Editing Sediment (section)

===Shape===
[[File:Rounding & sphericity EN.svg|thumb|300px|Schematic representation of difference in grain shape. Two parameters are shown: sphericity (vertical) and [[Roundness (geology)|rounding]] (horizontal).]]
The shape of particles can be defined in terms of three parameters. The ''form'' is the overall shape of the particle, with common descriptions being spherical, platy, or rodlike. The ''roundness'' is a measure of how sharp grain corners are. This varies from well-rounded grains with smooth corners and edges to poorly rounded grains with sharp corners and edges. Finally, ''surface texture'' describes small-scale features such as scratches, pits, or ridges on the surface of the grain.<ref>{{cite book |last1=Boggs |first1=Sam |title=Principles of sedimentology and stratigraphy |date=2006 |publisher=Pearson Prentice Hall |location=Upper Saddle River, N.J. |isbn=0131547283 |edition=4th |page=65}}</ref>

====Form====
{{See also|Sphericity}}
Form (also called ''sphericity'') is determined by measuring the size of the particle on its major axes. [[William C. Krumbein]] proposed formulas for converting these numbers to a single measure of form,<ref>{{cite journal |last1=Krumbein |first1=William C. |title=Measurement and Geological Significance of Shape and Roundness of Sedimentary Particles |journal=SEPM Journal of Sedimentary Research |date=1941 |volume=11 |pages=64–72 |doi=10.1306/D42690F3-2B26-11D7-8648000102C1865D}}</ref> such as

:<math>\psi_l = \sqrt[3]{\frac{D_S D_I}{D_L^2}}</math>

where <math>D_L</math>, <math>D_I</math>, and <math>D_S</math> are the long, intermediate, and short axis lengths of the particle.{{sfn|Boggs|2006|p=582}} The form <math>\psi_l</math> varies from 1 for a perfectly spherical particle to very small values for a platelike or rodlike particle.

An alternate measure was proposed by Sneed and Folk:<ref>{{cite journal |last1=Sneed |first1=Edmund D. |last2=Folk |first2=Robert L. |title=Pebbles in the Lower Colorado River, Texas a Study in Particle Morphogenesis |journal=The Journal of Geology |date=March 1958 |volume=66 |issue=2 |pages=114–150 |doi=10.1086/626490|bibcode=1958JG.....66..114S |s2cid=129658242 }}</ref>

:<math>\psi_p = \sqrt[3]{\frac{D_S^2}{D_L D_I}}</math>

which, again, varies from 0 to 1 with increasing sphericity.

====Roundness====
{{Main|Roundness (geology)}}
[[File:Rounding.gif|thumb|Comparison chart for evaluating roundness of sediment grains]]
Roundness describes how sharp the edges and corners of particle are. Complex mathematical formulas have been devised for its precise measurement, but these are difficult to apply, and most geologists estimate roundness from comparison charts. Common descriptive terms range from very angular to angular to subangular to subrounded to rounded to very rounded, with increasing degree of roundness.{{sfn|Boggs|2006|pp=66-67}}

====Surface texture====
Surface texture describes the small-scale features of a grain, such as pits, fractures, ridges, and scratches. These are most commonly evaluated on [[quartz]] grains, because these retain their surface markings for long periods of time. Surface texture varies from polished to frosted, and can reveal the history of transport of the grain; for example, frosted grains are particularly characteristic of [[Aeolian processes|aeolian]] sediments, transported by wind. Evaluation of these features often requires the use of a [[scanning electron microscope]].{{sfn|Boggs|2006|pp=68-70}}