The other and perhaps most familiar group of rocks is made of debris, waste and rubble. The geologist speaks of most of them as fragmental or clastic rocks. Clastic rocks cannot be the oldest, for by their very definition they have been formed from other rocks. Yet in many ways they are more interesting than any of the earliest rocks. Nobody knows what the surface of the earth was like when it was first formed. But if in those primeval times, rocks were broken up, weathered and washed away, the forces of nature such as we know them today must have been also present long ago.
Sedimentary rocks provide us with evidence that for at least a billion years - and perhaps more - conditions on the earths surface were very much the same as they are today. An atmosphere contained the suns heat. Rain, wind, running water, and the chemicals of the air changed the surface rocks. This process of weathering and erosion began as soon as the first rocks were formed, and it continues today. In older regions, snow changes into ice and ice forms glaciers, which move slowly over the land, wearing down the rocks and carrying fragments away. In past ages, great ice sheets spread out over much of North America, Europe, even covering parts of the African continent.
Even raindrops help wear away the rocks. But the water that runs over the land or through the cracks in the rocks wears them out even more. The heat of the sun splits rocks open. Frosts of winter splinter them, too. Plant roots, growing into cracks, swell and push the rocks apart. All these kinds of actions break the rocks into smaller and smaller fragments which are finally carried away by gravity, water and even by the wind.
Sooner or later these fragments move down to lower and lower levels because of the action of gravity. Rivers drop sand and mud in deltas at the sea shores. Lakes are gradually filled with washed-in sediment, so are the dry desert basins. Wherever sand, mud, silt and dust are deposited, they become cemented together, forming sedimentary rock.
Sedimentary rocks form in different ways in many places. But the rocks are always the formation of particles or fragments from older rocks. Sometimes fragments are stones and boulders, sometimes pebbles, sometimes fine sand or finer clay. Chemical action in shallow seas and in hot springs may form deposits of even finer chemical sediments. Sedimentary rocks make up about 75 per cent of the exposed land surface of the earth. But they make up less than 5 per cent of the total volume of the earths crust.
Sedimentary rocks are often classified according to the way they had been formed and to the size of the particles in them. Following this classification, we begin with fragments which were broken or worn off from older rocks. When such fragments are of pebbles size or even larger, then cementing they form a rock called conglomerate.
When the rock fragments are smaller, like grains of sand, the rock is called a sandstone. Sandstones may be fine or coarse, hard or soft. Usually they are composed of grains of sand or silica. Sometimes other minerals are present and then the sandstone is given a special name. Arkose, for example, is a sandstone formed under desert conditions. It contains grains of feldspar as well as grains of quartz. Sandstones form about 32 per cent of all sedimentary rock.
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When the rock particles are still smaller in size, so small that they are difficult to see, the rock is a shale. Shales are rich in clay minerals, though they may contain large amounts of silica. Shales are usually fine-grained, light or dark in color, and they often break in a typical way. Shales are the most common of sedimentary rocks, forming about 46 per cent of all sedimentary rocks known.
Limestone is a sedimentary rock which does not easily fit in with the others. Limestone has many origins; In some cases it is a clastic rock formed of fragments of shells piled on beaches and cemented together by the lime itself. Coquina is coarse limestone made of shelf fragments. Of the sedimentary rocks 22 per cent are limestones.
When clastic fragments are first deposited, nothing holds them together. They may remain in this state for hundreds of thousands of years. Big piles of sand and gravel were deposited 25,000 or more years ago during the great ice age. In only a few places have these begun to cement together.
Rock fragments will cement together faster when water circulates through them, especially if the water contains dissolved chemicals. Lime is such a chemical. Iron rust is another. It gives sandstone and shale a yellowish or reddish color. Silica is a third element, and when sandstones are cemented by silica they are likely to be hard and tough.
The other group of sedimentary rocks includes those of chemical origin. In shallow seas, lakes, and caves, and under certain special conditions, lime is deposited chemically. One form is oolitic limestone, formed of small, round grains about the size of grains of sand. Acid-bearing water trickling out through limestone dissolves some of it. The limestone is redeposited as the water evaporates in cracks or caves. Such drippings form stalactites and stalagmites. Dolomite, a magnesium limestone, halite (or salt) and gypsum may also be deposited chemically. All they form beds that are large enough to be classified as sedimentary rock.
Finally, plants and animals produce a few sedimentary rocks. Coal deposits begin as peat, a soft organic material that is formed from decaying ferns and swamp plants. As this material is buried by layers of sand and debris, water in the peat is squeezed out and gases are liberated. The amount of carbon gradually increases as the material changes from peat to lignite, a brown coal, and from the brown coal to soft, black coal. At each step, as the amount of carbon increases, the amount of hydrogen and oxygen in the material decreases. Coral and similar animals form deposits of organic lime in reefs.
Sometimes sedimentary rocks give a clue to the kinds of life and conditions which existed millions of years ago when the rocks were formed. Fossils are the remains or evidences of life buried in the rock. Sometimes the actual remains are buried; sometimes fossils are impressions, molds or casts. In nearly every case fossils are formed in sedimentary rock, when plants and animals have been buried under debris.
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Tough parts of plants and hard parts of animals produce the best fossils. Shells, bones, teeth, leaves, wood and bark are often preserved. Fossils tell the history of the earth and the development of life. In the oldest rock, only the simplest kinds of plants and animals are found. In more recent rocks the plants and animals are different and more complex, showing a great range of adaptations to many environments. It is mainly through the study of fossils that scientists have come to understand how the plant and animal life of today came to be.
Sedimentary rocks, especially those formed in shallow waters, contain ripple marks, formed by the action of waves stirring the bottom. Mud cracks mark the places where the shallow waters dried out, exposing the cracked mud. Later the cracks were filled with soft mud, preserving their pattern.
The layers of sedimentary rock often form parallel beds. A storm may wash down coarse material. During the winter when a lake is covered with ice, only the finest material is deposited. Sometimes the beds are tilted where a stream is depositing sand and mud at its mouth. All these features are preserved in sedimentary rocks and help to show how they were formed.
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NOTES:
* rubble , , ;
* silica , ;
* arkose , ;
* feldspar ;
* coquina - ;
* oolitic , ;
* lignite , .
