The lower Eocene Willwood Formation of the Bighorn Basin, northwest Wyoming, consists of about 770 m of alluvial rocks that exhibit extensive mechanical and geochemical modifications resulting from Eocene pedogenesis. Five arbitrary stages are proposed to distinguish these soils of different maturities in the Willwood Formation. An inverse relationship exists between soil maturity and short-term sediment accumulation rate. Illustrates several important principles of soil-sediment interrelationships in aggrading alluvial systems that have broad application to other deposits.-from Authors