maar
a low relief, broad volcanic crater formed by multiple, shallow explosive eruptions. It is surrounded by a crater ring in the form of low ramparts of gently dipping (i.e. < 25 degrees), well-bedded ejecta; may be partially or completely filled by water (maar lake). SW & GG

mafic rock
a general term for igneous rock composed chiefly of one or more ferromagnesian, dark-colored minerals; also said of those minerals. compare - felsic rock. GG

main scarp
the steep surface on undisturbed ground at the upper edge of a landslide, caused by movement of displaced material away from the undisturbed ground; it is visible a part of the surface of rupture (slip surface). compare - minor scarp, toe. CV & SW

marine deposit
sediments (predominantly sands, silts and clays) of marine origin; laid down in the salty waters of an ocean. compare - estuarine deposit. SW

marine terrace
A constructional coastal strip, sloping gently seaward, veneered by marine deposits (typically silt, sand, fine gravel). Compare - terrace, wave-built terrace. GG

marl
an earthy, unconsolidated deposit consisting chiefly of calcium carbonate mixed with clay in approximately equal proportions (35 to 65 percent of each); formed primarily under freshwater lacustrine conditions, but varieties associated with more saline environments also occur. HP

marsh
periodically wet or continually flooded areas with the surface not deeply submerged. covered dominantly with sedges, cattails, rushes, or other hydrophytic plants. compare - salt marsh, swamp, bog, fen. GSST

mass movement
a generic term for any process or sediments (mass movement deposit) resulting from the dislodgement and downslope transport of soil and rock material as a unit under direct gravitational stress. the process includes slow displacements such as creep and solifluction, and rapid movements such as landslides, rock slides, and falls, earthflows, debris flows, and avalanches. agents of fluid transport (water, ice, air) may play an important, if subordinate role in the process. HP

mass-movement till
(not preferred) refer to "till".

mass wasting
(not preferred) refer to mass movement.

mawae
(colloquial: Hawaii) a natural surface channel commonly found near the middle of an a'a lava flow, formed by the surficial draining of molten lava rather than by erosion from running water; a type of lava trench. compare - lava tube. MA

meander [streams]
one of a series of regular freely developing sinuous curves, bends, loops, turns, or windings in the course of a stream. GG

meander belt
the zone within which migration of a meandering channel occurs; the flood-plain area included between two imaginary lines drawn tangential to the outer bends of active channel loops. landform components of the meander-belt surface are produced by a combination of gradual (lateral and down-valley) migration of meander loops and avulsive channel shifts causing abrupt cut-offs of loop segments. landforms flanking the sinuous stream channel include: point bars, abandoned meanders, meander scrolls, oxbow lakes, natural levees, and flood-plain splays. meander belts may not exhibit prominent natural levee or splay forms. flood plains of broad valleys may contain one or more abandoned meander belts in addition to the zone flanking the active stream channel. HP

meandering channel
the term "meandering" should be restricted to loops with channel length more than 1.5 to 2 times the meander wave length. meandering stream channels commonly have cross sections with low width-to-depth ratios, cohesive (fine-grained) bank materials, and low gradient. at a given bank-full discharge, meandering streams have gentler slopes, and deeper narrower, and more stable channel cross sections than braided streams. compare - meander, braided stream, flood-plain landforms. HP & RR

meander scar
a) a crescent-shaped, concave or linear mark on the face of a bluff or valley wall, produced by the lateral erosion of a meandering stream which impinged upon and undercut the bluff; if it's no longer adjacent to the modern stream channel it indicates an abandoned route of the stream. SW;
b) (not recommended - refer to oxbow) an abandoned meander, commonly filled in by deposition and vegetation, but still discernable. GG

meander scroll
a) one of a series of long, parallel, close fitting, crescent-shaped ridges and troughs formed along the inner bank of a stream meander as the channel migrated laterally down-valley and toward the outer bank. compare - meander belt, point bar.
b) (not recommended; refer to oxbow lake) - a small, elongate lake on a flood plain in a well-defined part of an abandoned stream channel. GG

