Nonmarine lower Miocene rocks widely exposed in nearly continuous outcrop over approximately 3100 km² (1,200 mi²) of the Hartville Table in southeastern Wyoming and western Nebraska indicate a semiarid continental interior, with seasonal climate characterized by sandy ephemeral or intermittent braided streams, interchannel plains mantled by fine-grained volcaniclastic loess, and shallow ephemeral holomictic lakes. These paleoenvironments are recognized on the basis of distinctive sedimentologic, faunal, and taphonomic characteristics.

Stream sediments (10 percent or less of total outcrop) are primarily tuffaceous silty sandstones, deposited as reworked pyroclastic debris in wide shallow valleys. These valleys first filled with fluvial...