3. Sedimentary Rock
New layers of sediment
Water
Rock bed
New layers of sediment Compacted sediment
Sedimentary rock is formed by the deposition of multiple layers of
sediment in a process known as lithification. Once deposited, the
weight of new layers compacts previous layers with such a force that
they fuse together. Over millions of years the layers continuously build
up before geological processes leave them exposed and accessible for
quarrying operations.
4. Deposition of Sediment
Three methods of sediment disposal
are weathering, erosion and transport
(streams, rivers, glaciers and wind) Image by British Geological Survey
6. Types of Sedimentary Rock
Images by British Geological Survey
Sandstone is formed by the lithification of sand sized minerals and rock
grains which have been transported and deposited. Most sandstones
contain high levels of quartz or feldspar as these are the most common
minerals in the earths crust.
Limestone is predominantly formed from the remains of dead marine
organisms which contain calcium carbonate. Having died and fallen to
the bottom of the ocean, these creatures are compacted over millions
of years to form limestone.
7. Sedimentary Stone Variations
The variations in sedimentary stone colours and textures can be seen in
the image above. Note that this is only a small selection of British stones.
The selection of stone has been sourced from Tradstocks.co.uk
8. Natural Bedding
When stone is built on its natural bed its sediment remains on the same
plane as when it was formed. This allows the stone to maximise its
compressive strength and reduces weathering and erosion. Most stones
on a building are built on natural bed (ashlar, rybats, quoins, lintols etc).
9. Joint/Edge Bedding
When stone is built on its edge bed its sediment faces at 90 degrees to
the plane as when it was formed. This bedding plane is used for arch
stones and for overhanging stonework such as sting courses, band
courses, cornices and in freestanding work such as ballusters.
10. Face Bedding
When stone is built on its face bed its sediment faces at 90 degrees to
the plane as when it was formed and each layer of sediment is fully
exposed. This bedding plane is normally avoided as it is particularly
susceptible to rapid erosion and has lower compressive strength than
natural bedding. It can be used on ballusters and finnials.
11. Building to the Correct Bedding
This image shows ashlar stonework
built on a combination of natural and
edge bedding plane.
12. Igneous Rock
Water
Solidified molten rock
Molten rock from
volcanic activity
Rock bed
Igneous rock is formed by molten rock (magma) which has cooled and
solidified or by ash produced by a volcanic eruption. There are various
forms of igneous rock some of which are very hard and durable
(granite) whilst others are lightweight and brittle (pumice). The rock
can be formed due to volcanic eruptions or less visible forms of
volcanic activity (in underground magma chambers).
14. Types of Igneous Rock
Granite is formed by the cooling of magma and contains a minimum of
20% quartz. It is a very dense stone and has exceptionally high
compressive strength which makes it particularly suitable for plinths.
Tuff is formed from the sedimentary compaction of volcanic ash and so
is a lightweight rock which is suitable either as a building material or as a
pozzolan which is combined with a lime mortar for building.
15. Metamorphic Rock
Top layers of sediment
Sedimentary rock
Magma
New layers of sediment Compacted and heated sediment
Metamorphic rock is rock which has been subjected to intense heat
and pressure and through a metamorphic process has changed into
another type of rock. The two most commonly used metamorphic
rocks in the stonemasonry industry are marble (limestone) and slate
(shale).
16. Types of Metamorphic Rock
Slate is formed from the metamorphisism of shale and tends to retain a
heavy lithification which makes it easy to split. Due to its low
porosity, slate is often used as a roofing material.
Quartzite is metamorphisised sandstone. In the image above (centre) it
is clear that the bedding plane has been radically altered due to intense
heat and pressure.
Marble is metamorphisised limestone which has been crystallised. This
type of stone is commonly used for ornamental stonework such as
statues and intricate carving work.
17. Developed by The Stonemasonry Department
City of Glasgow College
2012
