OR/13/015 Survey details and outcomes
|Gillespie, M R, Everett, P A, Albornoz-Parra, L J, and Tracey, E A. 2013. A survey of building stone and roofing slate in Falkirk town centre. Nottingham, UK, British geological Survey. (OR/13/015).|
Scope, methodology and limitations
The survey of building stones and roofing slates encompassed all of the buildings that are contained within, and face onto, the Townscape Heritage Initiative (THI) area in Falkirk (Figure 2). The THI area includes much of the historic core of the town, including: the most historic sections of Cow Wynd, High Street, Vicar Street and Newmarket Street; all of Kirk Wynd; most of Manor Street, Glebe Street and Melville Street; and short sections of Bank Street and Princes Street (Figure 6). The southern half of the THI area (more or less south of Manor Street) is the oldest part of the town centre. This area is characterised by an irregular street pattern and includes the oldest surviving buildings. In the younger, northern half of the THI area, the streets and buildings have a more regular ‘planned’ layout.
The client supplied a GIS shape file in which the location of structures representing discrete buildings is illustrated using polygons (Figure 6). For the purposes of the survey, each polygon was treated as a single building. Information attributed to each polygon in the shape file included two means of uniquely identifying each polygon: an ‘FID_1’ identifier, which has twelve letters and numbers; and a ‘PRIMARY_KE’ identifier, which is a six or seven digit number.
182 polygons lie within, or face onto, the THI area. Most of these represent substantial buildings, but they include smaller structures like outhouses and a number of closes and vennels (ten polygons). Closes and vennels were not included in the survey, as they are not buildings. A few polygons have no associated construction of any sort; these presumably represent buildings that have been demolished. 172 discrete buildings/polygons were included in the survey.
Five of the buildings included in the survey are not in publicly accessible areas and do not have a street-facing elevation. These polygons were included in the survey but details for the building stone and roofing slate are not recorded.
Solid, natural stone masonry and roofing slate were assessed for the survey. Building stone used in thin panels as cladding or facing stone was excluded. Stone panels are relatively rare, but different building stones (granite, granite-like stone, and limestone) have been used as facing stone to create decorative base courses in several (typically relatively prestigious) buildings.
Masonry and roofing forming the street-facing elevation(s) of each building was assessed for the survey; other elevations were ignored. It should be borne in mind when using the survey data presented in this report (and in the data table that has been delivered separately), that data recorded against a polygon identifier (FID_1 and PRIMARY_KE) relate only to the street-facing elevation(s) of the associated building, not the building as a whole.
Survey data were captured digitally in the field on an Xplore iX104 tablet PC running Microsoft XP. Data were entered directly into a Microsoft Access form designed for the project (Figure 7). The form is divided into four sections – ‘Polygon’, ‘Stone’, ‘Slate’ and ‘Image’ — and each section contains one or more fields into which data are entered. The full list of field titles and field contents is presented in Table 1. The data content of most fields was constrained by a dictionary (i.e. a pre-determined list of terms). All the dictionaries are presented in Table 2. A ‘comment’ field in three sections of the form allowed the surveyor to record additional information as free text.
The ‘Polygon’ section of the form contains information about the building being described. This includes: a sequential identifying number assigned by BGS (ID); the two identifying numbers provided by the client (PRIMARY_KE and FID_1); a field for recording the date of construction, if recorded on the building (BUILDING_DATE); and a field for comments relating to the building or surveyed elevation (ELEVATION_COMMENT).
The ‘Stone’ and ‘Slate’ sections contain information relating to building stone and roofing slate, respectively. Three properties were recorded in each section: visibility, name and proportion. More information on these properties is presented below. The main building stone used in walling and dressings was also recorded.
The ‘Image’ section of the form contains a single field for recording the camera-assigned numbers of photographs taken at the time of the survey. A digital photograph of the surveyed elevation of each surveyed building has been delivered independently of this report. Each image file has been re-labelled with the PRIMARY_KE number of the pictured building, to allow easy cross-reference with the data table. Images have not been provided for five buildings that were in inaccessible locations.