medial moraine
a) an elongate moraine carried in or upon the middle of a glacier and parallel to its sides, usually formed by the merging of adjacent and inner lateral moraines below the junction of two coalescing valley glaciers.
b) a moraine formed by glacial abrasion of a rocky protuberance near the middle of a glacier and whose debris appears at the glacier surface in the ablation area.
c) the irregular ridge left behind in the middle of a glacial valley, when the glacier on which it was formed has disappeared. GG

melt-out till
till derived from slow melting of debris-rich stagnant ice buried beneath sufficient overburden to inhibit deformation under gravity, thus preserving structures derived from the parent ice. compare - flow-till, lodgement till. GG

mesa
a broad, nearly flat-topped, and usually isolated landmass bounded by steep slopes or precipitous cliff and capped by layers of resistant, nearly horizontal, rocky summit width greater than the height of bounding escarpments. (Colloquial: western USA; not preferred) also used to designate broad structural benches and alluvial terraces that occupy intermediate levels in stepped sequences of platforms bordering canyons and valleys. compare - butte, plateau, cuesta. HP & GG

metamorphic rock
rock of any origin altered in mineralogical composition, chemical composition, or structure by heat, pressure, and movement at depth in the earth's crust. nearly all such rocks are crystalline. examples: schist, gneiss, quartzite, slate, marble. HP

metasediment
a sediment or sedimentary rock that shows evidence of having been subjected to metamorphism. GG

meta-stable slope
(not recommended).

meteorite crater
an impact crater formed by the falling of a large meteorite onto the earth's surface; e.g., Barringer Crater (AZ). compare - crater, impact crater. SW & GG

microbiotic crust
a thin, surface layer (crust) of soil particles bound together primarily by living organisms and their organic byproducts; thickness can range up from < 1 cm up to 10 cm; aerial coverage of the ground surface can range from 10 - 100%. Crusts stabilize loose earthy material. other types of surface crusts include chemical crusts (e.g., salt crusts) and physical crusts (e.g., raindrop-impact crust). SW & SS

micro-depression
(not preferred) refer to micro-low.

microfeature [soil survey]
small, local, natural forms (features) on the land surface that are too small to delineate on a topographic or soils map at commonly used map scales (e.g. 1:24,000 to 1:10,000). examples include earth pillar, patterned ground, frost boil. compare - microrelief. SW

micro-high
a generic microrelief term applied to slightly elevated areas relative to the adjacent ground surface; changes in relief range from several centimeters to several meters; cross-sectional profiles can be simple or complex and generally consist of gently rounded, convex tops with gently sloping sides. SW

micro-knoll
(not preferred) refer to micro-high.

micro-low
a generic microrelief term applied to slightly lower areas relative to the adjacent ground surface (e.g. shallow depression); changes in relief range from several centimeters to several meters; cross-sectional profiles can be simple or complex and generally consist of subdued, concave, open or closed depressions with gently sloping sides. SW

microrelief
a) [soil survey] slight variations in the height of a land surface that are too small or intricate to delineate on a topographic or soils map at commonly used map scales (e.g. 1:24,000 through 1:10,000). examples include micro-high, micro-low. compare - microfeature. SW
b) (not preferred - refer to microfeature) generically refers to local, slight irregularities in form and height of a land surface that are superimposed upon a larger landform, including such features as low mounds, swales, and shallow pits. GG

midden
a mound or stratum of refuse (broken pots, ashes, food remains, etc.) normally found on the site of an ancient settlement. GG

mima mound
a term used for one of numerous low circular or oval domes composed of loose, unstratified, gravelly, silty, or sandy material. the basal diameter varies from 3 meters to more than 30 meters, and the height from 30 centimeters to about 2 meters. compare - pimple mound, patterned ground, shrub-coppice dune. GG

mine spoil or earthy fill [soil survey]
an accumulation of displaced earthy material, rock, or other waste material removed during mining or excavation. SW & GSST

minor scarp
a steep surface on the displaced material of a landslide, produced by differential movements within the sliding mass. compare - main scarp, toe. CV

