We estimate the thickness of Neogene basin fill along the junction of the Eastern California Shear Zone and the North Frontal thrust system of the San Bernardino Mountains using gravity data with geologic and well log constraints. The geometry of the basin fill is of interest for groundwater assessment and location of potential faults, as well as providing an upper bound on the thickness of any potential, buried Pliocene sediments. Nearly one thousand new gravity measurements were collected along the North Frontal thrust system from Hesperia to Johnson Valley. Three-dimensional inverse modeling of the new and existing gravity data shows that Neogene deposits are segmented into basins along the range front of the San Bernardino Mountains. The Helendale fault, a dextral fault in the Eastern California Shear Zone, separates shallower basement (approximately 300 m depth) beneath Lucerne Valley east of the fault from deeper basement (approximately 550 m or more) west of the fault. The thickest part of the basin fill is generally located near the San Bernardino Mountains and the basin shallows northward. The amount of throw on the North Frontal thrust appears to decrease eastward, as the gravity gradient associated with the fault diminishes in amplitude. The thickness of basin fill away from the North Frontal system and east of the Helendale fault is less than 100 to 200 m, except for local pockets generally developed along strike-slip faults of the Eastern California Shear Zone and local east-west oriented depressions associated with folding of the basin fill.