Extensive dunes of southwestern California and northwestern Sonora, Mexico, were derived mainly from Cenozoic Colorado River delta sediment. All dune sand and sandy delta sediment have K-feldspar in excess of plagioclase, abundant volcanic fragments, detrital Cretaceous Foraminifera, and abundant detrital calcite and dolomite. Sands derived from local crystalline bedrock and carried by the Whitewater River and major washes entering the Salton Basin lack these features.

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