base level . (a) The theoretical limit or lowest level toward which erosion of the Earth's surface constantly progresses but seldom, if ever, reaches; esp. the level below which a stream cannot erode its bed. The general or ultimate base level for the land surface is sea level, but temporary base levels may exist locally. The base level of eolian erosion may be above or below sea level; that of marine erosion is the lowest level to which marine agents can cut a bottom. Cf: base level of erosion. (b) A curved or planar surface extending inland from sea level, inclined gently upward from the sea and representing the theoretical limit of stream erosion. (c) The surface toward which external forces strive, at which neither erosion nor deposition takes place (Barrell, 1917); a surface of equilibrium.
GeoWord of the Day
The GeoWord of the Day is a free service of the American Geosciences Institute. All of the terms and definitions are from the Glossary of Geology, 5th Edition Revised.
uricite (u'-ri-cite). A white monoclinic mineral: C5H4N4O3 .
adventitious (ad-ven-ti'-tious). A plant part that arises from an unusual place, e.g. a root that arises from a leaf or stem rather than from a primary root.
meridian line . A line running accurately north and south through any given point on or near the Earth's surface; specif. a line used in plane surveying and defined by the intersection of the plane of the celestial meridian and the plane of the horizon.
cover head . A thick accumulation of debris, consisting of talus cones and alluvial fans, resting on an elevated marine terrace; the material is deposited during and after the emergence of the terrace.
pathfinder (path'-find-er). In geochemical exploration, a relatively mobile element or gas that occurs in close association with an element or commodity being sought, but can be more easily found because it forms a broader halo or can be detected more readily by analytical methods. A pathfinder serves to lead investigators to a deposit of a desired substance.
thalweg [streams] . (a) The line connecting the lowest or deepest points along a stream bed or valley, whether under water or not; the longitudinal profile of a stream or valley; the line of maximum depth. Syn: valley line. (b) The median line of a stream; the valley axis. (c) channel line. Etymol: German, old spelling "Thalweg" (now "Talweg"), "valley way". Pron: [taal-vehk].
hydrohetaerolite (hy''-dro-het-ae'-ro-lite). A dark brown to black tetragonal mineral of uncertain composition, possibly Zn2Mn3+4O8•H2O .
percent slope (per-cent'). The direct ratio between the vertical distance and the horizontal distance for a given slope; e.g. a 3-meter rise in 10 meters horizontal distance would be a 30 percent slope.
Bergeron-Findeisen process . Precipitation formation in cold clouds whereby ice crystals grow at the expense of supercooled water droplets in response to differences in vapor pressure relative to water and ice surfaces. Also known as the ice-crystal process.
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