Because stones are pieces of rock, we need to know how different kinds of rocks are made. Unlike the Moon, the Earth is chemically and physically active and constantly changing. In our short lives this may not be obvious, but buildings soon fall into ruin when not maintained. While this paragraph was written a passer–by was killed by masonry falling from an old building in Edinburgh, and people were narrowly missed by stone falling from two buildings in Perth. Think, then, of the changes that occur over hundreds of millions of years! During the vast periods of geological time not only do buildings decay but entire mountain ranges are reduced to sand and mud.
Even when not polluted by human action, Earth’s atmosphere is chemically reactive. Oxygen, which is necessary for our survival, is powerful enough to cause iron to decay – we call this rust – and the iron that is a common constituent of many rocks is not immune. We would not survive if the amount of oxygen in the atmosphere were much reduced, and a rather small increase could lead to spontaneous combustion of wood. Water, too, can in time destroy even the hardest rock. Wind and rain play their part in the destruction. Water expands on freezing. This process, many times repeated, enlarges cracks, causing pieces of rock to break off and fall under gravity to contribute to an accumulating scree slope below. Look up and see that in many buildings sizeable plants have taken root in crevices. Plants are partly responsible for dislodging masonry that occasionally falls from the rooftops.
At the surface of the Earth, rocks breakdown physically into smaller pieces and breakdown chemically into fine material such as clay and mud. This destructive process is necessary for the production of soil and is therefore essential for our existence. Under the action of gravity, running water carries the products of disintegration and decay to ever lower elevations as boulders, pebbles, sand grains, and clay. On their journey the loose pieces, colliding with one another, break into smaller pieces. Ultimately all these must be deposited as layers of sediment (sand and mud) on the sea floor. Given enough time all dry land will be destroyed.
Earth’s hot and active interior provides the power to move the crustal plates, and when plates collide new mountains are raised from the debris of former worlds. During this process the sedimentary debris is solidified by heat and pressure and by deposition of minerals (such as calcium carbonate) from water circulating between the grains. Thus sand is converted into sandstone and mud into shale. We make bricks by heating clay in kilns, and nature does something similar. If temperature and pressure are high enough, grains of quartz in a sandstone are recrystallised as quartzite; shale is converted into slate or, at even higher temperature and pressure, into glistening mica schist. This is how the rocks of the Highlands were made. The process is called "metamorphism" (i.e. change of form). At still higher temperature the rocks melt, and granite forms as the molten rock (magma) slowly cools deep below ground. Crystals of quartz, feldspar, mica, and other minerals grow large enough to become obvious to the naked eye – granite indeed means grained rock. Sometimes magma erupts from below the Earth’s crust, feeding volcanoes and often covering vast areas with great sequences of lava flows. The thickness of the lava flows in in Kinnoull Hill area is over 2400m! [BGS Geology of the Perth & Dundee Area, 1985, p.94] In Scotland these hard, black volcanic rocks (technically andesite, basalt, and dolerite) are often called whinstone.
Erosion and destruction of older rocks, followed by uplift and creation of new land, involve a remarkable recycling of material. The cycle has been repeated over and over again during the 4,500 million years the Earth has been in existence.
These are the processes that created our natural building stones. The available building stones near Perth are as follows:
For more information on their nature and distribution see the British Geological Survey maps 1:50 000 Series, Perth Sheet 48W Solid Edition, 1983, and Drift Edition, 1985; also Geology of the Perth and Dundee District, HMSO, BGS, 1985, 108p.
In the Bridge of Earn area and in the Carse of Gowrie as far as Dundee, red sandstones of Upper Devonian age (about 360 million years old) have been used for building stone and flagstones. At Clashbenny, east of St Madoes, they have yielded fossil freshwater fish. Late–glacial marine clays (about 14,000 years old) are used for brick–making at Errol.
Metamorphic rocks (about 500 million years old) north of the Highland Boundary fault include slates that were formerly worked at Dunkeld and Logiealmond for roofing in Perth.
