Slate.—A rock that by subjection to pressure within the earth has acquired the property of splitting smoothly into thin plates. The cleavage is smoother and more regular than the splitting of schist along its grain. It is a metamorphic rock nearly always derived from shale.
Soil.—The mantle of loose material resting upon bedrock, either in its place of origin or transported by water, wind, or ice.
Strata (or stratified rocks).—Sedimentary rocks which, by the sorting power of water (less often by wind), are arranged in more or less definite layers or beds separated by stratification surfaces.
Stratification.—The separation of sedimentary rocks into more or less parallel layers or beds.
Stratigraphy.—The branch of geologic science that deals with the order and relations of the strata of the earth’s crust.
Structure.—In geology, the forms assumed by sedimentary beds and igneous rocks that have been moved from their original position by forces within the earth, or the forms taken by intrusive masses of igneous rock in connection with effects produced mechanically on neighboring rocks by the intrusion. Folds (anticlines and synclines) and faults are the principal mechanical effects considered under structure. Schistosity and cleavage are also structural features.
Syncline.—A kind of folded structure in which strata have been bent downward. It is an inverted arch—the opposite of an anticline.
Talus (pronounced tāy’lus).—The mass of loose rock fragments that accumulates at the base of a cliff or steep slope.
Terrace.—A steplike bench on a hillside. Most terraces along rivers are remnants of valley bottoms formed when the stream flowed at higher levels. Other terraces have been formed by waves. Some terraces have been cut in solid rock, others have been built up of sand and gravel, and still others have been partly cut and partly built up.
Tertiary.—The earlier of the two periods of the Cenozoic era of geologic time.