Miocene
the epoch of the Tertiary Period of geologic time (from approximately 5.2 million to 23 million years ago), immediately following the Oligocene Epoch and preceding the Pliocene Epoch; also corresponding (time-stratigraphic) "series" of earth materials. HP

mogote
(colloquial: Caribbean Basin) an isolated, steep-sided, commonly asymmetrical hill or ridge composed of limestone, generally steeper on its leeward side (prevailing downwind side) and surrounded by nearly level to sloping coastal plain composed of marine and alluvial sediments; a type of karst tower. they range in height from a few feet (< 1 m) to over 150 ft (50 m). most are isolated and cover small areas but some form clusters of hills or ridges rising out of the surrounding blanket deposits. Mogotes are extensive in northern Puerto Rico. SW, Monroe ( 1976, 1980), & WW

monadnock
an isolated hill or mountain of resistant rock rising conspicuously above the general level of a lower erosion surface in a temperate climate representing an isolated remnant of a former erosion cycle in an area that has largely been beveled to its base level. compare - inselberg, nunatak. GG

monocline
a) [landform] a unit of folded strata that dips from the horizontal in one direction only, is not part of an anticline or syncline, and occurs at the earth's surface. this structure is typically present in plateau areas where nearly flat strata locally assume steep dips caused by differential vertical movements without faulting. compare - anticline, syncline, fold. SW & HP
b) [structural geology]- a local steepening in an otherwise uniform gentle dip. GG

moraine [glacial geology]
a) [material] a mound, ridge, or other topographically distinct accumulation of unsorted, unstratified glacial drift, predominantly till, deposited primarily by the direct action of glacier ice, in a variety of landforms.
b) [landform] a general term for a landform composed mainly of till that has been deposited by a glacier; a kame moraine is a type of moraine similar in exterior form to other types of moraines but composed mainly of stratified outwash materials. types of moraine include: disintegration, end, ground, kame, lateral, recessional, and terminal. SW

mossy organic materials
see organic materials.

moss peat [Soil Taxonomy]
an accumulation of organic material that is predominantly the remains of mosses (e.g. sphagnum moss). compare - Herbaceous peat, sedimentary peat, woody peat, peat, muck, and mucky peat. SSM

mound
a) a low, rounded natural hill of unspecified origin, generally < 3 m high and, composed of earthy material;
b) a small, human-made hill, composed either of debris accumulated during successive occupations of the site (e.g. tell) or of earth heaped up to mark a burial site (e.g. burial mound).
c) a structure built by colonial organisms (e.g. termite mound). GG

mountain
a generic term for an elevated area of the land surface, rising more than 300 meters above surrounding lowlands, usually with a nominal summit area relative to bounding slopes and generally with steep sides (greater than 25 percent slope) with or without considerable bare-rock exposed. a mountain can occur as a single, isolated mass or in a group forming a chain or range. Mountains are primarily formed by tectonic activity and/or volcanic action and secondarily by differential erosion. compare - hill, hillock, plateau, foothills, mountains. SW & HP

mountainbase
a geomorphic component of mountains consisting of the lowermost area, consisting of the strongly to slightly concave colluvial apron or wedge at the bottom of mountain slopes; composed of long-transport colluvium and slope alluvium sediment. it can extend out onto more level valley areas where it ultimately interfingers with, is buried by alluvium or is replaced by re-emergent residuum. compare - mountaintop, mountainflank, free face, geomorphic component. SW

mountainflank
a geomorphic component of mountains consisting of the side area of mountains, characterized by very long, complex backslopes with comparatively high slope gradients and composed of highly-diverse, colluvial sediment mantles, complex near-surface hydrology, mass movement processes and features (e.g., creep, landslides); rock outcrops or structural benches may be present. the mountainflank can be subdivided by the general location along the mountainside (i.e., upper third, middle third, or lower third mountainflank). compare - mountaintop, mountainbase, free face, geomorphic component. SW

