Flood-plain.—The nearly level land that borders a stream and is subject to occasional overflow. Flood-plains are built up by sediment left by such overflows.
Fold.—A bend in rock layers or beds. Anticlines and synclines are the common types of folds.
Formation.—A rock layer, or a series of continuously deposited layers grouped together, regarded by the geologist as a unit for purposes of description and mapping. A formation is usually named from some place where it is exposed in its typical character.
Fossil.—The whole or any part of an animal or plant that has been preserved in the rocks or the impression left on rock by a plant or animal. Preservation is invariably accompanied by some change in substance, and from some fossils the original substance has all been removed.
Geography.—The study of the distribution of the earth’s physical features, in their relation to each other to the life of sea and land, and human life and culture.
Geology.—The science which deals with the history of the earth and its inhabitants as revealed in the rocks.
Glacier.—A body of ice which slowly spreads or moves over the land from its place of accumulation.
Gneiss (pronounced nice).—A metamorphic, crystalline rock with mineral grains arranged with long axes more or less parallel, giving the rock a banded appearance. Derived from either igneous or stratified rocks well within the earth under conditions of pressure, and usually also heat and moisture.
Igneous Rocks.—Rocks formed by the cooling and solidification of a hot liquid material, known as magma, that has originated at unknown depths within the earth. Those that have solidified beneath the surface are known as intrusive rocks, or if the cooling has taken place slowly at great depth, as plutonic rocks, e.g. granite. Those that have flowed out over the surface are known as effusive rocks, extrusive rocks, or lavas, e.g., basalt. Volcanic rocks include not only lavas, but bombs, pumice, tuff, volcanic ash, and other fragmental materials or ejecta thrown out from volcanoes.
Joints.—Nearly all rocks, except very loose surface materials, are separated into blocks of varying size and shape by a system of cracks called joints. They may be caused by earth-crust movements, contraction during solidification of molten rocks, or contraction during drying out of sediments.