SEDIMENTATION It is well known that the original surface of the earth has under gone a great change. The gases of the atmosphere, such as nitrogen, oxygen, carbon dioxide and water vapor are very strong agents of erosion; the rocks being exposed to atmospheric influences are constantly denuded by the dissolving effect of percolating waters, the carving strength of strong winds and the splitting force of frost, and are worn away, and the eroded particles are carried by winds, waters and glaciers and deposited at various places. • The result of erosion upon the earth's surface is the formktion of hills and valleys.
The general level of the land is constantly changing (dias trophism); portions formerly under sea level being elevated and vice versa. Submergence of the strata permits the encroach ment of the sea, and the deposition of the eroded rock particles that are carried in suspension by the water (Fig. 5), forming nearly horizontal sediments, strata upon strata. Alternate rising and sinking of the land level allows for further erosion and deposition, so that former hills and valleys may be obliterated and new ones formed. This process might be continuous, or again be interrupted for a great length of time, causing a hiatus or unconformity between the sediments. (Fig. 6.) Deposition is a slow process, but during the great geological ages the amount of sedimentary rocks that have been formed this way are enor mous.
Certain geologists have represented the world as having a heaving crust—parts rising and sinking. Others represent the earth as a shrinking globe, but whatever the cause, the results upon the rocks are the same. The various thrusts and forces deform the almost horizontal sediments, tilting and folding them in every conceivable way. Changes of this kind in the rocks are accompanied by dislocations or faultS. (Fig. 7.) Supposing that after folding another rising and lowering of the land level takes place, further erosion and deposition may become possible upon the already deformed sediments, causing more and more complicated folding and unconformities, or perhaps obliterate previous folds. It is the folds and sometimes the faults as well,
that have a great bearing upon commercial oil and gas accumula tions, and hence the necessity of a careful study of folds and faults in the sedimentary rocks for a possible clue in locating oil and gas.
Climatic changes no doubt follow an extensive disturbance of the land level and such changes cause variation in the animal and plant life.
The Sedimentary Rocks The most common sedimentary rocks are: shales, sandstones, conglomerates, limestones, coals and glacial deposits. (Fig. 8.) Shale. Shale is formed by the consolidation of fine mud, deposited some distance from shore, and upon compacting forms a close-grained rock having an uneven and slaty structure. It is a fragile, argillaceous rock, of various colors such as gray, brown, red and black. As a shale is composed of closely packed fine particles its porosity is very small and therefore impervious to the movement of water, oil and gas to a great extent. Due to great pressure shales become more and more compact and less hydrous, thus forming slates.
Sandstone. A consolidation of sand grains due to the recementation of eroded particles, form sandstones. They are deposited in shallow water along the shore line, and the particles are cemented together by insoluble substances such as silicates, iron hydroxides and calcium carbonate. Sandstones are con sidered as porous rocks, the porosity varying with the shape of the grains, the kind and amount of cementing material holding them together, and the compacting of the sediments. Different figures have been given for the porosity of sandstones, which vary from 5 per cent up. Due to this porosity sandstones are very suitable rocks for oil and gas reservoirs, their porous portions permitting accumulations in commercial quantities. (Fig. 9.) Conglomerate. As the name implies, a conglomerate is a combination of rather large sand grains or pebbles, cemented together. It may also form a reservoir for oil and gas.