SEDIMENTARY ROCKS

5. SEDIMENTARY ROCKS - examples of types - formation and properties
A sedimentary rock is defined and what sedimentary rock formations look like. The formation of the sedimentary rocks sandstone, limestone, chalk, shale, mudstone, coal, salt deposits. The formation of fossils is described and why they mainly occur in sedimentary rocks and their usefulness to geological scientists studying the age of rocks and to biologists and zoologists studying the evolution of plant and animal species.

5. Sedimentary Rocks slowest to form, and weather the fastest!
5(a) A sedimentary rock bed is formed from plant/animal remains or weathered and eroded particles from pre-existing rocks. The material is deposited in layers that will eventually form the sedimentary rock. These may be transported, usually by water (or wind in the case of sand) anddeposited to form sediments. These become buried under later forming sediments and water or by major tectonic activity, and then become subjected to compression as enormous pressures are created deep in the crust from the weight of rocks or sediments above them.Over millions of years from the fragments of eroded pre-existing rocks, water is squeezed out and the particles cement together with the help of dissolved salts and silica crystallising out. Other changes come about depending on the type of material from which the sedimentary rock is formed. 5(b) Types of sedimentary rock

Shale and Mudstone is formed from relatively fine grained weathered rock material transported into seas and lakes before settling out as clay or mud sediment. It then becomes compressed under the weight of water and other sediments and the water is squeezed out and the particles cement together. These rocks are clearly layered and crumble easily. Shale can contain significant amounts of oil-like organic material.

Above is the pebble beach at Charmouth in southern England and the Jurassic fossil bearing shale cliffs in the distance (180-195 million years old). Fossils found include ammonites, nautilus, belemnites, crinoids, shells and bones etc. Shale cliffs are structurally weak and collapses of the cliff faces are regular, but they often reveal lots of good fossil specimens!

Limestone (above) is formed from the deposition of hard mineral remains of sea creatures and chemically is mainly calcium carbonate CaCO3. This sedimentary rock mineral contains the 'shelly' remains of marine organisms, including coral, that once lived in warm shallow fertile seas. Limestone is grey-white in colour and contains fossils and sand grains. The 'shelly' remains, including coral, get buried and compressed and cemented together by the weight of water and other sediments. Limestone tends to form beneath warm shallow seas rich in plant and animal life.
The picture above shows Gordale Scar in the Yorkshire Dales in the Pennines of the North of England. The scar was gouged out of the easily eroded limestone rock. Because limestone has not been subjected to high temperatures it often contains many good fossils. Limestone is relatively easily weathered and dissolves in acid rain so after a few thousand years deep valleys are formed like the one illustrated in the photograph. Medieval limestone buildings have suffered grievously at the hands of acid rain from the fossil fuel combustion smoke of the industrial revolution.

Chalk cliffs (above) are formed from the mineral remains of tiny marine organisms in the sea and is chemically relatively pure calcium carbonate and it contains microscopic fossils readily seen under a microscope. The picture above shows the chalk White Cliffs of Dover. Like limestone, chalk is quite easily eroded over thousands of years by wind rain, and waves if the cliffs are by the sea.
Sandstone is formed from weathered particles of igneous rock and these particles mainly consist of colourless silica (silicon dioxide, SiO2).
The rock particles are laid down in lakes,