_Dike._--A mass of igneous rock that has solidified in a fissure or crack in the earth's crust. _Drift._--Commonly called glacial drift. The rock fragments--soil, gravel, and silt--carried by a glacier. Drift includes the unassorted material known as till (ground moraine) and deposits made by streams flowing from a glacier. _Drowned River Valley._--When a land surface sinks enough to permit tidewater to enter the lower ends of its valleys to form estuaries, a good example being the lower Hudson Valley. _Era._--A name applied to one of the broadest subdivisions of geologic time (e.g. Paleozoic era). _Erosion._--The wearing away and transportation of materials at and near the earth's surface by weathering and solution, and the mechanical action of running water, waves, moving ice, or winds which use rock fragments as tools or abrasives. _Exfoliation._--The splitting off of sheets of rock of various sizes and shapes due to changes of temperature. It is a process of weathering. _Fault._--A fracture in the earth's crust accompanied by movement of the rock on one side of the break past that on the other. If the fracture is inclined and the rock on one side appears to have slid down the slope of the fracture the fault is termed a normal fault. If, on the other hand, the rock on one side appears to have been shoved up the inclined plane of the break, the fault is termed a reverse or thrust fault. _Fault-block._--A part of the earth's crust bounded wholly or in part by faults. _Fault-scarp._--The cliff formed by a fault. Most fault scarps have been modified by erosion since the faulting. _Fissure._--A crack, break, or fracture in the earth's crust or in a mass of rock. _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.