Before discussing particular stones and buildings, the following general points should be noted:
Until the arrival of the railway in 1848, Perth was largely confined to the flood plain of the Tay, of which the North and South Inches are extensions. Maps show that expansion, first to the west from the railway station, and slightly later towards the north of the old town, had taken place by 1893. [See A Vision of Perth, Denis Munro, Perth & Kinross Libraries, 2000]
An unusually "primitive" wall labelled Old City Wall forms the north side of Albert Close, between George Street and the north end of Skinnergate. This wall is on the line of the original defensive wall, which used the city lade as a moat, but the present wall lacks the strength required for a serious defensive structure. The Scottish Urban Archaeology Trust discovered the foundations of the city wall in excavations at the west end of the existing wall. The foundations, faced on the north side with ashlar (dressed rectangular stone blocks), were thicker than the present wall [personal communication, David Bowler, SUAT]. A conspicuous feature of the wall is the variety of rock types used in its construction. Pieces of sandstone in different sizes and shapes are common. Unlike the more massive sandstones common in other Perth buildings, the sandstones in this wall are mainly small slabs split along prominent bedding planes. The wall also contains rounded boulders of massive sandstone.
Of particular interest are clusters of water–rounded quartzites, green schist, and schist with large crystals of garnet. These rocks have come from the Highlands. Perhaps they were taken from the river – the beach at the north end of Friarton (Moncreiffe) island is composed of pebbles of similar rocks. I know no other Perth wall that incorporates Highland rocks, although the wall of the barn at Muirhall farm contains Highland rocks, doubtless taken from the deposits left by the retreating ice–age glaciers. The history recorded by these strange rocks – strange at least to Perth – is amazing! Once upon a time, perhaps a thousand million years ago, there were mountains, formed no doubt like present–day mountains by plate collisions. Weather and erosion slowly but surely wore away these mountains, and the debris of sand and mud was deposited in a long–vanished ocean. The unusual rocks in Albert Close are relics of this debris, and tell us something about the otherwise long–lost mountains.
Some 700 million years after the mountains were formed, the ocean that had received the debris was destroyed by another plate collision: the sediments on the sea floor were squeezed and heated; sand and mud reacted chemically to produce new minerals, such as garnet and mica, stable under the new conditions of high temperature and pressure The once flat layers of sand and mud became contorted layers of schist and quartzite, and slowly elevated to make a new mountain–range no doubt comparable to the present–day Alps or even the Himalaya. These "Caledonian" mountains have been eroded in their turn – our Highland hills are the remnants. The Tay and its tributaries continue the work of destruction, and pieces of Highland rock litter the bed of the River as it carries them to a grave on the sea floor. Some time in the future a new land may arise from this debris. Gaze, then, in awe and wonder at the wall in Albert Close!
Without crossing the river, the nearest source of building stone was on the high ground to the west. The ground rises abruptly from the flood plain to the level of the railway station and Whitefriars. It is underlain there by unconsolidated marine strata deposited towards the end of the Ice Age when the sea stood at higher levels than it does now and Moncreiffe Hill was an island. Look west across the North or South Inches, or up South Street, and notice the marked change in slopeapproximately at the line of Methven Street.
Beyond the line of Glover Street and Feus Road the ground rises steeply again. It is underlain there by Old Red Sandstone rocks which extend from Perth north as far as the Highland Line at Birnam and throughout Strathmore, north of the Sidlaws. The sandstones are here largely concealed by debris left by the ice–sheet that once covered Scotland, though sandstone was quarried on the Burghmuir (Newhouse Quarry) within living memory. Many samples seen in old walls (e.g. at Ochil Nursing Home, Ettrick Drive) are rich in flakes of mica which makes them easy to split along the bedding planes. Some large massive sandstone blocks are displayed in the park at Fittis Road and on West Mains Avenue near Soutar Crescent. Because transportation was downhill and did not involve crossing the river, Burghmuir Quarry was a possible source when stone was first used for Perth buildings. Huntingtower quarry (south of the Crieff Road) was only a little further away. Most of the stone for St Matthews Church on Tay Street came from Huntingtower in 1872 (Miss Rhoda Fothergill, personal communication). Being a continuation of the same “horizon” (layer), as at Quarrymill, rocks from the two sites are indistinguishable. Rocks from Burghmuir quarry (Newhouse) were probably also similar. Small outcrops of sandstone on the east side of the river are recorded in the Statistical Account of 1844.
