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natural levee
a long, broad low ridge or embankment of sand and coarse silt, built by a stream on its flood plain and along both sides of its channel, especially in time of flood when water overflowing the normal banks is forced to deposit the coarsest part of its load. it has a gentle slope away from the river and toward the surrounding floodplain, and its highest elevation is closest to the river bank. compare - levee, artificial levee, meander belt. GG
neck [volcanic]
a vertical, pipe-like tower of solidified lava or consolidated fragmental igneous rock that represents a former volcanic vent whose surrounding material (e.g. tuff and tephra) has been largely removed by erosion. compare - plug [volcanic] SW, GG, & GS
net (nonsorted)
(not preferred) refer to patterned ground.
net (sorted)
(not preferred) refer to patterned ground.
nivation
the process of excavation of a shallow depression or nivation hollow on a mountain side by removal of fine material around the edge of a shrinking snow patch or snow bank, chiefly through sheetwash, rivulet flow, and solution in melt water. freeze-thaw action is apparently insignificant GG
nivation hollow
a shallow, non-cliffed depression or hollow on a mountain side permanently or intermittently occupied by a snow bank or snow patch and produced by nivation. If the snow completely melts each summer the hollow is deepened; otherwise not; may be a cirque precursor if further enlarged and deepened by alpine glaciation. GG
nonsorted circle
a type of patterned ground whose mesh (shape) is dominantly circular and has a nonsorted appearance due to the absence of a boarder of coarse fragments. vegetation characteristically outlines the pattern by forming a bordering ridge. diameters commonly range from 0.5 to 3 m. nonsorted circles include mud boils, earth hummocks, turf hummocks, and frost boils. nonsorted circles have various origins. some, such as mud and earth hummocks and frost boils, involve cryoturbation activity and differential heave of frost-susceptible materials. others, such as mud boils, involve hydraulic pressures and diapir-like displacement of water-saturated sediments. compare - sorted circle, frost boil, patterned ground. NRC and GG
nonsorted polygon
(not preferred) refer to patterned ground.
nose slope [geomorphology]
a geomorphic component of hills consisting of the projecting end (laterally convex area) of a hillside, resulting in predominantly divergent overland water flow (e.g., sheet wash); contour lines generally form convex curves. nose slopes are dominated by colluvium and slope wash sediments (e.g., slope alluvium). slope complexity (downslope shape) can range from simple to complex. nose slopes are comparatively drier portions of hillslopes and tend to have thinner colluvial sediments and profiles. compare - head slope, side slope, free face, interfluve, crest, base slope. SW
notch
a) (colloquial - northeast USA) a narrow passageway, or short defile between mountains; a deep, close pass. compare - gap.
b) a breached opening in the rim of a volcanic crater. GG
nuée ardente
a swiftly flowing, turbulent gaseous cloud, sometimes incandescent, erupted from a volcano and containing ash and other pyroclastics in its lower part; a density current of pyroclastic flow. compare - lahar. GG
nunatak
an isolated hill, knob, ridge, or peak of bedrock that projects prominently above the surface of a glacier and is completely surrounded by glacier ice. compare - inselberg, monadnock. GG
ocean
the continuous salt-water body that surrounds the continents and fills the Earth's great depressions; also, one of its major geographic divisions. compare - sea. GG
offshore bar
(not recommended) use barrier beach.
Oligocene
the epoch of the Tertiary Period of geologic time (from 23.3 to 35.4 million years ago), following the Eocene Epoch and preceding the Miocene Epoch; also the corresponding (time-stratigraphic) "series" of earthy materials. SW
open depression
a generic name for any enclosed or low area that has a surface drainage outlet whereby surface water can leave the enclosure; an area of lower ground indicated on a topographic map by contour lines forming an incomplete loop or basin indicating at least one surface exit. compare - closed basin. SW
openpit mine
a relatively large depression resulting from the excavation of material and redistribution of overburden associated with surficial mining operations. compare - quarry, surface mine. SW & GG
organic materials
[soil survey] unconsolidated sediments or deposits in which carbon is an essential, substantial component. several types of organic materials (deposits) can be identified based on the composition of the dominant fibers (grassy organic materials, herbaceous organic materials, mossy organic materials, woody organic materials). compare - herbaceous peat, moss peat, sedimentary peat, woody peat. SW
outcrop
a) that part of a geologic formation or structure that appears at the surface of the earth.
b) [soil survey] an actual exposure of bedrock at or above the ground surface. compare - cliff. SW & GG
outwash [glacial geology]
stratified and sorted sediments (chiefly sand and gravel) removed or "washed out" from a glacier by melt-water streams and deposited in front of or beyond the end moraine or the margin of a glacier. the coarser material is deposited nearer to the ice. compare - pitted outwash, drift, esker, kame, till. SW & GG
outwash delta
a relict (inactive) delta composed of glaciofluvial sediments formed where a sediment laden outwash river emptied into an open lake, commonly a proglacial lake. sediment attributes include very gently dipping topset beds (coarser textures) and steeply dipping foreset beds (finer textures). SW & GM
outwash fan
a fan-shaped accumulation of outwash deposited by meltwater streams in front of the end moraine of a glacier. coalescing outwash fans form an outwash plain. GG
outwash plain
an extensive lowland area of coarse textured, glaciofluvial material. an outwash plain is commonly smooth; where pitted, due to melt-out of incorporated ice masses (pitted outwash plain), it is generally low in relief and largely retains it's original gradient. compare - outwash, pitted outwash plain, collapsed outwash plain, kettles. SW & HP
outwash terrace
a flat-topped bank of outwash with an abrupt outer face (scarp or riser) extending along a valley downstream from an outwash plain or terminal moraine; a valley train deposit. compare - kame terrace, valley train. SW
overbank deposit
fine-grained sediments (silt and clay) deposited from suspension on a flood plain by floodwaters that cannot be contained within the stream channel. GG
overburden
a) the upper part of a sedimentary deposit, compressing and consolidating the materials below.
b) the loose soil or other unconsolidated material overlying bedrock, either transported or formed in place (synonym for regolith). GG
overflow stream channel
a watercourse that is generally dry but conducts flood waters that have overflowed the banks of a river, commonly from large storms, annual meltwater, or glacial meltwaters. SW
overprinting
the process of superimposing a new set of features over a preexisting set due to a shift in environmental conditions such as a change in climate or local hydrology. the resulting composite morphology retains features that would not form under present conditions. compare - overprinted soil. SW
overprinted soil
a soil in which new soil morphology has developed and is superimposed upon that of a pre-existing soil due to a shift in pedogenic conditions such as a change in climate or hydrology; the composite morphology retains some relict features that would not form under present day conditions. (not preferred) sometimes called welded soil. SW
overthrust
a low angle thrust fault of large scale, with displacement generally measured in kilometers. GG
oxbow
a closely looping stream meander having an extreme curvature such that only a neck of land is left between the two parts of the stream. (colloquial: northeastern U.S.A.) the land enclosed, or partly enclosed, within an oxbow. compare - meander belt, oxbow lake, bayou. GG
oxbow lake
the crescent-shaped, often ephemeral body of standing water situated by the side of a stream in the abandoned channel (oxbow) of a meander after the stream formed a neck cutoff and the ends of the original bend were silted up. compare - meander belt, oxbow. GG
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