The survey of Falkirk buildings was conducted by Paul Everett (BGS) in January and February 2013. All data recorded during the survey are presented in a single table (Falkirk THI Area Stone Survey Data). Data recorded for each polygon are presented in a single row of the table.
|Field name||Field content|
|PRIMARY_KE||Primary ID number for each surveyed polygon; from polygon shapefile provided by the client|
|FID_1||Unique ID number assigned to polygons; from polygon shapefile provided by the client|
|ADDRESS_LABEL||A ‘summary’ address for the building to act as an aide-mémoire for the surveyor|
|BUILDING_DATE||Date of the surveyed building, where known|
|ELEVATION_COMMENT||Comment regarding the surveyed elevation of each polygon/building (free text)|
|STONE_PRESENT||A record of whether natural stone is present in the street-facing elevation|
|STONE_VISIBILITY||A record of the visibility of building stone at the time of survey|
|STONE_MAJOR||Name of major building stone, forming >50% of stone in the street-facing elevation|
|STONE_MINOR_1||Name of minor building stone, forming 10–50% of stone in the street-facing elevation|
|STONE_MINOR_2||Name of minor building stone, forming 10–50% of stone in the street-facing elevation|
|STONE_TRACE_1||Name of trace building stone, forming <10% of stone in the street-facing elevation|
|STONE_TRACE_2||Name of trace building stone, forming <10% of stone in the street-facing elevation|
|MAIN_WALLING_STONE||Name of building stone forming the largest proportion in walling of the street-facing elevation|
|MAIN_DRESSING_STONE||Name of building stone forming the largest proportion in dressings of the street-facing elevation|
|STONE_COMMENT||Comment regarding building stone (free text)|
|SLATE_PRESENT||A record of whether natural roofing slate is present in the surveyed elevation|
|SLATE_VISIBILITY||A record of the visibility of roofing slate at the time of survey|
|SLATE_MAJOR||Name of major roofing slate, forming >50% of slate in the street-facing roof elevation|
|SLATE_MINOR_1||Name of minor roofing slate, forming 10–50% of slate in the street-facing roof elevation|
|SLATE_TRACE_1||Name of trace roofing slate, forming <10% of slate in the street-facing roof elevation|
|SLATE_COMMENT||Comment regarding roofing slate (free text)|
|IMAGE_NUMBER||Camera-assigned image number(s)|
|Field title||Dictionary terms|
|Buff sandstone 1|
|Buff sandstone 2|
|Buff sandstone 3|
|Buff sandstone 4|
|Buff sandstone 5|
|Buff sandstone 6|
|Buff sandstone 7|
|Modern buff sandstone 1|
|Modern buff sandstone 2|
|Modern buff sandstone 3|
|Modern buff sandstone 4|
|Modern buff sandstone 5|
|Modern buff sandstone 6|
|Buff sandstone, undifferentiated|
|Orange sandstone 1|
|Orange sandstone 2|
|Orange sandstone, undifferentiated|
|Scottish Highland Border slate|
|Scottish West Highland slate|
|Scottish slate, undifferentiated|
|Welsh slate, purplish|
|Welsh slate, grey|
|Welsh slate, undifferentiated|
The list of stone names in this table is the Dictionary of building stones.
The list of slate names is the Dictionary of roofing slates.
The terms ‘Not applicable’, ‘Not entered’, and ‘Unknown’ are included in all dictionaries.
The level of information available to the surveyor at the time of the survey was recorded as a simple, qualitative assessment (‘none’, ‘very poor’ ‘poor’, ‘moderate’ or ‘good’) under the heading ‘Visibility’. Visibility was recorded separately for building stone and roofing slate, and takes into account two main variables: the degree to which the stone or slate is exposed; and the distance between the surveyor and the stone or slate. For example, visibility was typically recorded as ‘good’ where the stone is fully exposed and can be examined at ground level (allowing close inspection of fine detail), whereas it may have been recorded as ‘poor’ where the stone is largely concealed beneath a covering of paint or render, or where the stone is only exposed above ground level (in which case fine details cannot be examined).
