Magnetic and gravity anomalies, with typical amplitudes of 1,000 γ and 50 mgal, respectively, occur over the Great Valley of California. Aeromagnetic data were combined to make a composite aeromagnetic map of most of the Great Valley and adjoining areas. Good correlation between local details of the gravity and magnetic anomalies suggests that dense, magnetic rock is the source of the anomalies. A nearly continuous magnetic high and weak gravity highs occur where serpentinite is exposed along the Coast Range thrust fault. Magnetic highs, but no associated gravity highs, occur where serpentinite crops out in the western Sierra Nevada metamorphic belt. The metavolcanic rocks are relatively non-magnetic. Major magnetic highs with strong associated gravity highs are caused by gabbro at three places in the western Sierra Nevada. Two of these gabbro occurrences are probably part of ophiolite complexes. Gravity, magnetic, and drill-hole data were used to construct a map of basement rock types. Gabbro and similar mafic rocks are abundant beneath the crest of the Great Valley anomalies. Ultramafic rocks are very rare in drill holes that reach basement. A major break in the anomaly patterns suggests a possible east-trending fault in the basement rocks near Fresno. A two-dimensional crustal model was made across central California through the use of seismic refraction data and gravity and magnetic modeling. Magnetic rock comes to within 2.5 km of the surface just east of the center of the Great Valley and dips steeply to the west beneath the western side of the valley. If gabbro is the source of the anomalies, then sialic crust must be virtually nonexistent beneath the Great Valley. Analogy with the ophiolite complexes of the Sierra Nevada foothills suggests that the source of the Great Valley anomalies is a tectonically emplaced fragment of oceanic crust. In Late Jurassic time, an eastward-dipping subduction zone in the western Sierra Nevada foothills became detached and stepped westward to the present position of the Coast Range thrust fault, leaving behind a fragment of oceanic crust. I propose that this fragment, covered by Tithonian and younger strata, causes the Great Valley magnetic and gravity anomalies.