OR/15/053 Summary and conclusions

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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.

This study of the provenance of building stones in galley castles has produced the following conclusions. A summary of the conclusions is presented in Table 6.

  • The main bedrock in Argyll is a thick sequence of sedimentary strata with intrusions and extrusions of igneous rock (the Dalradian Supergroup). The entire sequence was folded and metamorphosed during a major geological event (the Caledonian Orogeny) 470 million years ago involving the closure of an ocean and collision of continents.
  • All the metamorphosed rock in the Dalradian Supergroup has a weak to strong foliation (a planar, parallel arrangement of the constituent minerals along which the rock splits preferentially, producing tabular blocks).
  • Most of the Dalradian Supergroup consists of metamorphosed sedimentary rocks, but metamorphosed intrusions of silica-poor igneous rock (metamafite) are relatively abundant around Loch Awe and in Knapdale.
  • Outcrops of sandstone are relatively rare in Argyll, but small exposures occur in several localities close to the mainland coast and on some of the outlying islands. Beds of Devonian sandstone crop out close to the coast between Dunstaffnage and Knapdale. In places, sandstone strata of Carboniferous, Permian, Triassic, Jurassic and Cretaceous age crop out in coastal exposures where they are preserved beneath beds of Palaeogene lava that erupted due to a period of rapid crustal thinning as the Atlantic Ocean opened 60 million years ago. A sequence of sandstone beds of Carboniferous, Permian, Triassic, Jurassic and Cretaceous age crops out at Ardtornish and Inninmore Bay, in Morvern. Jurassic and Cretaceous sandstones crop out nearby, at Lochaline. A small outcrop of Carboniferous sandstone occurs at Bridge of Awe. Permian sandstones also crop out in small exposures on the west coast of Kintyre (the Bellochantuy Bay Formation) and quite extensively on the coast of Arran (the Corrie Sandstone). Jurassic and Cretaceous sandstones crop out at Carsaig on the south coast of Mull.
  • In all of the Argyll castles, the main walling stones are derived from the local Dalradian Supergroup bedrock and/or from loose (superficial) deposits lying on top of the Dalradian bedrock. In each case, the walling stones very probably would have been sourced close to the castle.
  • In Castle Sween the walling stone consists mainly of blocks of metamorphosed sandstone and metamafite (both Dalradian). The castle sits on an outcrop of Dalradian rocks that has been quarried quite extensively.
  • In Skipness Castle the walling stone consists mainly of blocks of metamorphosed muddy sandstone; this is typical of the local Dalradian bedrock (the Beinn Bheula Schist Formation).
  • In Dunstaffnage Castle the walling stone consists mainly of blocks of metamorphosed sandstone (Dalradian), granite (sourced from superficial deposits but probably originally from the Ben Cruachan Granite Pluton), and lava (sourced from superficial deposits but originally from the nearby Lorne Plateau Lava Formation).
  • In Kilchurn Castle the walling stone consists mainly of blocks of metamafite. The castle sits on an outcrop of metamafite that has been quarried quite extensively. The walling stone was probably sourced from this outcrop, from other outcrops of metamafite around the north end of Loch Awe, and from nearby superficial deposits.
  • In Kilchurn Castle, virtually all of the decorative stones consist of metamorphosed igneous rock. There may be two types: metamorphosed intrusive mafic igneous rock, and metamorphosed pyroclastic-rock. Both were probably sourced from outcrops around the north end of Loch Awe. A few blocks of pink sandstone are recorded in Kilchurn Castle (not seen by BGS). These are believed to have come from a castle on nearby Fraoch Eilean and originally from the outcrop of Carboniferous sandstone at Bridge of Awe.
  • In the three other Argyll castles most of the decorative stone is sandstone, but a small proportion is flaggy metamorphosed rock.
  • In Castle Sween tabular blocks of metamafite used mainly to form lintels and sills were almost certainly sourced from outcrops close to the shore of Loch Sween, possibly including the nearby quarries at Doide. The main decorative stone is light brownish grey, poorly consolidated aeolian (desert) sandstone. An outcrop of stone with similar characteristics is described in BGS records at Muasdale on the west coast of Kintyre; this is considered to be the likeliest source of the sandstone dressings in Castle Sween, though further evaluation is required to test this.
  • In Skipness Castle tabular blocks of the local bedrock stone (Beinn Bheula Schist Formation) have been used to form lintels and sills. The original (and main) decorative stone is pink sandstone which is typical of Permian sandstones and almost certainly came from Arran. Blocks of white sandstone were used in the 16th century to form structural corbels on the tower and scattered replacements for pink sandstone dressings. This sandstone shares several characteristics with the pink sandstone and may be a ‘bleached’ Permian sandstone from Arran.
  • The oldest sandstones in Dunstaffnage Castle a white to buff and a light buff sandstone are likely to be Carboniferous sandstones and probably come from the Inninmore Bay/Ardtornish area of Morvern. Tabular blocks of purplish Devonian sandstone, probably from nearby outcrops of the Kerrera Sandstone Formation, were introduced in the 16th century and used to form arched openings, lintels and sills and some walling stone blocks. Blocks of white sandstone have been used to form two fireplaces in the 16th century dwelling house, and a large column (recycled lintel?) now supporting a more recent stairway. This sandstone may have been sourced from the Loch Aline White Sandstone Formation, at Loch Aline in Morvern. A single tabular block of pyrite-bearing metamorphosed mudstone has been used to form a lintel in the 16th century dwelling house; this is likely to have come from the Easdale Slate Formation, probably from the Easdale and ‘slate islands’ area.
  • A sample of the decorative stone from Kisimul Castle is very strongly foliated (mylonitic) metamafic rock. The intensity of deformation is typical of rocks from the Outer Isles, and the rock was probably sourced from an outcrop near to Kisimul Castle.
  • The evidence from this study suggests that metamorphic sedimentary and igneous rocks forming part of, or emplaced into, the Dalradian Supergroup were selected for their functional properties (splitting along foliation) during the construction of the Argyll galley castles; they are not from a common source and probably were collected from the nearest available source of strongly foliated stone (good for forming tabular blocks) that was known to the castle builders.
  • The sandstones have come from many sources, probably all of which were in the Argyll coast and Arran area. The castle builders clearly were prepared to transport these decorative stones over considerable distances by sea.
Table 6 Summary of building stone provenance in Argyll galley castles, based on geological evidence

