Ultramafic rock: An igneous rock consisting dominantly of mafic minerals, containing less than 10 percent feldspar. Includes dunite, peridotite, amphibolite, and pyroxenite.

Unconformity: A surface that separates two strata. It represents an interval of time in which deposition stopped, erosion removed some sediments and rock, and then deposition resumed (see also Angular unconformity ).

Unconsolidated material: Nonlithified sediment that has no mineral cement or matrix binding its grains.

Unit cell: The smallest contiguous group of atomic structural units in a mineral that can be repeated in three directions to form a crystal.

Uplift: A broad and gentle epeirogenic increase in the elevation of a region without a eustatic change of sea level.

Upwelling current: The upward movement of cold bottom water in the sea, which occurs when wind or currents displace the lighter surface water.

U-shaped valley: A deep valley with steep upper walls that grade into a flat floor, usually eroded by a glacier.