Sediment comprising the Cambrian(?) Van Horn Sandstone was derived from a highland source area of rhyolite, granite, and metamorphic and sedimentary rocks. Detritus ranging from boulders to silt was transported southward through canyons by high-gradient streams under rapid-flow and surge-flow conditions. South of the canyon mouths, the sediment was spread across the fan surface by shallower, less-confined braided streams; mud-flow deposits are absent. Three gradational facies, from north to south, are recognized in the fan deposits.

Proximal facies are deposited in the feeder canyons and near their mouths. This facies consists of massive cobble and boulder beds overlain by thinner, horizontally bedded pebble, cobble, and boulder gravels.

Mid-fan facies consist of gravels and sands. Gravel lenses are interpreted as longitudinal bars (parallel-bedded, convex-upward deposits) and channel fills (convex-downward deposits). Mid-fan sands occur as channel fills, across the tops of gravel bars, and in lows flanking them. Foreset and trough crossbeds are the dominant sedimentary structures in these sands which are interpreted as transverse bars with dunes.

Distal facies is a sand sequence in which three subfacies have been delineated: (1) braided mainstream deposits containing both foreset and trough crossbeds; (2) braided distributary deposits characterized by a relatively high content of muddy sand units, well-preserved channel cross sections, some ripple cross-laminae, and soft-sediment and injection structures; and

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(3) braided interlobe deposits which are nearly mud-free, thick sand sequences made up almost exclusively of small trough crossbeds.

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