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 fine-grained volcaniclastics, but with the cessation of streamflow in the region, filling was completed by air-fall volcaniclastic loess that blanketed both valleys and interchannel reaches. Fluvial sediments within the valleys include much spatially dispersed mammal bone that had been scavenged and subaerially weathered prior to burial. Waterholes, situated in or adjacent to the valleys, filled with tuff and carbonate mud containing freshwater ostracods, pulmonate gastropods, diatoms, and charophyte algae. These tuffaceous waterhole muds intertongue with fluvial volcaniclastic sediments and are the locus of major mammalian bone beds, the best known preserved at Agate Fossil Beds National Monument. Bones of chalicothere, rhinoceros, and entelodont are common in waterhole bone beds and in fluvial sediments in the region.

Massive tuffaceous air-fall silty sandstones (87 percent of outcrop) punctuated by silcrete paleosols were deposited in the interchannel reaches; mammal remains are commonly represented by widely scattered, isolated bones and partial skeletons of young and aged ungulates, chiefly oreodonts and camels, indicative of attritional deaths over time. No bone beds occur.

Thin silicified carbonate mudstones (about 2 percent of outcrop) with ostracods, plant debris, and aquatic pulmonate gastropods (but without fish or other aquatic vertebrates) indicate shallow, holomictic, ephemeral lakes that filled with homogeneous micrite mud. These lakes were isolated sheet-like bodies of water unassociated with stream sediments. Following desiccation, lacustrine sediments were commonly overprinted by pedogenic features.

Eolian transport of fine pyroclastic detritus into the North American midcontinent was essential to preservation of these sedimentary environments and their rich fossil record. In the Americas and in Africa during the Cenozoic, fine-grained volcaniclastic sediments blanketed large geographic areas within the continental interiors, preserving significant temporal intervals of the vertebrate fossil record. If volcanism had not occurred, these intervals would exist as major hiatuses in our knowledge of vertebrate, particularly mammalian, evolution. The important role of fine-grained volcaniclastics in preservation of mammalian faunas and their associated depositional environments in the Americas and in Africa during the Cenozoic deserves greater emphasis.