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ultimate base level	Sea level; the level below which a trunk stream cannot cut.
ultramafic	A term used to describe igneous rocks or magmas that are rich in iron and magnesium and very poor in silica.
unconfined aquifer	An aquifer that intersects the surface of the Earth.
unconformity	A boundary between two different rock sequences representing an interval of time during which new strata were not deposited and/or were eroded.
unconsolidated	Consisting of unattached grains.
undercutting	Excavation at the base of a slope that results in the formation of an overhang.
undersaturated	A term used to describe a solution capable of holding more dissolved ions.
unsaturated zone	The region of the subsurface above the water table.
unstable air	Air that is significantly warmer than air above and has a tendency to rise quickly.
unstable ground	Land capable of slumping or slipping down- slope in the near future.
unstable slope	A slope on which sliding will likely happen.
updraft	Upward-moving air.
upper mantle	The uppermost section of the mantle, reaching down to a depth of 400 km.
upwelling zone	A place where deep water rises in the ocean, or hot magma rises in the asthenosphere.
U-shaped valley	A steep-walled valley shaped by glacial erosion into the form of a U.