Other survey data, in particular the names recorded for stone and slate, should always be considered in the context of the visibility record. Survey data captured when visibility was recorded as ‘good’ will have a higher level of confidence (or certainty) than those captured when visibility was ‘poor’. Visibility may have been recorded as ‘none’ if a building stone is entirely concealed (e.g. beneath a coating of render), or if roofing slate cannot be seen from any publically accessible area. In such situations the stone or slate generally has been recorded as ‘undifferentiated’ unless there is separate evidence that allows the identity to be constrained (in which case the ‘Comment’ field will contain a note describing the evidence). Examples of ‘poor’ and ‘good’ visibility are presented in Figure 8.
Shared use of a cherry-picker with architects from Gray, Marshall & Associates on 17th February 2013 provided access to the upper levels of around a dozen surveyed buildings. In some cases, the view from the cherry-picker offered better visibility of the stone and/or slate than was available from street level; such instances were noted in the relevant ‘Comment’ field of the data table.
The formal names of building stones typically combine the names of the quarry source and the rock type; for example, sandstone sourced from Locharbriggs Quarry in Dumfriesshire is known as Locharbriggs Sandstone, and granite from Kemnay Quarry in Aberdeenshire is known as Kemnay Granite. However, several thousands of different building stones have been used in UK buildings, many of which have similar or overlapping geological characteristics; it is therefore commonly not possible to identify the quarry source of the stone used in a building through field examination alone. For that reason, the various building stones in the survey area were initially distinguished according to their rock type and rock properties.
Several visits to Falkirk were made to distinguish, describe and photograph the range of building stones that is represented in the survey area. Every building was examined to ensure that the full range of building stones was included in this exercise. The resulting ‘Dictionary of building stones’ (Table 2) is a list of all the stones that can be distinguished by properties and features that are observable in the field. An assessment of the likely quarry source of each building stone was conducted at a later stage in the project (described in section 5), with the benefit of information from a review of local quarrying history and a stone matching assessment.
Unlike building stones, roofing slates used in UK buildings come from a small number of sources, and slate from each source has a set of geological and man-made properties that can be used to identify it (see section 3.3). The ‘Dictionary of roofing slates’ (Table 2) is therefore simply a list of the names of the slates that traditionally have been used in the UK.
The dictionaries for building stones and roofing slates contain a number of low precision names to ensure that an accurate name could always be assigned during the survey. These names have the word ‘undifferentiated’ in them; for example, ‘Buff sandstone, undifferentiated’, and ‘Scottish slate, undifferentiated’ (Table 2).
Clockwise from top left: A) A building in which stone visibility is ‘good’: exposed stone can be examined at ground level. B) A building in which slate visibility is ‘good’: the angle of view is favourable and the viewing distance to the roof is short. C) A building in which slate visibility is ‘poor’; the angle of view is acute, the viewing distance to the roof is relatively long, and biogenic growth partly conceals the slate. D) A building in which stone visibility is ‘poor’: a shop front at ground level increases the viewing distance to the stone masonry, and paint conceals most of the masonry; small areas of stone are visible where paint has flaked off.
The proportion (or ‘amount’) of each building stone in a building was recorded semi- quantitatively. Any stone forming more than 50% (by area) of all the stone in the surveyed elevation was recorded as the major component. A stone forming between 10 and 50% of the total was recorded as a minor component, and one forming less than 10% was recorded as a trace component. An individual building can have only one major stone, but one or more other stones can be present in minor or trace proportions.
The same approach was used for roofing slates.
The following points should be borne in mind when using the survey data and supporting maps.
- The properties of natural stone and slate (including colour, grain-size and fabric) can vary considerably, even in material sourced from a single quarry. There is therefore always some degree of uncertainty in deciding whether or not the stone in different buildings is related using field observation alone.
- The natural stone in some buildings is partially or wholly concealed beneath an applied coating such as paint or render, or biogenic growth (moss or lichen); in such cases the confidence attached to observations is reduced.
- The survey was undertaken from publicly accessible areas at street-level, and observations are limited accordingly.