Kinnoull Hill, like most of the Sidlaws and the Ochils, consists of a succession of north–dipping andesite lava–flows (commonly called whinstone or trap), which in the Sidlaws reaches a thickness of about 1,500m. These rocks outcrop in the grounds of Kinnoull Primary School (the outcrop on the road is concealed by cement) and near the toll–house on the Dundee road. There is also a small outcrop opposite 33 Dundee Road. The rock was formerly quarried south of Scone at Muirhall farm and at a number of smaller sites. The same rock is worked south of Perth at the large Friarton Quarry. These rocks were formed by volcanic action, but volcanic activity was not confined to the Sidlaws and Ochils – it included the Oban area, Ben Nevis and Glencoe, Blackford Hill, and the Pentland Hills (south of Edinburgh), St Abb’s Head, and the Cheviot Hills. Today this type of volcanism is associated with subduction; i.e. one crustal plate being pushed underneath another. The great volcanoes of the Andes (hence andesite) are of this type because the Pacific Plate is currently being subducted under South America. Consequently the rocks of Kinnoull Hill are evidence that plate subduction took place in Scotland some 380–400 million years ago.
This black volcanic rock is very hard and tough. Because it breaks into sharp angular pieces it make good road metal. It has been used extensively in walls along the Dundee Road between the Smeaton Bridge and Branklyn Garden, though sandstone is used for Murray Royal and Friarton Prison the high retaining wall of the Garden itself. Here and there surprisingly rectangular blocks are incorporated in the walls along the road, but it has rarely been used for building in Perth. Cottages in the village of Glencarse, however, are almost exclusively built of the volcanic rock, and are well worth examining. Whinstone has also been used for houses in Abernethy, though cornerstones and the surrounds of doors and windows are usually freestone that a mason can work freely in any direction. Excellent examples of freestone can be examined in the south wall of the bridge that carries the Glasgow Road across the railway. Although the blocks are layered (stratified), the shape of individual blocks is independent of the layering.
A small abandoned quarry at Corsiehill, above St Mary’s monastery on Kinnoull Hill, is in a volcanic dyke formed when molten rock (magma) erupted from depth through a vertical fissure. Because the magma cooled slowly below ground, the resulting rock (dolerite) is coarser grained than common whinstone. I have not seen any record of how it was used, but having fewer irregular cracks it may have been suitable for making setts for paving streets, or even shaped into rectangular building blocks. Some of the setts in the streets beside St John’s Kirk and the City Hall are fine–grained with steam–holes formed by escaping bubbles of gas as the magma reached the surface. Other setts are distinctly crystalline, coming from the centre of a lava flow or dyke where cooling was slow enough to allow crystal growth. A few other setts, such as some of those in Flesher’s Vennel and its extension into South St John’s Place, are blocks of granite. These granite setts have, of course, been imported – indeed I am informed that the granite setts now being laid on the west pavement of Tay Street came from China! It seems likely that at least some of the setts in the City Centre have been brought for re–use from other streets (possibly from other cities).
The 50 million year old Corsiehill dyke is a member of a swarm of east–west dykes. Others of the swarm cut the youngest Carboniferous (coal–bearing) strata between Edinburgh and Glasgow, and must be younger than the volcanic rocks of Kinnoull Hill. As the molten rock was intruded up vertical, east–west trending cracks in the Earth’s crust, Scotland at that time must have been stretched and pulled apart by forces directed north and south. Another dyke crosses the Tay at Campsie Linn, and being resistant, creates a water fall in the river to the delight of canoeists. This dyke has been quarried at Wolfhill, a little more than a kilometre east of the river.
