sand, gravel, and boulders varying widely in size and 

 shape. (See ablation till and basal till.) HP 



Glaciofluvial deposits — Material moved by glaciers and 

 subsequently sorted and deposited by streams flowing 

 from the melting ice. The deposits are stratified and 

 may occur in the form of outwash plains, valley trains, 

 and deltas, kames, eskers, and kame terraces. (See 

 glacial drift and glacial outwash.) HP 



Glaciolacustrine deposits — Material ranging from fine 

 clay to sand derived from glaciers and deposited in gla- 

 cial lakes by water originating mainly from the melting 

 of glacial ice. Many are bedded or laminated with 

 varves. HP 



Gorge — (a) A narrow, deep valley with nearly vertical 

 rocky walls, enclosed by mountains, smaller than a 

 canyon (geomorph), and more steep-sided than a ra- 

 vine; especially a restricted, steep-walled part of a can- 

 yon, (b) A narrow defile or passage between hills or 

 mountains. GG 



Gulch — A small stream valley, narrow and steepsided in 

 cross section, and larger than a gully. Gulch is a re- 

 gional term of the western U.S. A more generally used 

 synonym is ravine. (See draw.) HP 



Gully — A very small valley with steep sides cut by running 

 water and through which water ordinarily runs only 

 after a rain or ice or snow melt. The distinction between 

 a gully and a rill is one of depth. A gully generally is an 

 obstacle to wheeled vehicles and is too deep to be oblit- 

 erated by ordinary tillage; a rill is of lesser depth and 

 can be smoothed over by ordinary tillage. (See gulch, 

 arroyo, wash, and draw.) HP 



Hanging valley — A tributary valley whose floor at the 

 lower end is notably higher than the floor of the main 

 valley in the area of junction. GG 



Headland (coast)— (a) An irregularity of land, especially 

 of considerable height with a steep cliff face, jutting 

 out from the coast into a large body of water (usually 

 the sea or a lake); a bold promontory or a high cape. 

 Synonyms include head and mull, (b) The high ground 

 flanking a body of water, such as a cove, (c) The steep 

 crag or cliff face of a promontory. GG 



Headwall — The steep slope at the head of a valley; espe- 

 cially the rock cliff at the back of a cirque. GG 



Hill — A natural elevation of the land surface, rising as 

 much as 1,000 ft (300 m) above surrounding lowlands, 

 usually of restricted summit area (relative to a table- 

 land) and having a well defined outline; hill slopes gen- 

 erally exceed 15%. The distinction between a hill and a 

 mountain is often dependent on local usage. (See foot- 

 hills.) HP 



Hillslope— The steeper part of a hill between its summit 

 and the drainage line, valley flat, or depression floor at 

 the base of the hill. In descending order geomorphic 

 components of a simple hillslope may include shoul- 

 der, backslope, footslope, and toeslope. However, all 

 of these components are not necessarily present in any 

 given hillslope continuum. In addition, complex hill- 

 slopes may include two or more backslope to toeslope 

 sequences, (fig. B-2) HP 



Hogback — A sharp-crested, symmetric (homoclinal) 

 ridge formed by highly tilted resistant rock layers; pro- 

 duced by differential erosion of interlayered resistant 

 and weak rocks with dips greater than about 25 ° (45%). 

 (See cuesta.) HP 



Holocene — The second epoch of the Quaternary Period 

 of geologic time, extending from the end of the Pleisto- 

 cene Epoch (about 10 to 12 thousand years ago) to the 

 present; also the corresponding (time-stratigraphic) 

 "series" of earth materials. Two synonyms of holo- 

 cene are post-glacial and Recent.) HP 



Homoclinal (structural geomorphology) — Pertaining to 

 strata that dip in one direction with a uniform angle. 

 (See cuesta and hogback.) HP 



Hummock — A rounded or conical mound or knoll, hillock 

 or other small elevation. Also, a slight rise of ground 

 above a level surface. GG 



Igneous rock — Rock formed by solidification from a 

 molten or partially molten state; major varieties in- 

 clude plutonic and volcanic rocks. (See intrusive and 

 extrusive. Examples are andesite, basalt, and granite.) 

 HP 



Interfluve — The elevated area between two fluves (drain- 

 ageways) that sheds water to them. FFP 



Intermittent stream — A stream, or reach of a stream, that 

 flows for protracted periods only when it receives 

 ground-water discharge or long-continued contribu- 

 tions from melting snow or other surface and shallow 

 subsurface sources. (See ephemeral stream.) HP 



Intermontane basin — A generic term for wide structural 

 depressions between mountain ranges that are partly 

 filled with alluvium and are called "valleys" in the ver- 

 nacular. Intermontane basins may be drained inter- 

 nally (bolsons) or externally (semi-bolson). FFP 



Intrusive — Denoting igenous rocks derived from molten 

 matter (magma) which invaded pre-existing rocks and 

 cooled below the surface of the earth. (See extrusive.) 

 HP 



Joint (geology)— A surface of actual or potential fracture 

 or parting in a rock, without displacement; the surface 



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