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Before the advent of plants with spores and body structures that are easily preserved in the fossil record, the evidence for terrestrial ecosystems is limited to observations of their effects on the chemistry of weathering surfaces. Seven paleosols and paleosaprolites from southeast South Dakota and northwest and central Iowa have been analyzed to assess the extent of abiotic weathering and the presence of biosignatures in the Midwestern US during the Middle Cambrian. Evidence for extensive weathering exists on a variety of basement material, from the 1800 Ma Harris granite to the 1100 Ma Keweenawan basalts. All of the weathering profiles are overlain by the Middle Cambrian Mt. Simon Sandstone, placing their most likely time of development shortly before the advent of vascular plants. All weathering horizons show high chemical indices of alteration, typically> 85, and considerable leaching of mobile cations …