- The survey was conducted in January and February, and observations will to some extent have been affected by the low light levels at that time of year.
Twenty different building stones were identified within the survey area: fifteen are sandstone, one is limestone, and four are granite. Each was assigned a stone code and a stone name, for the purposes of the survey. The codes and names together with a brief description and representative photographs for all twenty building stones are presented in Appendix 1. An assessment of the source of each building stone is presented in section 5.
Building stone character
The sandstones can be divided readily on the basis of their colour: thirteen are buff and two are orange. Six of the buff sandstones were placed in buildings relatively recently, after all the building stone quarries in the local area had closed. These ‘modern’ stones have in most cases been used to make selective repairs (indents) to, or to construct extensions on, older buildings, but three relatively recent (post-1930) buildings have been constructed entirely of ‘modern’ stone.
Sandstones have a number of properties that are visible to the unaided eye and can be used individually or collectively to distinguish them. Some of these properties, notably colour, grain-size, primary sedimentary structure, and composition, are intrinsic features (i.e. they are present in all sandstones). Others may or may not be present, and these have been grouped in Table 3 and Appendix 1 under the heading ‘Distinctive features’. Summary information for the various properties that have been used to describe and distinguish the various sandstones in surveyed Falkirk buildings is presented in Table 3.
Simple descriptions and the criteria that were used to distinguish the various sandstones in the field are presented in Appendix 1. The terms for sandstone properties that are included in Table 3 are used throughout Appendix 1.
|Property||Description or comment|
|Intrinsic features||Colour||Recorded as ‘base colour’ (in this case buff or orange), with a qualifier term (e.g. greyish) and/or a shade term (e.g. light) if applicable, to create terms like ‘light greyish buff’.|
|Grain-size||Refers to the average diameter of sand grains, as follows: fine- sand-grade = 0.125-0.25 mm; medium-sand-grade = 0.25-0.5 mm; coarse-sand-grade = 0.5-1 mm; very-coarse-sand-grade = 1-2 mm; granule-grade = 2-4 mm.|
|Primary sedimentary structure||Refers to structure in sandstone formed at the time the sand was deposited. If no structure is visible the stone is said to be uniform. Coarse layering (layers >1 cm thick) is referred to as bedding, and fine layering (layers <1 cm thick) is lamination. Parallel layers = parallel bedding or parallel lamination. Cross-stratified layers = cross-bedding or cross-lamination. Irregular layers = convoluted bedding or convoluted lamination.|
|Composition||Refers to the relative proportions of sand grains composed of quartz, feldspar and rock fragments. Sandstone with a large proportion of quartz grains is siliceous, sandstone containing a moderate proportion of quartz is impure, and quartz-poor sandstone is very impure.|
|Distinctive features||Micaceous||Refers to sandstone containing a significant and conspicuous proportion of sand grains formed of mica minerals (muscovite and/or biotite). These minerals form flat platelets rather than rounded grains, and are typically aligned parallel to bedding in the stone. They are silvery (muscovite) and black (biotite).|
|Carbonaceous matter||Plant fragments (wood, bark, leaves) that have been deposited with the sand grains and have been converted by geological processes into black, carbon-rich matter.|
|Iron nodules||Nodules of iron that form in the sandstone. Nodules are usually dark brown to black, spherical to ovoid, and up to several centimetres across.|
|Coloured spots||Typically spherical to ovoid, brown to orange patches up to several centimetres across that form around iron-rich minerals or nodules as they break down.|
|Liesegang bands||Parallel and often concentric, coloured bands of iron-rich minerals that typically cut across bedding in sandstone.|
|Granulation seams||Thin (<2 mm wide) bands of crushed and healed sand grains representing small geological faults.|
|Speckled||Describes sandstone characterised by numerous small (mm- scale), more-or-less evenly distributed coloured spots. The spots are usually chemically altered grains of one or more of the constituent minerals in the sandstone.|
|Shell fragments||Whole shells or shell fragments that were deposited with the sand grains and were subsequently fossilised.|
|Gritty||A texture in which the sand grains are of conspicuously unequal sizes.|
The single example of a limestone building stone in the survey area consists of pale grey, fine- sand-grade to medium-sand-grade limestone with abundant shell fragments up to 10 cm long.
