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U-shaped valley
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A deep valley with steep upper walls that grade into a flat floor, usually eroded by a glacier.
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Uniformitarianism, Principle of
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The concept that the processes that have shaped the Earth through geologic time are the same as those observable today.
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ultramafic rock
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An igneous rock consisting dominantly of mafic mineral, containing tiron and magnesium and with much less silicon and aluminum than most crustal rocks. Most come from the Earth’s mantle. Includes dunite, peridotite, amphibolite, and pyroxenite.
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unconformity
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"The contact between older rocks and younger sedimentary rocks in which at least some erosion has removed some of the older rocks before deposition of the younger. An angular unconformity shows that the older rocks have been deformed and eroded before the younger sedimentary rocks were deposited; there is an angle between the bed of the older and the younger."
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unconsolidated
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Loosely arranged, not cemented together, so particles separate easily.
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unit cell
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The smallest contiguous group of atomic structural units in a mineral that can be repeated in three directions to form a crystal.
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unsaturated zone
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Zone between the land surface and the water table in which fluid pressures are less than atmospheric.
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uplift
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A broad and gentle epeirogenic increase in the elevation of a region without a eustatic change of sea level.
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upwelling current
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The upward movement of cold bottom water in the sea, which occurs when wind or currents displace the lighter surface water.
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