Sedimentary rocks are formed from material which was once a sediment. Therefore they are normally formed under a mass of water, making up a series of layers, or strata. These strata, originally horizontal, can often become folded or cracked (faulted) by earth movements. Older rocks will form lower strata, with more recent rocks making up the strata above, having been deposited upon those already there.
The most commonly known sedimentary rocks are limestone, chalk, sandstone, clay, shale, coal and conglomerate.
One way of classifying sedimentary rocks is according to their age, with older rocks forming below younger ones. They can also be classified according to the type of material from which they were created, or in terms of the particle size and type within them. For example conglomerates are made of large material, sandstones of smaller sand particles, and clay and shale of very fine mud particles.
Finally these rocks can be classified according to the medium in which they were deposited, water or ice, for example.
Some sediments were formed below moving masses of ice during ice ages, and these are usually made up of a mix of material called till or boulder clay, which comprises various rock particles within a clay matrix. The type of rock making up the particles in the clay will depend on the areas crossed by the ice.
For example, the most common boulder clay of East Anglia, UK., is made up of chalk fragments, broken from the chalk of the East Anglian Heights as the ice moved over them. Hence this example is called Chalky boulder clay.
Sediments can form at the bottom of a lake, but these are rarely compressed to the extent that they become rocks. Most sedimentary rocks are formed below an ocean, the depth of the ocean along with the type of material determining the type of rock which is created.
If the example of a river flowing to the sea is considered, the river will be carrying a large amount of varied material within its current, ranging from the relatively large to the tiny. When the river enters the sea its current ceases and the sediment it carried is gradually deposited on the sea bed. The heavier material will be deposited first, therefore nearest to the coast, whereas the finer material will be carried further out to sea before it sinks. By this process sedimentary rocks made up of gravelly material can be said to have been formed just offshore on a sea bed. Sand, being lighter, will have been formed in deeper sea as its particles are smaller and, sinking more slowly, are carried further away from the shore. Mud particles, being the finest of all, are carried out furthest before they settle out.
The classification of these rocks will therefore relate to particle size and the depth of sea in which they were formed. The larger material will form conglomerates, sand will form sandstones, and mud particles will produce shales and clay.
In the deepest oceans, beyond the reach of a river’s current, the only material to be deposited will be the remains of dead sea creatures, essentially calcium material from their skeletons or shells. The sedimentary rocks limestone and chalk, made in this way, were therefore formed under very deep oceans, way beyond the reach of sands, clays or pebbles, and can also be classified by the fact that they are made of organic material.
Coal is a sedimentary rock which was also formed from organic material, in this case dead forest material. These forests, found for the most part in marshy coastal areas, became submerged by a rise in sea level, and the dead and decaying forest matter was compressed first into peat, then into lignite or brown coal, and eventually into black coal. This material, being organic, is carbon rich, as are the deposits of oil and natural gas which can be found trapped between other sedimentary rocks.
There are various ways, therefore, to classify sedimentary rocks; by their age, as shown by their depth in a series of strata; by their particle size; by the means of their formation, whether ice, river, lake or sea; by the depth of sea in which they were formed; and by their chemical composition, whether they were formed of organic or inorganic material.