The middle Tertiary Valley Springs Formation is characterized over much of its surface and subsurface extent in the northern San Joaquin Valley by yellowish- to greenish-gray claystone with crude wavy bedding and common clay-lined partings, fractures, and tubules. Common glass shards and pumice grains in this lithofacies have led previous workers to interpret it as altered vitric tuff or welded tuff, but the presence of unaltered glass in the claystone and in the interbedded vitric tuff argues against such a simple genesis.

Analysis of the mineralogy, chemistry, fabric, and organic content of a 26 ft (8 m) thick section at Wallace, Calaveras

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County, has revealed a complex history. The presence of abundant freshwater microfossils (megaspores, chrysomonad cysts, diatoms, sponge spicules), together with only limited evidence of authigenesis, indicates that most of the claystone originated as detrital clay deposited in shallow ponds or marshes. Weathered horizons, at least one of which may represent a remnant of a fossil soil, and other evidence of surficial processes, such as root tubes (and roots) and nearly ubiquitous illuvial-clay coatings in pores, indicate that the ponds dried up periodically.

The dominant claystone lithofacies of the Valley Springs Formation, together with interbedded fine-grained sandstone (channel deposits?) and tuff, can be interpreted as the deposits of a poorly drained coastal plain that was occasionally blanketed by ash deposits and that extended westward from the present Sierra Nevada foothills to at least the present Coast Ranges foothills.

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