There are four major dune shapes: crescent, linear, star, and parabolic. The shapes are determined by the direction and variability of the wind and by the supply of sand. A seif dune is a linear, or longitudinal, dune (sayf is “sword” in Arabic); seif dunes form in ridges, parallel to one another and to the direction of the wind, so that the prevailing wind sweeps through the troughs between them. They are narrow dunes with steep crests and slip faces on either side. The origin of seif dunes is a matter still under study, but it is generally agreed that they are formed where the wind comes strongly and perhaps seasonally from at least two different directions. The Algodones Dunes in the Imperial and Coachella Valleys contain the largest stretch of linear dunes in California. They are under more or less regular assault from the off-road vehicle lobby. A spectacular pair of seif dunes can be seen at Sand Mountain in Nevada, off Highway 50, sixteen miles southeast of Fallon.