A large east–west dyke crosses the main road from Perth to Stirling about 1km beyond the turnoff to Tibbermore. The dyke is exposed in the road cut [046 208]. Being close to the main road it is difficult and even dangerous to study it. The rock, having cooled slowly from a molten state, consists of a mass of intergrown crystals visible to the naked eye on a freshly broken surface. The Gask road continues west and crosses the same dyke, which is then seen to the north of the road and nearly parallel to it. Because the dyke is more resistant to erosion than the surrounding sandstones it is a conspicuous feature looking like an abandoned railway embankment. The BGS [Geology of Perth and Dundee District, 1985, p95] says that this dyke was quarried at “Lamberkine” [058 208] – although the topographic map shows “Lamberkine” as an area some 2.5km to the NE of this locality.
Records show that Lamberkin quarry was once an important source of building stone.The parking area immediately behind the Sheriff Court building on Tay Street was formerly the site of the County Prison built from Lamberkin stone [Miss Rhoda Fothergill, personal communication]. The rock, which is preserved in walls adjacent to three sides of the parking area, is probably dolerite, which though black, weathers to a rusty surface.
The Murray Royal Hospital is built of similar rock, though one would assume that in that case the stone would have come from the Muirhall Quarry, only a short distance up the hill. If so its grain size would suggest that the source was one of the thicker lava flows two flows were formerly exposed in Muirhall Quarry [BGS Geology of Perth & Dundee District, 1985, p.32, 39]. This quarry – like so many others – is now filled in and no specimens are available. Mr John Dow of Muirhall farm is, however, of the opinion that the Murray Royal was not built from Muirhall stone. Whatever its source (Muirhall and Lamberkin have been suggested), the stone of this large building has been carefully trimmed into regular rectangular blocks.
The volcanic rocks forming Kinnoull Hill are in layers, each resulting from an individual volcanic eruption some 400 million years ago. The layered structure can be detected by looking from Perth at the sloping grassy areas on the west side of the hill, and is seen most clearly in the south–facing cliff above West Kinfauns when viewed from beside the well–known tower (“folly”) at the top of the cliff. The contrast between the precipitous south side of the hill and the gentle north slope is due to the fact that the sequence of lava flows, originally horizontal, have been tilted towards the north. As a result younger rocks, which originally were deposited on top of the lava flows after volcanic activity ended, are now found in the lower ground north of Kinnoull Hill. The rocks of the Old Red Sandstone extend from New Scone north to the Highland Boundary Fault, which is seen from Perth as a long straight–line feature running from ENE to WSW. These rather massive sandstones were quarried along the Annaty Burn at the appropriately named Quarrymill. It is said that there were formerly extensive quarries in the Kincarrathie area between the Isla Road and Scone Road, but I have not found any evidence in old maps or in the Statistical Accounts. Moreover the British Geological Survey’s map  shows no sites in that area where bedrock has been recognised at or near the surface.
Dr Douglas Simpson’s account of St John’s Kirk states that in 1328 "King Robert the Bruce asked permission for hewn stones to be taken from the quarries of Kinharrachie [sic] and Balcormac, belonging to the Abbey of Scone for the edification of the Church of Perth [St John’s Kirk]" [1958, p.16]. William Barclay’s history of the structure of St John’s Kirk says that the rebuilding of the nave in the 15th century "was done from the quarry of Kincarrathie under the liberty obtained by Bruce’s request to the monks of Scone" [1920, p.10]. The etymology of Kincarrathie is uncertain: perhaps it is related to Ceann ("head" – there is no K in Gaelic) and Carraigeach ("rocky" or "craggy"). A map dated 1792 shows that the Annaty Burn Woodland Park (Quarrymill) lies within the Kincarrathie estate (John Dodds, Factor: personal communication,), and there can be little doubt that this is the quarry that was at one time called "Kincarrathie Quarry". At Kincarrathie House a doocot (dovecote), built of obviously local stone, bears the date 1694; it is less than half a mile (0.7km) from the quarry in Quarrymill Woodland Park.
