) and metamafic (igneous) rocks. The walling is generally coursed rubble style. Larger, sometimes rounded, blocks commonly of metasandstone are held with smaller, tabular pinnings of metamudstone and metamafic rock (Figure 9b); the latter rock types also occur in a range of shapes and sizes throughout the rubble walling. These rock types occur in rock outcrops by the castle that have obviously been quarried in the past, and much of the walling stone was probably sourced from outcrops and superficial deposits near to the castle.

A sample of the metamafic rock was collected from a small block that had become detached from the rubble walling. A thin section was prepared and is described in Appendix 1. The rock is foliated metamafic rock, almost certainly from an intrusion in the Dalradian Supergroup. This rock type is mapped immediately underlying Castle Sween on BGS geology maps. The bedrock c.200 m to the southeast of the castle is assigned to the Crinan Grit Formation (which consists mainly of metasandstone), and the metasedimentary rocks in the walling will have come from this unit.

Decorative stone

Flaggy metamafite

Tabular blocks of flaggy, strongly foliated, greenish-grey metamafite have commonly been used to form lintels and sills around door and window openings, some of which can also feature sandstone quoins (Figure 8c). This flaggy metamafic rock has similar geological characteristics to the stone at Doide quarry (Appendix 1). This observation agrees with the assessment by Walker (2000), who cited Doide quarry as a source of decorative stone used in Castle Sween:

“In medieval times the quarries at Doide on Loch Sween supplied stone slabs of the green, chlorite-albite schist for numerous crosses and tombstones, and it was also used in building for pinnings and lintels, as at Castle Sween… This outcrop has vertical joints which allow the rock to be split into slabs a few centimetres thick. It was easily carved and was the preferred stone for this purpose for many centuries”.

However, the rock is essentially closely similar to some of the walling stone, and some at least may have been sourced from one or more other sites in the vicinity.

Figure 8 Plans for Castle Sween with periods of construction and alteration. Source: RCAHMS (1992) , pages 246–7.

Light brownish grey sandstone

Blocks of uniform, fine- to medium-grained, light brownish grey sandstone have been used to form quoins and dressings, including those forming the archway at the main entrance to the castle. (Figure 9a, d). All of the sandstone dressings in the castle appear to be of the same stone, which is present in all of the stages of construction. In some blocks the sandstone contains thin quartz veins, iron-stain banding and concretions. Many of the sandstone dressings are only around 20 cm deep, and are placed around doors and window surrounds as well as quoins. Some sandstone blocks are experiencing rapid material loss (through scaling and granular disintegration), and some show signs of alveolisation (salt-weathering) in places Figure 9b, d).

Figure 9 Decorative stone in Castle Sween.
a – detail of weathered sandstone block on the E side of the main doorway in the SW elevation of Castle Sween. b – sandstone quoins showing alveolisation (deep pitting), at the SE corner of Castle Sween. c – a lintel and sill formed of tabular blocks of metamafite on the NW-facing elevation of the N side of Castle Sween. d – sandstone dressings around the main doorway in the SW elevation of Castle Sween.

A thin section of the light brownish grey sandstone was prepared from a fragment of stone that had fallen to the ground. The thin section reveals that the stone contains rounded, wind worn-grains of metamorphic rock that are partly enclosed in a mineral cement of calcite (Appendix 1). The wind-worn sand grains indicate the sandstone was deposited in a desert environment, and therefore is probably of Permian or Triassic age. The pale grey colour is unusual for Permian or Triassic sandstone, but greyish white Permian-Triassic sandstone with a calcite cement has been described at Muasdale on the west coast of Kintyre (see Permian-Triassic sandstone). Based on the available evidence, this is the likeliest source of the sandstone in Castle Sween.