mountain range
a single, large mass consisting of a succession of mountains or narrowly spaced mountain ridges, with or without peaks, closely related in position, direction, orientation, formation, and age; a component part of a mountain system. compare - mountain system, mountains. GG

mountains
a region or landscape characterized by mountains and their intervening valleys; a generic name for any group, cluster, or sequence of mountains or narrowly spaced mountain ridges, with or without peaks, closely related in position, orientation, direction, formation, or age, and whose summits commonly exceed 300 m (approx. 1000 ft). compare - foothills, hills, mountain range, mountain system. SW

mountainside
(not recommended) use mountain slope.

mountain slope
a part of a mountain between the summit and the foot. compare - mountainflank, hillslope. GG

mountain system
a group of mountain ranges exhibiting certain unifying features, such as similarity in form, structure and alignment, and presumably originating from the same general causes; especially a series of mountain ranges belonging to an orogenic belt. compare - mountain range, mountains. GG

mountaintop
a geomorphic component of mountains consisting of the uppermost, comparatively level or gently sloped area of mountains, characterized by relatively short, simple slopes composed of bare rock, residuum, or short-transport colluvial sediments. In humid environments, mountaintop soils can be quite thick and well developed. compare - mountainflank, mountainbase, free face, geomorphic component. SW

mountain valley
a) any small, externally drained V-shaped depression (in cross-section) cut or deepened by a stream and floored with alluvium, or a broader, U-shaped depression modified by an alpine glacier and floored with either till or alluvium, that occurs on a mountain or within mountains. several types of mountain valleys can be recognized based on their form and valley floor sediments (i.e., V-shaped valley, U-shaped valley). compare - valley. SW
b) (colloquial: Basin and Range, USA) a relatively small, structural depression within a mountain range that is partly filled with alluvium and commonly drains externally to an intermontane basin, bolson, or semi-bolson. compare - valley flat. SW & FFP

muck
unconsolidated soil material consisting primarily of highly decomposed organic material in which the original plant parts are not recognizable (i.e. "sapric" in Soil Taxonomy). it generally contains more mineral matter and is usually darker in color, than peat. compare - peat, mucky peat, herbaceous peat. GSST

mucky peat
unconsolidated soil material consisting primarily of organic matter that is in an intermediate stage of decomposition such that a significant part of the original material can be recognized and a significant part of the material can not be recognized (i.e. "hemic" in Soil Taxonomy). compare - peat, muck, herbaceous peat. SSM

mud flat
a relatively level area of fine grained material (e.g. silt) along a shore (as in a sheltered estuary) or around an island, alternately covered and uncovered by the tide or covered by shallow water, and barren of vegetation. compare - low marsh, tidal flat, tidal marsh. GG

mudflow [mass movement] the process, associated sediments (mudflow deposit) or resultant landform characterized by a very rapid type of earthflow dominated by a sudden, downslope movement of a saturated mass of rock, soil, and mud (more than 50 % of the particles are < 2 mm), that behaves as much as a viscous fluid when moving. compare - debris flow, flow, landslide. SW & DV

mud pot
a type of hot spring containing boiling mud, usually sulfurous and often multicolored, as in a paint pot. mud pots are commonly associated with geysers and other hot springs in volcanic areas, especially in Yellowstone Natl. Park, WY. compare - geyser, hot spring. GG

mudstone
a) a blocky or massive, fine-grained sedimentary rock in which the proportions of clay and silt are approximately equal;
b) a general term that includes clay, silt, claystone, siltstone, shale, and argillite, and that should be used only when the amounts of clay and silt are not known or cannot be precisely identified. GG

muskeg
a bog, usually a sphagnum bog, frequently with grassy tussocks (hummocks), growing in wet, poorly drained boreal regions, with deep accumulations of organic material, often in areas of permafrost; a moss-covered muck or peat bog of boreal regions. GG and HP