The Old Red Sandstone (ORS) is so–called because it is older than the strata containing the coal seams, whereas the New Red Sandstone (at Dumfries) is younger than the coals. The red colour is iron oxide (hematite) produced when iron silicates in rocks such as granite are oxidised in contact with the atmosphere. Marine sediments are grey or even greenish – never red. The ORS is the major locally derived building stone in Perth. It varies greatly in quality, but in general it has a pleasant pale purplish colour and an interesting texture. We have already pointed out that it underlies Strathmore between the Sidlaws and the Highland line. Kirriemuir and other Strathmore villages have a richer red stone than Perth, and often an attractive coarser texture. Close to the Highland line the sandstone is replaced by conglomerate (pudding–stone) which is well exposed on the coast at Stonehaven and Dunottar, and at Cargill’s Leap on the River Ericht upstream from Blairgowrie. Some of the boulders exposed south of the harbour at Stonehaven are comparable to boulders in the Colorado River at the bottom of the Grand Canyon, and are witness to the former existence of a great river with its source in the former Caledonian Mountains.
The west abutment of the Smeaton Bridge is the best place to see the local sandstone. Many water–rounded pebbles of older rocks are scattered through the red sandstone. These are typical fluviatile deposits; i.e. the sands accumulated by braided rivers, rather than by deposition on the sea floor free from atmospheric oxygen. The rocks of Smeaton's Bridge have all the characteristics of flash–flood deposits; they are massive rather than thinly bedded and poorly sorted. Rapidly running water carried the sand and pebbles and then suddenly dispersed and dropping its burden. These are all features of deposits forming today in desert environments as in Death Valley, California.
During the deposition of the Old Red Sandstone, 350–400 million years ago, Scotland was about as far south as Madagascar. It was arid and no land plants protected loose material from erosion. When torrential rains released a flash flood, the water spread on large alluvial fans and petered out on the desert floor as happens today in similar situations. When running water loses energy it drops its load, and in a desert this is likely to happen in a sudden and haphazard way. At its furthest point the muddy water will no longer be carrying boulders or even sand, and finally the water leaves behind a layer of mud which soon dries up with a characteristic pattern of roughly hexagonal mud–cracks. The hot sun dries the platelets of mud which curl up round the edges like sheets of wet paper. When the wind blows, or the next flood arrives, the flakes of dried mud are carried away and dispersed. But sometimes, without being transported more than a short distance, they are covered with fresh sand and gravel. This explains the stringers of red flakes of dried mud that are sometimes exposed when the masons cut sandstone blocks. These features are common in Perth’s older buildings: good examples are visible on the north wall of Oliphant’s Vennel; the second buttress from the east end of the exterior north wall of St John’s Kirk, and the south–east corner of the base of the great pillar at the north–west corner of the crossing; on the front wall of Kinnoull Primary School; and on the west side of the wall along Platform five – the oldest platform in Perth railway station.
It is impossible on lithological evidence alone to say from which quarry or quarries in the Lower Old Red Sandstone these rocks have come, but despite the near five hundred year span of building history, the conditions of deposition of the original sands were very similar. Architects obviously took more care to choose sandstone free of pebbles when stone was selected for an important building like St John’s Kirk than for the Smeaton Bridge, but there are nevertheless plenty of examples of pebbles in the exterior walls of the Kirk, and there are even some in the interior walls; e.g. to the left of the door leading from the nave to the Halkerston Tower.
When a small delta is being built, sand grains are carried along the top surface until they tumble down the front slope of the delta. In this way the front of the delta moves forward. As a result the successive former delta fronts will be seen in cross–section as "cross bedding" at a steeper angle than the bed itself.
|The growth of a small delta:
Water moving in the direction of the arrow carries sand to the front of the delta. A layer of sand deposited on the steep frontal slope of the delta becomes tangential to the floor.
