Metamorphic Rocks.

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Metamorphic rocks such as slates and schists are those changed from igneous, sedimentary or from soils into metamorphic by pressure or heat or both. These rocks vary from dense slates in which the layers of the material are barely visible to schists in which the layers of various minerals are clearly visible and may readily split into thin plates. Because of the mode of the formation of these rocks the layers or planes rarely lie horizontal in the ground and so generally provide an unsatisfactory or poor foundation.

Sedimentary Rocks.

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Sedimentary rocks, such as limestone and sandstone, are those formed gradually over thousands of years by the settlement of particles of calcium carbonate or sand to the bottom of bodies of water where the successive layers of deposit have been compacted as beds of rock by the weight of water above. Because of the irregular and varied deposit of the sediment, these rocks were formed in layers or laminae. In dense rock beds the layers are strongly compacted and in others the layers are weakly compacted and may vary in the nature of the layers and so have poor compressive strength. Because of the layered nature of these rocks the material should be laid as a building stone with the layers at right angles to the loads.

Many of the beds of sound limestone and sandstone in this country have been quarried for the production of natural building stones such as Portland and Bath limestones and Darly Dale and Crosland Hill sandstones.

The suitability of sound limestone and sandstone as a foundation may be determined by the need to use a pneumatic drill to level the material ready for use as a foundation. As with igneous rock it is usual to cast a concrete base on the roughly levelled rock for the convenience of building. 

Rocks - Groups.

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Rocks may be divided into three broad groups as igneous, sedimentary and metamorphic.

Igneous Rocks.

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Igneous rocks, such as granite, dolerite and basalt, are those formed by the fusion of minerals under great heat and pressure. Beds of strong igneous rock occur just below or at the surface of ground in Scotland and Cornwall as Aberdeen and Cornish granite. The nature and suitability of such rocks as a foundation may be distinguished by the need to use a pneumatic drill to break up the surface of sound, incompressible rock to form a roughly level bed for foundations.

Because of the density and strength of these rocks it would be sufficient to raise walls directly off the rock surface. For convenience it is usual to cast a bed of concrete on the roughly levelled rock surface as a level surface on which to build. The concrete bed need be no wider than the wall thickness it supports.