The OGG Guide to Sedimentary Rocks
Sediments & sedimentary rocks
Sedimentary rocks are the products of Earth processes and accumulate over time. Sedimentary rocks are formed from sediment grains deposited by water, wind or ice. They are always formed in layers, called “beds” or “strata”, and quite often contain fossils.
The crust accounts for only the outer 0.3% of the Earth's radius, and sedimentary rocks make up a relatively minor proportion of the crust.
Sediments cover more than 80% of the Earth’s surface and they record important information on surface conditions and processes over geological history. Information about climate, ecosystems, sea level rise and fall, chemistry of the oceans, tectonics and bolloid impacts can all be preserved in sedimentary record. The surface rocks of Oxfordshire are all sedimentary.
Principle Sedimentary Rock Types
Sedimentary rocks can be grouped very broadly into two categories: siliciclastic rocks and carbonates + evaporates (chemical precipitates).
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Conglomerates are made of pebbles deposited on beaches or in river channels. The pebbles have become cemented together by minerals precipitated from groundwater after the sediment was laid down.
Sandstone is made of sand grains (0.05mm to 2mm) that may have been deposited in the sea, by rivers, or in deserts, and later cemented together by minerals precipitated from groundwater.
Most sandstones are made up largely of quartz grains, because quartz is a very hard and chemically-resistant mineral.
Many sandstones contain some grains of other minerals like calcite, clay, or mica.