Sedimentary Rocks - Sandstone, Shale & Limestone
Sedimentary rocks are secondary in their origin, the materials of which they are composed having been derived from the decay and disintegration of some previously existing rock mass. They have been formed by a deposition of sediments in a body of water.
They may be divided into two classes, depending upon whether their origin has been mechanical or chemical in its nature. In the case of the sedimentary rocks of a mechanical origin, their constituent particles have been transported by stream into a large body of quiet water, where they have been deposited in practically horizontal layers. Sedimentary rocks of chemical origin have had the materials of which they are composed dissolved by waters circulating through the rocks and brought ultimately by these waters into a sea , where trough some chemical change they are precipitated upon its floor, also in horizontal layers. These horizontal beds of sediments are ultimately consolidated into the masses known as sedimentary rocks.
Sedimentary rocks are therefore characterized by a parallel arrangement of their constituents particles into layers and beds which are to be distinguished from each other by differences in thickness, size of grain and often in color. It is to be noted, further, that sedimentary rocks in general are composed of an aggregate of individual minerals particles, each of which stands out in a way by itself and does not have that intimate interlocking relation with the surrounding particles which is to be seen in the minerals of an igneous rock. In all the coarse-grained sedimentary rocks there is some material which, acting as a cement, surrounds the individual mineral particles and binds hem together. This cement is usually either silica, calcium carbonate or iron oxide. The chief minerals to e found in sedimentary rocks are quartz and a carbonate, calcite or dolomite. These give it the two chief types of sedimentary rocks, the sandstone and the limestone. A brief description of these rocks follows.