The delta advances to the left, its internal structure being a series of cross beds.
|A more energetic water current will erode sand already deposited. The diagram shows the structure after half the sand has been removed.|
|Erosion and sedimentation may alternate. This diagram shows a second crossbedded layer (from a different source) with layers tangential to the truncated cross beds below.|
|This example shows younger horizontal layers deposited on older truncated crossbeds.|
Click here for examples of cross bedding
The cross bedding common in Perth’s local Old Red Sandstone tends to involve coarse sand and pebbles, showing that deposition was spasmodic.
Evidence provided by the British Geological Survey [Scottish Journal of Geology, 34, 1998, p.145–152] includes petrological descriptions of sandstone from Quarrymill, Lethendy, Huntingtower, Letham House, Crossgates, and Kingoodie (Invergowrie), and is consistent with the suggestion that the famous Stone of Scone (Stone of Destiny) came from Quarrymill [the story of the Stone is given by Pat Gerber in Stone of Destiny. Edinburgh: Canongate, 1997, 204p]. The BGS authors wrote: "A few small old quarries also exist but are in most cases overgrown or infilled. Perth’s infilled Burghmuir Quarry and the now poorly exposed Quarrymill/Kincarrathie quarries are the only sandstone workings in the Scone (Perth) area of the appropriate antiquity to have supplied the Stone of Destiny" (p.151). They show Kincarrathie on their map (fig.4) as a place name and not either as a quarry or as a bedrock outcrop. The authors make the following observations: "It has to be acknowledged that even a close lithological match can only indicate a probable origin, and that comparable rocks may well be present in the ORS [Old Red Sandstone] or in other formations, while features such as colour, porosity and proportion of lithic grains [pieces of rock] may vary over small distances at outcrop. The best that can be hoped for is to be able to state that a particular sample site might have been the source, and to exclude certain other sites" (p.151). For the same reasons any attempt to match building stones with geological sources is uncertain if builders’ records are not available – and they seldom are.
Most of the walls along the Isla Road between Bridgend and Upper Springland are built of either pale red and poorly sorted sandstones of local origin or of whinstone. The association suggests that the locality is close to the contact between the volcanic rocks and the overlying sandstones.
Stone from Quarrymill was shipped downstream to build Smeaton’s Bridge [Dr Iain Robertson, personal communication]. Work began in 1766 and the bridge was completed in 1771 [George Penny. Traditions of Perth, 1836; Statistical Account, 1844]. Although the quarry is now overgrown there are some poor exposures at Quarrymill, the local walls display the nature of the lock rock, and the bridge itself displays magnificent samples to which there is easy access at the west abutment.
St John’s Kirk, the oldest building preserved in Perth, was granted to Dunfermline Abbey in 1126, but nothing of the earlier building remains above ground. If the three consecration crosses in the choir are correctly identified, that part of the Kirk had been built before 1242. The stones in the choir are smaller, less regular in shape, and less well finished that those in the nave. All appear, however, to be of local sandstone. In many places in the building – and notably between the crossing and the nave – there is evidence that old walls have been modified, patched, and rebuilt, no doubt old stones being reused. The pillars in the nave are noticeably simpler than those in the choir.
About 1400 the Chapel of St James, on the south side of the Kirk, was repaired, and in 1440 funds were assigned for "building the choir and porch of the parish church" [Simpson, p.20]. About the same time the names of Bailie John Fuller and his wife – no doubt donors – were commemorated on the pillar towards the south–east end of the choir, and a reference in 1448 to "the new choir of the parish church" shows that this part of the Kirk was completed by about 1450. Dr Douglas Simpson says that the nave was completed by 1500 and the central tower before 1511 [Simpson, p.20, 39]. He dates the vaulting in the porch of the Halkerston Tower as 15th century [Simpson, between p.22 & 23]. A programme of repairs to the nave began in 1598 [Simpson, p.26], and inspection shows that many walls were modified after they were first built. The north transept was shortened as late as 1823 in order to widen St John’s Place, and about the same time the exterior of the choir was refaced [Simpson, p.31–32]. Sir Robert Lorimer was responsible for a major restoration of the Kirk between 1923–1926.
