Rock FormationIntroduction for Stonemasons Stonemasonry Department 2012
The Rock Cycle Image by British Geological Survey
Sedimentary Rock New layers of sediment Water Rock bed New layers of sediment Compacted sedimentSedimentary rock is formed by the deposition of multiple layers ofsediment in a process known as lithification. Once deposited, theweight of new layers compacts previous layers with such a force thatthey fuse together. Over millions of years the layers continuously buildup before geological processes leave them exposed and accessible forquarrying operations.
Deposition of SedimentThree methods of sediment disposalare weathering, erosion and transport(streams, rivers, glaciers and wind) Image by British Geological Survey
Deposition of Sediment Image by British Geological Survey
Types of Sedimentary Rock Images by British Geological SurveySandstone is formed by the lithification of sand sized minerals and rockgrains which have been transported and deposited. Most sandstonescontain high levels of quartz or feldspar as these are the most commonminerals in the earths crust.Limestone is predominantly formed from the remains of dead marineorganisms which contain calcium carbonate. Having died and fallen tothe bottom of the ocean, these creatures are compacted over millionsof years to form limestone.
Sedimentary Stone Variations The variations in sedimentary stone colours and textures can be seen inthe image above. Note that this is only a small selection of British stones. The selection of stone has been sourced from Tradstocks.co.uk
Natural BeddingWhen stone is built on its natural bed its sediment remains on the sameplane as when it was formed. This allows the stone to maximise itscompressive strength and reduces weathering and erosion. Most stoneson a building are built on natural bed (ashlar, rybats, quoins, lintols etc).
Joint/Edge BeddingWhen stone is built on its edge bed its sediment faces at 90 degrees tothe plane as when it was formed. This bedding plane is used for archstones and for overhanging stonework such as sting courses, bandcourses, cornices and in freestanding work such as ballusters.
Face BeddingWhen stone is built on its face bed its sediment faces at 90 degrees tothe plane as when it was formed and each layer of sediment is fullyexposed. This bedding plane is normally avoided as it is particularlysusceptible to rapid erosion and has lower compressive strength thannatural bedding. It can be used on ballusters and finnials.
Building to the Correct BeddingThis image shows ashlar stoneworkbuilt on a combination of natural andedge bedding plane.
Igneous Rock Water Solidified molten rock Molten rock from volcanic activity Rock bedIgneous rock is formed by molten rock (magma) which has cooled andsolidified or by ash produced by a volcanic eruption. There are variousforms of igneous rock some of which are very hard and durable(granite) whilst others are lightweight and brittle (pumice). The rockcan be formed due to volcanic eruptions or less visible forms ofvolcanic activity (in underground magma chambers).
Formation of Igneous Rock Image by greenanswers.com
Types of Igneous RockGranite is formed by the cooling of magma and contains a minimum of20% quartz. It is a very dense stone and has exceptionally highcompressive strength which makes it particularly suitable for plinths.Tuff is formed from the sedimentary compaction of volcanic ash and sois a lightweight rock which is suitable either as a building material or as apozzolan which is combined with a lime mortar for building.
Metamorphic Rock Top layers of sediment Sedimentary rock Magma New layers of sediment Compacted and heated sedimentMetamorphic rock is rock which has been subjected to intense heatand pressure and through a metamorphic process has changed intoanother type of rock. The two most commonly used metamorphicrocks in the stonemasonry industry are marble (limestone) and slate(shale).
Types of Metamorphic RockSlate is formed from the metamorphisism of shale and tends to retain aheavy lithification which makes it easy to split. Due to its lowporosity, slate is often used as a roofing material.Quartzite is metamorphisised sandstone. In the image above (centre) itis clear that the bedding plane has been radically altered due to intenseheat and pressure.Marble is metamorphisised limestone which has been crystallised. Thistype of stone is commonly used for ornamental stonework such asstatues and intricate carving work.
Developed by The Stonemasonry Department City of Glasgow College 2012