Lunate

Lunate/Crescentic:

The word lunate is derived from the Latin word luna which means “crescent-shaped” (or moon).

Lunate refers to a structure having the shape of a crescent moon, and is a term used in various fields, including geology, archaeology, biology, engineering, and medicine.

A structure resembling the figure of the moon as it appears between its first or last quarter and the new moon, with concave and convex edges terminating in two narrow pointed ends.

Physical drivers of lunate landforms include aeolian forces, currents and wave action, glacial movement, and others.

 (Further definition references are listed at the bottom of the Word of the Day home page)

Where in California:

Lunate sand ripples, or wave ripples:

Sand ripples and wave ripples are found in most shallow marine depositional environments, and develop in sandy or sedimentary media of small grain size, under increasing hydrologic velocity of unidirectional currents and/or wave motion. Sediment or sand ripples appear in repetitive symmetrical or asymmetrical bands, displaying pronounced downstream (lee) form/angles, and modest upstream (stoss) form/angles. Sand ripple form is defined, or classified, by the shape of the crest, including lunate formed sand ripples/wave ripples. Once ripples take form, they migrate downstream.       

Barchan and Parabolic Dunes:

As with submarine sand ripples, sand dunes form under similar physical forces of directional flow, but by aeolian transport. Dunes also develop pronounced, or steep, angles on the lee side, and gently sloped angles on the upstream/wind-facing stoss slope. Dune form is classified by crest shape, and both Barchan and Parabolic dunes are crescentic, or lunate, and form by unidirectional wind flow. Barchan dune crests are formed facing into the wind, and Parabolic dune crests face away, or in the same directional flow of the wind. Primary environments for dune formation in California are coastal dune fields and deserts.  

Glacial landforms:

Glaciers have a knack for producing lunate landforms, both small and large. In California’s alpine environments glacial forces have carved immense crescentic cirque valleys out of mountain ranges, plowed-up giant ridges of rubble into lunate terminal moraines, and have left perfectly crescent-shaped erosional marks on rock faces known as lunate fractures. Lunate fractures also display gently sloped crescents facing up-glacier/upstream, and steep faces on the downstream side.