Granites have a number of properties that are visible to the unaided eye and can be used individually or collectively to distinguish them. Some of these, notably colour, grain-size, texture, and composition, are intrinsic features (i.e. they are present in all granites). Others may or may not be present, and these have been grouped in Table 4 and Appendix 1 under the heading ‘Distinctive features’. Summary information for the various properties that have been used to describe and distinguish granites in surveyed Falkirk buildings is presented in Table 4.
The four granite building stones are readily distinguishable on the basis of colour and texture. Two (G1 and G2) are grey and foliated, and two (G3 and G4) are pink and massive. One grey granite (G2) and one pink granite (G3) are inequigranular and have mafic enclaves. The other grey granite (G1) and the other pink granite (G4) are equigranular and contain no mafic enclaves.
|Property||Description or comment|
|Intrinsic features||Colour||Recorded as ‘base colour’ (in this case grey or pink), with a qualifier term (e.g. orangeish) and/or a shade term (e.g. very light) if applicable, to create terms like ‘very light orangeish grey’.|
|Grain-size||Refers to the average diameter of crystals in the rock. Granite is, by definition, coarse-crystalline (average crystal-size = 2–16 mm). In igneous rocks, the ‘coarse-crystalline’ size division is divided, as follows: coarse-crystalline (fine division) = 2–4 mm; coarse- crystalline (medium division) = 4–8 mm; coarse-crystalline (coarse division) = 8–16 mm.|
|Texture||Refers to geometric aspects of, or mutual relations among, the constituent crystals. Granite is said to be equigranular if the constituent crystals are of approximately equal size and inequigranular if they are not. Porphyritic texture is a form of inequigranular texture in which crystals of one mineral are significantly larger than the typical crystal size in the rock; the term feldspar-phyric refers to a porphyritic texture in which crystals of feldspar are significantly larger than the typical crystal size. In a foliated rock, some or all of the constituent crystals have a preferred orientation, whereas in a massive rock the constituent crystals have no preferred orientation.|
|Composition||All granitic rocks have substantial proportions of the minerals quartz, alkali feldspar and plagioclase feldspar. Those containing more alkali feldspar than plagioclase feldspar are granite, while those containing more plagioclase feldspar than alkali feldspar are granodiorite. The names of other prominent constituent minerals (usually mica minerals) are usually included in the name of the rock, e.g. biotite granite.|
|Distinctive features||Micaceous||Refers to granite containing a significant and conspicuous proportion of mica minerals (muscovite and/or biotite). These minerals form platelets and are usually silvery (muscovite) and black (biotite).|
|Mafic enclaves||Typically black, round to irregular patches up to several centimetres across consisting of dark minerals (usually mainly biotite and/or hornblende).|
Building stone distribution
Figure 9 shows the distribution of buildings in which natural stone is present in the surveyed elevation. Natural stone is present in 121 buildings, not present in thirty-eight buildings, and in thirteen buildings it could not be established if natural stone was present.
Figure 10 shows the visibility recorded for building stone in all surveyed buildings in which natural stone is present. Visibility was recorded as ‘good’ in thirty-six buildings, ‘moderate’ in thirty-three, ‘poor’ in eight, ‘very poor’ in thirty-four and ‘none’ in twenty. As noted earlier, the confidence or certainty attached to other survey data is largely determined by the visibility record, and other survey data should always be considered in that context. Survey data captured when visibility was recorded as ‘good’ will have a higher level of confidence than those captured when visibility was ‘poor’.
Reduced visibility was commonly due to a coating of paint and/or render/harling. Where this was the case, a note was made in the STONE_COMMENT field of the data entry form. Figure 11 has been generated from this dataset.
Maps illustrating the distribution of the various building stones are presented in Appendix 2. Figure A2_1 shows which stone was recorded as the ‘major’ stone (>50% of the total) in all buildings. Figures A2_2 and A2_3 show which stones were recorded as the main walling stone and main dressing stone, respectively. Figures A2_4 to A2_25 illustrate the distribution of each of the twenty building stones.