A house in the Watergate bears the date 1725, and although commemoration stones of this kind can be reused at later dates, there is no reason to doubt that that this is indeed the construction date. Unfortunately, here, as in a number of other buildings, the walls have been plastered and painted so that the stone work is concealed. The date 1780 is found high on the gable of the first building on the west side of George Street at its junction with the High Street. Once again the stone work is concealed. Uncoursed walls made of random rubble (rough and ready local sandstone of poor quality) can be seen in the backs of some buildings in the city centre; for example, some with frontages on George Street, St John’s Street, and St John’s Place would do poorly in a moderate earthquake.
With increasing affluence, Perth’s builders looked further afield for better quality stone. The earliest record is in 1379 when King Robert II arranged for stone for his own monument to be brought from Holyrood Abbey to Leith and then carried by sea to Perth [Simpson, 1958, p.18]. As the Tay was navigable upstream as far as the High Street, it would have been relatively easy to bring stone from Kingoodie, on the shore of the Tay near Invergowrie (though this is Old Red Sandstone), or from the Lothians.
The New Town of Edinburgh is built of grey sandstones of Carboniferous age (about 350 million years old) from Craigleith, Granton, Hailes, Cramond, Redhall (Slateford), Binny (Uphall), Dalmeny, and other local quarries [For information on these quarries as sources of building stone see Andrew McMillan (Editor): Building Stones of Edinburgh, Edinburgh Geological Society, 2nd edition 1999]. The statue of Prince Albert on the North Inch (1864) is carved from Redhall stone (Miss Rhoda Fothergill, personal communication). I do not know the date of the oldest Perth building using imported stone, Tay Street, however, provides many examples of buildings constructed of high quality grey sandstone presumably of Carboniferous age. On the east side of King Street and on Marshall Place, for example, houses with fine facades of grey sandstone have side walls of local red sandstone. It should be mentioned that the dark grey colour is superficial: after buildings in Edinburgh’s new town have been cleaned the exposed colour is often brilliantly white or yellowish. There are places in Perth where corners have recently been damaged and the broken surfaces gleam surprisingly white and sugary in appearance; e.g. the buttresses of the south wall of St Matthews Church on Tay Street and the north west corner of the Pullar Building on Kinnoull Street. It doesn’t take long, however, for the accumulation of dirt to darken the rock again.
Unlike the Old Red Sandstone (Devonian age) the sandstones of Carboniferous age (Fife and the Lothians) were deposited in shallow–water marine deltas; this is why they lack the oxidised iron that gives a reddish or purplish colour to Perth’s local rocks. Between 1826 and 1873 several enormous fossil tree trunks (up to 14m long and 1.8m thick) were found in Edinburgh’s Craigleith quarry, which is now filled in. The trees had apparently been swept into the delta during a flood and buried before the wood had rotted. [see references in Building Stones of Edinburgh]
A characteristic feature of these shallow–water marine sediments is the prevalence of "cross bedding". This is a little different from the typical cross bedding in the Old Red Sandstones. The cross layering within a bed of sandstone is finer and more sigmoid; that is, it begins with a gentle slope, then steepens before flattening out again tangential to the base of the main bed. Moreover the upper part of a cross–bed has often been eroded away before the next bed has been deposited. Consequently the cross bedding is tangential to the base of the bed but is truncated at the top. Observing this one can tell whether a block of cross–bedded sandstone is the "right way up" or whether the builder has turned it upside down. Many of Perth’s grey sandstones show these features, but they are particularly obvious on the east and south walls of the Sheriff Court building on Tay Street.
The original grains of sand (mainly quartz) are held together by a cement deposited by percolating water over millions of years. The commonest cement is lime (calcium carbonate), but is sometimes iron carbonate or even silica. The durability of the stone depends very much on the efficiency of the natural cement, silica being the best. One can therefore see considerable differences in the amount of weathering of different stones under nearly the same conditions. Examples of sandstones that are uniformly well cemented and unweathered (though discoloured) freestone can be seen on the capitals at nos.50–52 and 63–66 (the former museum) Tay Street; the corner of the Old City Chambers at High Street and Tay Street; the High Street fašade of the Council Building; and at the SE corner of the High Street and King Edward Street. (The light coloured stone on the Council Building is from Woodburn Quarry, Cumberland: Miss Rhoda Fothergill, personal communication). There are many examples of other grey sandstones that have suffered considerable weathering; e.g. pillars on South Street at Tay Street.
