Sedimentary Rocks

Sedimentary rocks are produced by the weathering and erosion of older rocks. In the earliest geological time these would have been the original igneous rocks, but subsequently other sedimentary and meta-morphic rocks too will have been reworked. Weathering action by water, ice and wind breaks the rocks down into small fragments which are then carried by rivers and sorted into size and nature by further water action. Most deposits are laid down in the oceans as sedimentary beds of mud or sand, which build up in layers, become compressed and eventually are cemented together by minerals such as calcium carbonate (calcite), quartz (silica), iron oxide or dolomite (magnesium and calcium carbonate) remaining in the groundwater. The natural bedding planes associated with the formation of the deposits may be thick or thin but are potentially weak; this is used to advantage in the quarrying process. In masonry, to obtain maximum strength and durability, stones should be laid to their natural bed except for cornices, cills and string courses which should be edge-bedded. Stones which are face-bedded will tend to delaminate (Fig. 9.2). When quarried, stones contain quarry sap and may be worked and carved more easily than after exposure to the atmosphere.

Stone bedding planes

Stone bedding planes

Appropriate application of bedding planes

Natural

Lintel

Fig. 9.2 Natural stone bedding planes

Sandstones

Deposits of sand cemented together by calcium carbonate, silica, iron oxide and dolomite produce calcareous, siliceous, ferruginous and dolomitic sandstones respectively. Depending upon the nature of the original sand deposit, the sandstones may be fine or coarse in texture. Sandstones range in colour from white, buff and grey through to brown and shades of red depending upon the natural cement; they are generally frost-resistant. Some common UK sandstones are listed in Table 9.2. Typical finishes are sawn, split faced and clean rubbed, although a range of tooled finishes including broached and droved can also be selected (Fig. 9.3). For cladding, sandstone is normally 75 mm to 100 mm thick and fixed with non-ferrous cramps and corbels. Sandstones are quarried in Scotland, the North ofEngland, Yorkshire and Derbyshire; they include the old and new red sandstones, York Stone and Millstone Grit. Sandstone is imported from Spain and Italy from where Pietra Serena is sourced.

Calcareous sandstone

Calcareous sandstones are not durable in acid environments, which may cause the slow dissolution of the natural calcium carbonate cement of the stone. Pure calcite is white, so these sandstones are generally white in colour.

Siliceous sandstone

Siliceous sandstones are predominantly grains of silica (sand) cemented with further natural silica, and are therefore durable even in acid environments. Siliceous sandstones are generally grey in colour.

Ferruginous sandstone

Ferruginous sandstones are bound with oxides of iron which may be brown, ochre or red. They are generally durable.

Dolomitic sandstone

Dolomitic sandstones are bound with a mixture of magnesium and calcium carbonates, and therefore do not weather well in urban environments. They are generally off-white and buff in colour.

Limestones

Limestones consist mainly of calcium carbonate, either crystallised from solution as calcite or formed from accumulations of fossilised shells deposited by various sea organisms. They are generally classified according to their mode of formation. Many colours are available ranging from off-white, buff, cream, grey and blue. Limestones are found in England in a belt from Dorset, the Cotswolds, Oxfordshire and Lincolnshire to Yorkshire. Limestone is also imported from Ireland, France and Portugal to widen the palette of colours. Some common UK limestones are listed in Table 9.3. The standard finishes are fine-rubbed, fine-dragged and split-faced, although tooled finishes are also appropriate. Externally, limestones must not be mixed with or located above sandstones, as this may cause rapid deterioration of the sandstone.

Oolitic limestone

Ooitic limestones are formed by crystallisation of calcium carbonate in concentric layers around small fragments of shell or sand, producing spheroidal grains or ooliths (Fig. 9.4). The ooliths become

Table 9.2 Typical UK sandstones and their characteristics

Name

Colour

Source

Characteristics

Doddington

purple/pink

Northumberland

fine to medium-grained

Darley Dale - Stancliffe

buff

Derbyshire

fine-grained

Birchover gritstone

pink to buff

Derbyshire

medium to coarse-grained

York Stone

buff, fawn, grey,

Yorkshire

fine-grained

light brown

Mansfield Stone

buff to white

Nottinghamshire

fine-grained

Hollington

pale pink, dull red

Staffordshire

fine to medium-grained

pink with darker stripe

St. Bees

dark red

Cumbria

fine-grained

Blue Pennant

dark grey/blue

Mid-Glamorgan

fine-grained

Fig. 9.3 Typical tooled-stone finishes

cemented together by the further deposition of calcite to produce the rock. Typically the ooliths are up to 1 mm in diameter, giving a granular texture to the stone, which may also incorporate other fossils.

Oolitic limestone (x 20) (ofter Arkwell, W.J. 1946: Oxford stone, Fober & Faber) Fig. 9.4 Roach limestone and oolitic limestone (X 20)

Oolitic limestones are very workable and include Bath Stone and Portland Stone. Clipsham Stone and Ketton Stone have been widely used at Oxford and Cambridge respectively, including the recent Queen's Building of Emmanuel College, Cambridge (Fig. 9.5), which is built of load-bearing Ketton limestone, with appropriately massive columns and flat voussoir arches to the colonnade and window openings. Lime mortar is used to ensure an even spreading of the load between stones. In the case of Foundress Court, Pembroke College, Cambridge (Fig. 9.6), the Bath Stone (Monks Park) is built up three storeys from ground level as a well-detailed cladding, with restraint back to the load-bearing blockwork inner skin. The flexibility of lime mortar is used to reduce the number of visible movement joints.

Organic limestone

Organic limestones are produced in bedded layers from the broken shells and skeletal remains of a wide variety of sea animals and corals. Frequently clay is

STONE AND CAST STONE Table 9.3 Typical UK limestones and their characteristics

Name

Colour

Source

Characteristics

Ancaster

cream to buff

Lincolnshire

oolitic limestone - variable shell

content; freestone available

Bath Stone

pale brown

Avon

oolitic limestone

to light cream

- Westwood Ground

coarse-grained - buff coloured

- Monks Park

fine-grained - buff coloured

Clipsham

buff to cream

Rutland

medium-grained oolitic limestone

with shells; some blue stone;

best quality stone is durable

Doulting

pale brown

Somerset

coarse textured; fossils uncommon

Hopton Wood

cream or grey

Derbyshire

carboniferous limestone containing

many attractive fossils; may be

polished

Ketton

pale cream to

Lincolnshire

medium-grained oolitic limestone;

buff and pink

even-textured; durable stone

Portland Stone

white

Dorset

exposed faces weather white,

protected faces turn black.

- Roach

coarse open-textured shelly stone;

weathers very well

- Whitbed

fine-grained - some shell fragments;

durable stone

- Basebed

fine-grained with few shells;

suitable for carving

Purbeck

blue/grey to buff

Dorset

some shells; durable stone.

incorporated into organic limestones and this adversely affects the polish which can otherwise be achieved on the cut stone.

Crystallised limestone

When water containing calcium bicarbonate evaporates, it leaves a deposit of calcium carbonate. In the case of hot springs the material produced is travertine, and in caves stalactites and stalagmites or onyx-marble result.

Dolomitic limestone

Dolomitic limestones have had the original calcium carbonate content partially replaced by magnesium carbonate. In general this produces a more durable limestone, although it is not resistant to heavily polluted atmospheres.

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