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

UNCONFINED AQUIFER: An aquifer that is not overlain by an aquiclude, thereby causing the level of water in a well that penetrates the aquifer to be at the level of the surrounding groundwater table. (Compare Confined aquifer.)

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

UNCONSOLIDATED MATERIAL: Unlithified sediment that has no mineral cement or matrix binding its grains.

UNIFORMITARIANISM, PRINCIPLE OF: The concept that the processes that have shaped Earth through geologic time are the same as those observable today.

UNSATURATED ZONE: The region in the ground between the surface and the groundwater table in which pores are not completely filled with water.

UPWARPED MOUNTAINS: Mountains elevated by uplift of broad regions without faulting.

U-SHAPED VALLEY: A deep valley with steep upper walls that grade into a flat floor; typical shape of a valley eroded by a glacier.