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Parting lineation
Parting lineation (also known as current lineation or primary current lineation) is a subtle sedimentary structure in which sand grains are aligned in parallel lines or grooves on the surface of a body of sand (or lithified as a sandstone).〔Boggs, S., Principles of Sedimentology and Stratigraphy, 3rd Ed., pgs. 125-126〕 The orientation of the lineation is used as a paleocurrent indicator, although the precise flow direction (i.e. upstream vs. downstream) is often indeterminable. They are also the primary indicator of the lower part of the upper flow regime bedform.
== Description ==
Parting lineation is a sedimentary structure commonly found on the surface of parallel-laminated sandstones. It is aligned with the current direction, the alignment holding out in places over several square meters.〔Allen, J. R. L. (1964a). Sedimentology, 3, page 89 – 108〕 The lineation is formed by flat, parallel ridges which are separated by depressions or grooves. The height of the ridges rarely exceeds several grain diameters. In profile the depressions are flat-bottomed and the ridges are rounded. Ridges and depressions are arranged en echelon, so that in downstream direction the ridges pass into depressions and vice versa. The crosswise distance between individual ridges typically varies between 0.59 and 1.25 centimeters. Length as well as spacing of the ridges augments with increasing grain size: in fine-grained sands the ridges are 3.5 to 12 centimeters long, whereas in medium-grained sands they can reach 30 centimeters in length. The long dimension of the ridges is therefore 5 to 20 times their crosswise spacing. The coarse(r) sediment fraction accumulates in the ridges, whereas dark heavy minerals and mica trails take up an intermediate position between the ridges and the grooves.
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