Abstract The Martinez Mountain rock avalanche, tentatively dated as early Holocene, comprises approximately 3.8 x 10 8 m 3 of granitic gneiss that broke away from the mountainside at elevation 1,926 m and flowed to elevation 49 m. The length of the slide debris is 7.6 km, and the width ranges from 1,070 to 1,370 m. Rock in the area surrounding the slide is closely jointed, with northwest-trending faults cutting under the slide mass. Two dominant sets of high-angle joints intersect at the crown of the slide and are believed to have controlled formation of the broad V-shaped scarp. A third joint set dips down-slope at 15° to 30°, and it locally controlled the basal surface of the slide. Because of the crystalline nature of the rock involved in sliding, water was probably not a significant factor in the triggering mechanism; rather, it is thought that a major seismic event caused the slide. Such seismic triggering is consistent with historic occurrences of massive rockslides and in the seismically active Coachella Valley such an event would provide the energy needed to disaggregate the rock and start it moving. An understanding of the structural geology and probable triggering mechanism provides a basis for comparison with similar geologic terranes and assessment of the potential for catastrophic landsliding that might affect urban developments.