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Ripple marks
In geology, ripple marks are sedimentary structures (i.e. bedforms of the lower flow regime) and indicate agitation by water (current or waves) or wind.
== Defining ripple cross-laminae and asymmetric ripples ==
* ''Current ripple marks'', ''unidirectional ripples'', or ''asymmetrical ripple marks'' are asymmetrical in profile, with a gentle up-current slope and a steeper down-current slope. The down-current slope is the angle of repose, which depends on the shape of the sediment. These commonly form in fluvial and aeolian depositional environments, and are a signifier of the lower part of the Lower Flow Regime.
* Ripple cross-laminae forms when deposition takes place during migration of current or wave ripples. A series of cross-laminae are produced by superimposing migrating ripples. The ripples form lateral to one another, such that the crests of vertically succeeding laminae are out of phase and appear to be advancing upslope. This process results in cross-bedded units that have the general appearance of waves in outcrop sections cut normal to the wave ''crests''. In sections with other orientations, the laminae may appear horizontal or ''trough''-shaped, depending upon the orientation and the shape of the ripples. Ripple cross-laminae will always have a steeper dip downstream, and will always be perpendicular to paleoflow meaning the orientation of the ripples will be in a direction that is ninety degrees to the direction that current if flowing.〔Boggs Jr., Sam. (2006) Principles of Sedimentology and Stratigraphy, 4th ed. 〕 Scientists suggest current drag, or the slowing of current velocity, during deposition is responsible for ripple cross-laminae.〔Potter, Pettijohn. (1977) Paleocurrents and Basin Analysis〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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