Abstract Voluminous serpentinite strata interfinger with Lower Cretaceous terrigenous clastics of the Great Valley sequence along the western edge of the Sacramento Valley. These deposits are exposed in the core of a regional, southeast-plunging structure, the Wilbur Springs anticline. The sedimentary serpenti-nites form a distinctive lithologic unit of foliate serpentinite breccias in road cuts along Highway 20 just west of its intersection with Highway 16. Extensive serpentinite masses of sedimentary origin were first reported in the area of Wilbur Springs by Taliaferro (1943). He recognized their association with fault-bounded serpentinite of the ophiolite belt but was unable to establish sufficient criteria to distinguish between the two types in many cases. Thus the origin of much of the exposed serpentinite remained unresolved. Lawton (1956), citing numerous fossil discoveries within serpentinite breccia and gradational contacts with surrounding Great Valley strata, suggested that all of the serpentinite bodies south of Wilbur Springs were the products of sedimentary processes. Although he noted textural variation between chaotic and stratified units, he did not attach genetic significance to these facies changes. He considered all the deposits detrital accumulations despite the poor stratification, chaotic nature, schistose fabric, and the admitted lack of internal clastic textures in many of the bodies. The foliation was attributed to later tectonism, which he thought had obliterated original depositional features. Moiseyev (1966), in his study of the mercury mineralization of the Wilbur Springs District, concurred with Lawton's findings and provided further documentation of the detrital nature of some of the ultramafic rocks. Stressing the inherent difficulty in discriminating sedimentary serpentinites, he concluded that lateral gradation of foliate breccia into coarse elastics of ultramafic composition and the presence of fossils in the serpentinite are the only unambiguous criteria of sedimentary origin (Moiseyev, 1970).