The largest deposit of gypsum in California occurs in the Miocene Split Mountain Formation. It is located at the north end of the Fish Creek Mountains, in Imperial and San Diego Counties, about 70 mi (113 km) east of San Diego. The deposit consists of up to 200 ft (65 m) of massive rock gypsum, lying at the top of the Split Mountain Formation. Selenite and celestite occur in scattered pockets. Anhydrite is present as erratic lenses in the gypsum, and interbedded clay occurs near the top and bottom contacts of the deposit. Minor impurities in the gypsum include varying concentrations of chloride salts and fine-grained, opaque manganese and iron oxides. The deposit is underlain by nonmarine gray conglomeratic sandstone, and in complete sections is overlain by marine shale a d poorly consolidated sandstone of the Miocene Imperial Formation. Both contacts appear conformable and gradational.

The gypsum outcrops as erosional remnants, which have been preserved in a shallow synclinal basin 3 mi (4.8 km) long and 1 mi (1.6 km) wide. The general dip is 15 to 30° toward the synclinal axis, with sharp local contortions. The overlying Imperial Formation has been eroded away making estimates of the original thickness impossible. Overburden is nonexistent and the gypsum forms smoothly rounded hills, capped by a thin layer of gypsite.

The origin of this deposit appears to fit the "modified bar hypothesis," which suggests that calcium sulfate was precipitated in seacoast lagoons where evaporation took place rapidly, and periodic influx of seawater across shallow bars added new increments of salts. Gradual sinking of the lagoon allowed accumulations of great thicknesses.

The rock is mined in a side-hill bench quarry, crushed at the quarry site, and then shipped by private narrow-gauge railway to a calcining and wallboard plant at Plaster City, 25 mi (40 km) to the south.

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