The main outcomes of the building stone distribution data are summarised below; reference should be made to the relevant maps in Appendix 2.
- The major stone in twenty-three buildings was recorded as ‘Sandstone, undifferentiated’. Most of these buildings face onto High Street. In many cases, a coating of harling or paint concealed or largely concealed the stone, preventing a precise identification, but there was always enough information to identify the building stone as sandstone. Buildings at the east end of High Street are generally old and in the vernacular style, and it is likely these buildings consist of ‘Buff sandstone 1’.
- ‘Buff sandstone 1’ is the most commonly recorded building stone (forming the major building stone in thirty-five buildings) and has the most widespread distribution. It appears in at least one building on nearly all of the streets within the survey area. Date panels on several buildings indicate this stone was used from at least the late 18th Century (pre-1787) until at least the late 19th Century (1880). The stone has been used in buildings displaying a range of architectural styles, from relatively humble, cottage style buildings (e.g. east end of High Street) to relatively prestigious structures such as the Burgh Buildings and Fa’ Kirk. The architectural style of some tenement buildings suggests this stone continued to be used into the earliest part of the 20th Century. ‘Buff sandstone 1’ in a roughly worked rubble walling style has been used in the side elevations of many buildings for which the main street-facing elevation consists of a different building stone.
- ‘Buff sandstone 2’ and ‘Buff sandstone 4’ were recorded only in the northern part of the THI area where they have been used mainly in Victorian tenement buildings. Date panels on some buildings range from 1862 to 1895. These buildings, which line much of Melville Street, Vicar Street, and Lower Newmarket Street, occupy a section of the town centre that seems to be a result of late Victorian town planning.
- Orange sandstone is relatively rare. It was recorded in nine buildings that are scattered along the north-south spine of the THI area. Seven buildings to the north of High Street (on Kirk Wynd, Vicar Street and in St Andrews Church West on Upper Newmarket Street) were built between 1896 and 1908. Two buildings on Cow Wynd were built in the 1920s. All the buildings are relatively formal and prestigious.
- ‘Buff sandstone 3’ was recorded in only three buildings, two of which have date panels recording 1832 and 1845; the third was probably constructed in the 1920s. All three are commercial buildings.
- ‘Buff sandstone 5’ was recorded in only four buildings, all in the central part of the THI area and with dates ranging from 1909 to 1914. All were probably constructed for mainly residential use; in each case the ground floor is today a shop.
- ‘Buff sandstone 6’ was recorded in only three buildings, two of which are adjacent on Manor Street. This is a relatively poor-quality building stone; the stone is spalling in places, particularly where blocks have been face-bedded. These buildings have no date panels, but their style and locations suggest they pre-date 1890.
- ‘Buff sandstone 7’ was recorded in only three buildings. Two are essentially the same tenement building on Williamson Street, the other is a largely brick building at the opposite end of the THI area. In each case, the stone has been used only in dressings.
- The six modern building stones (‘Modern buff sandstone 1’ to ‘Modern buff sandstone 6’) were recorded in a total of nineteen buildings. Two of these contain two ‘modern’ building stones, while the remainder contain only one. Modern buff sandstones are present mainly as indents to the original masonry, but two buildings have new shop fronts
- constructed of modern stone and three (possibly four) buildings (the oldest dated 1930) consist entirely of modern buff sandstone.
- Limestone was recorded in only one building, at the east end of High Street. The large 20th Century building features marble cladding around the main door.
- Four different granite building stones were recorded in a total of six buildings. ‘Granite 3’ has been used in three buildings, whereas ‘Granite 1’, ‘Granite 2’ and ‘Granite 4’ each appear in only one building. Granite masonry forms the base course in four buildings. Granite has been used as the plinth for the South African War Memorial on Newmarket Street and as decorative columns framing the main door of the Burgh Buildings.
- The same stone has been used for walling and dressings in all but one of the surveyed buildings (a tenement forming the east side of Williamson Street). Traditional Scottish buildings commonly feature one stone in the walling of the building, and another (typically a higher quality and/or differently coloured stone) in the dressings, and the scarcity of this building style in the survey area is unusual.