The City Hall is made of a uniform, rather fine–grained greyish sandstone with numerous excellent examples of truncated cross bedding and even one or two examples of contorted beds resulting from the escape of water during the compaction of what were once loose sand layers at the bottom of the sea. The best places to look for these sedimentary features is on the south and east walls, especially on the parts of these walls towards the SE corner of the building. The texture and colour of this rock differs from that of the Sheriff Court, or indeed of other buildings on Tay Street. According to The Perthshire Advertiser, 1911, the City Hall is "built of bluish grey stone from Leoch Quarries, Forfarshire." [Miss Rhoda Fothergill, personal communication. The Leoch quarry [NO359361] was still being worked in 1952 [BGS Geology of Perth and Dundee District, 1985, p.94]
About 250 million years ago Scotland was near the equator and the conditions were like the Sahara today. Some of the great sand dunes from that time are preserved in the New Red Sandstone (Permian) of the Mauchline and Dumfries areas, notably in the Corncockle and Locharbriggs quarries, where many footprints of early reptiles have been found (Robert Boyle, Trans. Geological Society of Glasgow, vol.13, 1909, p.344–384; M.E. Brookfield, Sedimentology, vol.24, 1977, p.303–332 & Scottish Journal of Geology, vol.15, 1979,p.81–96; P.J. McKeever, Scottish Journal of Geology, vol.30, 1994, p.11–14). Sand dunes in the desert are constantly changing. The wind sifts the grains and drives them up the gentle slope of the dune. When the sand reaches the summit of the dune it cascades down the steep front, and the dune slowly migrates downwind. This produces dune (aeolian) cross–bedding which is similar to, but not the same as, stream cross–bedding. Unlike sand grains in a river (fluviatile) grains carried by wind are better sorted and, not being buffered by water, are more rounded and have a frosted appearance. Each grain is commonly coated with iron oxide (hematite), which accounts for the uniform red colour of the rock.
The opening of the railway in 1848 opened the possibility – abundantly fulfilled before the end of the century – of transporting the superior Locharbriggs red sandstone from Dumfries (Peter F. Marshall, The Scottish Central Railway: Perth to Stirling, Usk, Mon: The Oakwood Press, 1998, 248pp. p.94). The red stone, which was used for the steps below the Statue of Liberty in New York, is a conspicuous feature in many parts of Perth: the Sandeman building, and the former Gloag building on Kinnoull Street along to the High Street, and up to Caledonian Road; the Salvation Army building on South Street; the buildings on South St. John’s Place; the Caledonian Road School; Perth Christian Centre on the Glasgow Road, the former Middle Church on Tay Street (now flats), and some buildings, including the Perth Theatre in the High Street. These are some of the buildings built of stone that was formed in great sand dunes on an arid land. The characteristic desert sand–grains can be seen with a handlens – though more easily if a broken piece is available. The pillars of the new flood gates on the North Inch adjacent to the Smeaton Bridge contain beautiful freshly–cut Dumfries Stone (presumably Locharbriggs) displaying the fine layering and uniform constitution found in a sand dune. Examples of truncated cross–bedded dune sands are common in many of Perth’s red sandstone buildings; see especially the north wall of the Sandeman building, the north side of the High Street immediately east of Methven Street, and the lower part of the wall at Peddie’s at the top of the High Street east of Caledonian Road.
If we look around with open eyes and open minds, it is truly marvellous what we can see in old stones. As John Playfair said of a geological field trip he attended in 1788, "The mind seemed to grow giddy by looking into the abyss of time; – we became sensible how much farther reason may sometimes go than imagination can venture to follow."
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