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...