Roofing slate character
Roofing slate from four countries — Scotland, Wales, England and Spain - has been used widely in Scotland. Unlike stone, slate from each source can generally be distinguished (depending on visibility) by a combination of two sets of properties: geological properties include colour, lustre, and surface texture; man-made properties include size, thickness, edge roughness, shape, number of nail holes, and laying pattern.
Scottish slate was traditionally sourced from three areas of the country - the Highland Border area, West Highlands (Ballachulish and Easdale), and Aberdeenshire - and slate from each of these areas can be identified by its properties. Two variants of Welsh slate - purplish and grey - are distinguished from each other by their colour. The name and code used for each roofing slate, and a summary of the geological and man-made characteristics that were used to distinguish them in the survey of Falkirk buildings, is presented in Table 5.
A photograph showing a typical example of each slate is presented in Figure 12.
Name and code
|Geological properties||Man-made properties|
|Scottish slate, undifferentiated (SLSu)||Scottish slate, Highland Border (SLSHB)||Grey, purplish grey and greenish grey, occasionally variegated, with a lustre and moderately smooth surfaces. Commonly displays ribboning (banding on the slate surface which is caused by the intersection of bedding with the metamorphic cleavage plane on which the slate has parted).||Variable size|
Laid in diminishing courses and random widths
West Highland slates are typically larger and thicker than Highland Border slates.
|Scottish slate, West Highland (SLSWH)||Black to grey, with rough, variably crenulated surfaces and commonly with scattered cubes of pyrite. Slate from the Easdale quarries is typically more strongly crenulated than slate from the Ballachulish quarries.|
|Welsh slate, undifferentiated (SLWu)||Welsh slate, purplish (SLWp)||Purple, uniform, smooth matt surfaces.||Often in one standard size|
Laid in regular courses
Commonly two holes
|Welsh slate, grey (SLWg)||Grey, uniform, smooth matt surfaces.|
|English slate, Cumbrian (SLEc)||Bluish grey to greenish grey, with typically rough, matt surfaces. The distinctive colour and texture reflects the volcanic origin of the original sedimentary material.||Size is not diagnostic|
Can be laid in regular courses or diminishing courses
|Spanish slate (SLS)||Black, blue-black and dark grey, commonly with a slight oily lustre, particularly when new. Scattered crystals of pyrite are common. Surfaces can be crenulated, but not as strongly as Scottish West Highland slate.||Uniform size|
Thin to moderately thick
Laid in regular courses
Roofing slate distribution
The distribution of buildings in which roofing slate is present in the surveyed elevation is shown in Figure 13. Roofing slate is present in 122 surveyed buildings, not present in forty-one buildings, and in nine buildings it could not be established if roofing slate was present.
Figure 14 shows the visibility recorded for roofing slate in all surveyed buildings in which slate is present. Visibility was recorded as ‘good’ in nine buildings, ‘moderate’ in fifty-four, ‘poor’ in thirty-nine, ‘very poor’ in thirteen and ‘none’ in eleven. As noted earlier, the confidence or certainty attached to other survey data is largely determined by the visibility record, and other survey data should always be considered in that context. Survey data captured when visibility was recorded as ‘good’ will have a higher level of confidence than those captured when visibility was ‘poor’.
Maps illustrating the distribution of the various roofing slates are presented in Appendix 3. Figure A3_1 shows which slate was recorded as the ‘major’ slate (>50% of the total) in all buildings. Figures A3_2 to A3_10 illustrate the distribution of each roofing slate.
The main outcomes of the roofing slate distribution data are summarised below; reference should be made to the relevant maps in Appendix 3.
- The major slate was recorded as ‘Slate, undifferentiated’ in thirty-seven of the buildings in which slate is present.
- For the remaining buildings in which slate is present, Scottish slate is the most commonly recorded (47) followed by Welsh slate (26) Spanish slate (9) and English slate (3).
- There are no obvious patterns in the spatial distribution of different slates.