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.