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.