Glossary

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Scroll down to see the "U" terms

ultimate base level

Sea level; the level below which a trunk stream cannot cut at its mouth.

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 moving downslope in a matter of seconds to weeks.

unstable slope

A slope on which sliding will likely happen.

updraft

Upward-moving air.

uplift

336.

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.