Site

Walling stones

Decorative stones

Main stone type(s) Bedrock unit Source area Sample Stone type Bedrock unit Source area Sample Masonry elements
Castle Sween meta-sedimentary rock Crinan Grit Formation probably mainly from outcrops and superficial deposits close to the castle N/A foliated
metamafite
Dalradian – 
‘Metabasaltic-rock’
Loch Sween coast, possibly including Doide quarry N/A lintels and sills
metamafite Dalradian Supergroup ED11449 light brownish grey sandstone Permian – Bellochantuy Bay Formation Muasdale, Kintyre ED11441 quoins, window surrounds and door surrounds
Skipness Castle variably foliated meta-sedimentary rock







occasional rounded blocks of granite

Beinn Bheula Schist Formation probably mainly from outcrops and superficial deposits close to the castle ED11448 foliated meta-
sedimentary rock
Beinn Bheula Schist Formation probably mainly from outcrops close to the castle N/A lintels on some doors and windows; smaller blocks used to form archways
pink sandstone Permian-Triassic – Corrie Sandstone Arran N/A quoins, window and door surrounds, archways, parapet slabs; the main dressing stone
white sandstone Permo-Triassic – New Red Sandstone Supergroup Arran or Kintyre N/A corbels on the tower and random dressings (repair?)
Dunstaffnage Castle original walling

















later additions/ modifications

volcanic rock – lava






granite





pasmmite



pyritic meta- mudstone


Devonian sandstone

Lorn Plateau Volcanic Formation





Cruachan Granite Pluton





Dalradian Supergroup

Dalradian Supergroup



Kerrera Sandstone Formation

probably mainly from outcrops and superficial deposits close to the castle N/A white to buff sandstone Carboniferous – Scottish Coal Measures Group Inninmore Bay, Morvern ED11439 (castle);

ED11440 (chapel)

quoins, window and door surrounds, arrow slit surrounds and archways; the original dressing stone, much recycled
light buff sandstone Carboniferous – Scottish Coal Measures Group Inninmore Bay, Morvern/ N/A the main doorway surround and nearby quoins in the late 15th/ early 16th C
flaggy purplish sandstone Devonian – Kerrera Sandstone Formation probably local N/A lintels and arches in 16th C alterations
light grey sandstone Jurassic – Loch Aline White Sandstone Formation Lochaline area, Morvern or Carsaig? N/A two first floor fireplaces in the dwelling house and a large block (recycled lintel?) supporting a stairway
pyritic metamudstone Easdale Slate Formation the ‘slate islands' around Easdale N/A an alcove lintel in the kitchen of the dwelling house
gritty buff sandstone Carboniferous not known N/A first floor window dressings of gatehouse
uniform buff sandstone Carboniferous not known; possibly Midland Valley N/A dressings in the gatehouse tower
Kilchurn Castle variably foliated metamafite outcrop immediately adjacent to the castle N//A well foliated pyritic metamafite Tayvallich Volcanic Formation/Dalradian Supergroup unidentified quarry in Loch Awe area N/A quoins, lintels, sills, corbels on tower, door & window surrounds