nation of organic and inorganic processes and include 

 chemical and clastic (soluble and insoluble) consti- 

 tuents; many contain fossils. HP 



Lithification — The conversion of a newly deposited, 

 unconsolidated sediment into a coherent and solid 

 rock, involving processes such as cementation, com- 

 paction; desiccation, crystallization, recrystallization, 

 and compression. It may occur concurrent with, or 

 shortly or long after deposition. HP 



Lithologic— Pertaining to the physical character of a 

 rock. HP 



Lodgment till— A basal till commonly characterized by 

 compact fissile structure and containing stones oriented 

 with their long axes generally parallel to the direction 

 of ice movement. GG 



Loess — Fine-grained wind-deposited material, dominantly 

 of silt-size. HP 



Marl — An earthy, unconsolidated deposit consisting 

 chiefly of calcium carbonate mixed with clay in ap- 

 proximately equal proportions (35 to 65 °7o of each); 

 formed primarily under freshwater lacustrine condi- 

 tions, but varieties associated with more saline environ- 

 ments also occur. HP 



Mass wasting (mass movement) — Dislodgement and 

 downslope transport of earth (regolith and rock) mate- 

 rial 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 ava- 

 lanches. Agents of fluid transport (water, ice, air) may 

 play a subordinate role in the process. HP 



Meander, meandering channel (flood-plain landforms)— 

 A meander is one of a series of sinuous loops, with 

 sine-wave form, in the course of a stream channel. The 

 term "meandering" should be restricted to loops with 

 channel length more than 1.5 to 2 times the length of 

 the wave form. Meandering stream channels commonly 

 have cross sections with low width to depth ratios, 

 (fine-grained) cohesive bank materials, and low gra- 

 dient. At a given bank, full discharge meandering 

 streams have gentler slopes, and deeper, narrower and 

 more stable channel cross-sections than braided streams. 

 (See flood-plain landforms.) HP 



Meander belt — The bottomland zone within which mi- 

 gration of a meandering channel occurs; the flood- 

 plain area included between two imaginary lines drawn 

 tangentially 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 evulsive 



channel shifts causing abrupt cut-offs of loop seg- 

 ments. Forms flanking the sinuous stream channel 

 include: point bars, scars of abandoned meanders and 

 flanking point bars, meander scrolls, oxbow lakes, nat- 

 ural levees and flood-plain splays. Many meander belts 

 do not exhibit prominent natural levee or splay forms. 

 Flood plains of broad alluvial valleys may contain one 

 or more abandoned meander belts in addition to the 

 zone flanking the active stream channel. HP 



Meander scroll — Individual ridge-swale pairs. (See 



meander belt.) HP 



Mesa — A broad, nearly flat-topped and usually isolated 

 upland mass characterized by summit widths that are 

 greater than the heights of bounding erosional escarp- 

 ments. A tableland produced by differential erosion of 

 nearly horizontal, interbedded weak and resistant 

 rocks, with the latter comprising caprock layers. As 

 summit area decreases relative to height, mesas are 

 transitional to buttes. In the western states mesa is also 

 commonly used to designate broad structural benches 

 and alluvial terraces that occupy intermediate levels in 

 stepped sequences of platforms bordering canyons and 

 valleys. (See plateau and cuesta.) HP 



Metamorphic rock — Rock of any origin altered in miner- 

 alogical composition, chemical composition, or struc- 

 ture by heat, pressure, and movement at depth in the 

 earth's crust. Nearly all such rock are crystalline. (Ex- 

 amples are schist, gneiss, and quartzite.) HP 



Metastable slope — A slope that is relatively stable at the 

 present time, but may become active if the environ- 

 mental balance is disturbed, for instance, by road con- 

 struction or destruction of vegetation. A metastable 

 slope is often related to base levels of former geomor- 

 phic episodes. The regolith is generally moderately 

 deep, may contain stone lines, or relict evidence of 

 slope alluvium, slope gradients usually range from 15 

 to 45 percent. (See active slope.) HP 



Mima mound — A term used in the NW U.S. for one of 

 numerous low, circular or oval domes composed of 

 loose, unstratified, gravelly silt and soil material, built 

 upon glacial outwash on a hogwallow landscape; the 

 basal diameter varies from 3 m to more than 30 m, and 

 the height from 30 cm to about 2 m. GG 



Monocline— A unit of folded strata that flexes from the 

 horizontal in one direction only, and is not part of an 

 anticline or syncline. This structure is typically present 

 in plateau areas where nearly flat stata locally assume 

 steep dips caused by differential vertical movements 

 without faulting. HP 



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