Homocline

In structural geology, a homocline or homoclinal structure (from old  = same, cline = inclination), is a geological structure in which the layers of a sequence of rock strata, either sedimentary or igneous, dip uniformly in a single direction having the same general inclination in terms of direction and angle.Jackson, JA, J Mehl and K Neuendorf (2005) ''Glossary of Geology.'' American Geological Institute, Alexandria, Virginia. 800 pp. Huggett, JR (2011) ''Fundamentals of Geomorphology,'' 3rd ed., Routledge, New York. 516 pp. A homocline can be associated with either one limb of a fold, the edges of a dome, , slice of thrust fault, or a tilted fault block. When the homoclinal strata consists of alternating layers of rock that vary hardness and resistance to erosion, their erosion produces either cuestas, homoclinal ridges, or hogbacks depending on the angle of dip of the strata.Thornbury, W. D., 1954, ''Principles of Geomorphology'' New York, John Wiley & Sons, 618 pp. Twidale, C.R., and E.M. Campbell (1993) ''Australian Landforms: Structure, Process and Time.'' Gleneagles Publishing, Adelaide, South Australia, Australia. 568 pp. Gerrard, J (1988) ''Rocks and landforms.'' Allen & Unwin Inc.,Winchester, Mass. 319 pp. On a topographic map, the landfroms associated with homoclines exhibit nearly parallel elevation contour lines that show a steady change in elevation in a given direction. In the subsurface, they characterize by parallel structural contour lines.Reynolds, SJ, and GH Davis (1996) ''Structural geology of rocks and regions.'' John Wiley, New York, New York. 864 pp.

Unicline and Uniclinal are obsolete and currently uncommon terms that are defined and have been used by geologists and geomorphologists in an inconsistent and contradictory manner. They are terms that have been used in a mutually exclusive manner as a synonym for either a homoclineThomas, G, and A Goudie (2000) The Dictionary of Physical Geography, 3rd ed. Blackwell Publishing Ltd., Maiden, Massachusetts. 624 pp. or monoclineU.S. Bureau of Mines Staff (1996) ''Dictionary of Mining, Mineral, & Related Terms.'' Report SP-96-1, U.S. Department of Interior, U.S. Bureau of Mines, Washington, D.C.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, J.A., eds. (2005) ''Glossary of Geology'' (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. depending the author. The meaning of this term has been further confused by Grabau, who redefined ''uniclinal,'' not as a geological structure, but as a general term for ridges produced by erosion of anticlines.Grabau, AW (1920a) ''Unicline; a term proposed for monoclinal ridges of erosion.'' Geological Society of America Bulletin. 31(1):153.Grabau, AW (1920b) ''A Comprehensive Geology, vol. 1'' D. C. Heath & Company, New York, New York. 804 pp.

The erosion of tilted sequences of either stratified sedimentary or igneous rock, homoclines, of alternating resistance to erosion produce distinctive landforms that form a gradational continuum from cuestas through homoclinal ridges to hogbacks. Less resistant beds are preferentially eroded creating valleys that lie between ridges created by the erosion of more resistant beds. For example, the erosion of homoclines consisting of resistant beds of either limestone, sandstone, or both interbedded with weaker, less resistant beds of either shale, siltstone, marl, or combination of them will produce either cuestas, homoclinal ridges, or hogbacks depending on the angle of dip of the strata. The greater the difference in the resistance to erosion, the more pronounced the structural control and relief between valley and ridge crest.

Etymology

According to Whitney in 1890Whitney, WD (1890) ''The Century Dictionary and Cyclopedia: An Encyclopedia Lexicon of the English Language.'' vol. VI, The Century Company, New York, New York. and Kelley in 1955,Kelley VC (1955
''Monoclines of the Colorado Plateau.''
Geological Society of America Bulletin. 66(7):789-804. Charles Darwin used the term ''uniclinal'' prior to 1843 to describe to strata dipping uniformly in one direction. Later in 1843, Rogers and RogersRogers, WB, and HD Rogers (1843) ''On the physical structure of the Appalachian chain, as exemplifying the laws which regulated the elevation of great mountain chains generally.'' Association of Geographers and Naturalists. 1:474-531. created the term ''monocline'' and used it to describe beds dipping uniformly in one direction within the Appalachian Mountains. However, starting with Powell in 1873,Powell, JW (1873) ''Geological structure of a Grand Canyon of the Colorado.'' American Journal of Science 3rd series, 5(5):456-465. geologists also used ''monocline'' to specifically describe a step-like fold in otherwise uniformly dipping strata while other geologists still used it to describe beds dipping uniformly in one direction. Thus, the term ''monocline'' was used to describe both beds dipping in one direction and a one-limbed flexure with different authors making incompatible distinctions between them. Because of this confused usage of ''monocline'', DalyDaly, RA (1915) ''A Geological Reconnaissance between Golden and Kamloops, B.C., along the Canadian Pacific Railway.'' memoir no. 68. Canada Geological Survey, Canada department of Mines, Ottawa, Canada. 260 pp.Daly, RA (1916) ''Homocline and Monocline.'' Geological Society of America Bulletin 27:89-92. formally introduced the term ''homocline'' and in 1915 and 1916 defined it in its current usage and redefined ''monocline'' as a one-limbed flexure as it is currently used.

References

{{Structural geology

Structural geology
Geomorphology