The lower to middle Eocene Ione Formation in southern Madera County was deposited in an alluvial-fan, braided-stream complex bordering the ancestral Sierra Nevada. Sandstone and conglomerate occur as a thin veneer unconformably overlying a deeply weathered Mesozoic igneous and metamorphic basement. Two facies are recognized: a braided-stream sandstone and a proximal to midfan conglomerate with sandstone interbeds.

Braided-stream facies display both sandstone and thin matrix-supported conglomerate (debris flows). Sandstone units contain tabular cross-bedded and parallel-laminated bedding units, which may be capped by small-scale trough cross-beds. This longitudinal-bar sequence may locally occur adjacent to large-scale festoon cross-beds of channel origin. Locally, lebensspuren (trace fossils) are abundant in the sandstone units and these traces represent biologic activity on emergent longitudinal bars or inactive channels.

The proximal to midfan facies consists of interbedded conglomerate and sandstone. Matrix-supported conglomerate units were deposited by debris flows and cross-bedded sandstone units were deposited in alluvial channels. Framework-supported conglomerate may have resulted by reworking of the sediment, removing the clay (forming sieve deposits), or as alluvial channel gravels.

The Ione Formation in Madera County is part of a long, narrow alluvial plain between the ancestral Sierra Nevada and the ocean. This part of the Ione Formation differs from the mixed marine and nonmarine environments of the Ione at its type section.

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