Detailed gravity profiles across linear gravity gradients 20 to 30 km long indicate 1 to 2 km of structural relief in zones more than 5 km wide. The gravity anomaly is attributed to low-density alluvial basin fill, the density of which is assumed to decrease basinward from the fault as a result of blending of piedmont-slope alluvial with fluvial sedimentary facies. A direct inverse solution of gravity for lateral density variation is derived. The gravity data are compatible with a single large fault and a density contrast increasing from 0 to 0.4 g/cm3 in a 5-km-wide zone. The data are also compatible with a constant density contrast and a gently dipping fault, although this seems unlikely, geologically, because most observed faults are high angle. Geologic and gravity data considered together suggest the presence of a relatively few, widely spaced, long, steeply dipping (at shallow levels), mostly buried “master” faults, to which most observed faults are secondary and sympathetic.