The History Of Masonry
Director of Technical, Conservation, Research and Education, Historic Scotland.
The introduction of farming into Scotland during the Neolithic period created a demand for permanent structures from about 4000BC. Dating from c.3500 – 3100BC the stone building at Knap of Howar, Papa Westray, Orkney is one of the oldest surviving houses in NW Europe. Using local material gathered from the immediate vicinity, the rubble dry-stone construction illustrates the builder’s remarkable understanding of the structural use of stone, and an ability to exploit its natural properties to advantage.
The strength of dry-built rubble stonework relies upon each stone physically touching its adjacent and underlying neighbour. Each successive layer of masonry has its vertical joints staggered to “break bond” so that the weight of every stone is effectively borne by those lying immediately below. This was spectacularly achieved at Knap of Howar where a further sophistication is the form of the figure-of-eight plan-shape employed in each of the two adjacent blocks. Here stones marking the neck of the eight in each apartment are neatly aligned to form a slot into which vertically set thin stone slab partitions are located. The curvilinear form of construction adds to the structural strength of the building whilst simultaneously retaining the mounded earth fill behind the wall faces.
Dating from c.3100 – 2200BC, the best preserved sequence of Neolithic building developments is at Skara Brae, Orkney. Continuing the Knap of Howar constructional techniques, the juxtaposition of curvilinear low stone-walled houses, and interconnecting streets, created an integrated village community. Exposed by a storm in 1850 the complex utilizes the free flowing nature of dry-built rubble work and contains sophisticated stone furniture, stone-on-edge partitioning, with door, window and passageway openings spanned by slab lintels. The various voids between the house walls were infilled with compacted midden material to create an effective waterproofed wall core.
Readily available in Caithness and the Orkney Islands, Old Red Sandstone is an ideal material for effecting sophisticated masonry construction. This is nowhere better illustrated than inside the Neolithic Maes Howe chambered tomb of c.2700BC, where a 4.6 metres square and 3.8 metres high central roofed chamber was created by corbelling out successive courses of large parallel-sided slabs. Above a datum of 1.4 metres, the diminishing plan dimensions occurring on all 4 sides of the chamber eventually permitted the placing of a single slab to cap the central void. To help stabilise the structure, buttressing monoliths were positioned at each of the four corners and braced against the walling by a tightly bonded horizontal infilling of carefully chosen stonework. Earth cover over the entire central masonry chamber, and 11 metre long entrance passageway, create a mound 35 metres in diameter and 7 metres high. Additionally, another natural material, clay, was applied as a protective waterproof layer over the masonry work to help keep the internal chamber dry.
The largest and most extensive group of prehistoric cup-and-ring marked rock carvings in Scotland exists at Achnabreck in Argyll. Whist a variety of enigmatic concentric circles, alignments and other features are cut into three exposed rock outcrops the examples are representative of the first true form of stone art to be found in Scotland. Although no full understanding of what the purpose of this art form was, similarities can be identified in the overall form and plan shape of Broch sites some millennia later.
Standing 13 metres high with a 5 metre internal diameter, the double walled dry-stone Mousa Broch on Shetland represents the pinnacle of rubble-stone building construction. Erected c.200BC – 100AD, the tower is the best preserved example of Iron Age architecture in Scotland. Utilising roughly dressed stone, the concentric walls are carefully built in tightly placed horizontal courses. The cylindrical face of the internal skin rises almost vertically, whilst the external skin is slightly inclined to produce a truncated conical exterior. The two walls are linked by a series of interconnecting stair and passageways which spiral up to the wall top. Larger sized stones were carefully chosen to serve as stair treads and passageway lintels and, like the rubble work, are roughly dressed. Similar forms of Broch construction are found throughout the northern Isles and mainland of Scotland, extending as far south as the central belt.
Discovered in 1929, the Broch of Gurness in Orkney is of similar construction to Mousa, and at c.20 metres in diameter, might originally have stood 8-10 metres high. Set within a series of concentric outer defensive ditches and walls, a free flowing, irregularly planned village of interconnected houses surrounds the broch. Continuous runs of thin stone slabs on edge were used to create subdividing partitioning within each house. Constructed between 500 – 200BC the site remained in use until c.100AD.
Continuing the pattern of building in rubble masonry, the multi-period site at Jarlshof, Shetland spans from c.2000BC – 17th century AD. The earliest buildings on the site consist of small oval shaped houses, followed by a Bronze Age smithy, and an Iron Age broch and village of round houses with two small souterains or earthhouses. These were overlaid with the remains of four first millennium AD wheelhouses,
a Norse settlement,
a medieval farm, a 16th century Laird’s house and, finally,
a 20th century visitors office –
all constructed in stone. Spanning this range, the site provides a unique snapshot of how horizontally coursed dry stone rubble construction first seen at Knap of Howar continued across the ages. In emphasis of the relevance of this technique, the same process of building two concentric sets of walls infilled with loose material was still in use almost 4000 years later in the construction of traditional Hebridean Black Houses on Lewis during the 1860’s.