The Algodones dune belt, which is 40 miles long in a northwesterly direction, 3–6 miles wide, and has individual dunes 200-300 feet high, lies along the southeastern border of the Cahuilla Basin, a structural depression in southeastern California. Much of the basin is below sea level and includes Imperial and Coachella valleys. The lowest portion of the basin is now occupied by the Salton Sea.
Most detritus enters the basin from the high mountains to the west and northwest; little material is contributed by the lower, drier desert ranges to the east.
The dunes appear unsystematic from the ground, but as seen from the air they consist of long northwest-trending ridges on the west which individually curve eastward and disappear in a complex of prominent south-facing slip faces. In the central and southern part of the dune area some of the slip faces are 200–300 feet high and overlook large flat-floored, sand-free depressions which are interpreted as exposed parts of the desert floor over which a succession of large, closely spaced complex barchans is advancing. Present conditions favor destruction of the depressions by encroachment of sand in the form of small linear ridges and barchan dunes from the transverse dunes.
Color and degree of rounding of the sand grains indicate that the dunes are now less active than in the past and that the amount of sand is not increasing significantly.
The dunes were probably produced by wind transport of sand inshore from the beaches of Lake Cahuilla, a much larger forerunner of the Salton Sea. The large volume of material in these ancient beaches indicates vigorous wave and current action along the northeastern shore of the lake.