ABSTRACT The Marias River canyon in north-central Montana, incised into Upper Cretaceous strata of the Great Plains during latest Pleistocene to Holocene time, served as a locus of human activity tied to the unique floral and faunal resources it provided ancient peoples. Erosion of the main canyon walls resulted in deposition of tributary junction alluvial fans characterized by debris-flow and hyperconcentrated flow sediment transport processes where side canyons emerged onto the alluvial valley floor. These alluvial-fan deposits preserve Late Precontact archaeological remains accessible due to their postburial exposure where partially eroded by the meandering channel of the Marias River (Goose Bill site complex). Archaeological materials are also preserved high on the dissected canyon walls where movement of hillslope sediment and colluvium by sheetwash led to their burial (Sparrowhawk site). Deposition of sediment of contrasting physical attributes (grain size, sorting, clay mineral content) within the canyon influences soil substrate properties, favoring growth of specialized plant communities in both alluvial-fan and fluvial environments. These relations exemplify the strong geoecological connections among depositional environment and sediment characteristics, substrate properties, and vegetation community development. The plant communities, interacting in conjunction with the physical landscape, provided a range of habitats utilized by such large mammals as plains bison, Rocky Mountain elk, Audubon sheep, pronghorn, Rocky Mountain mule deer, and white-tailed deer. Four major bison bone layers, some including bison hair and other soft tissues, with associated stone arrow points and stone flakes (artifacts) and evidence of bone processing are preserved in alluvial-fan deposits exposed along the bank of the Marias River at the Goose Bill site complex. These relations illustrate: (1) human activities dating to ~100-595 yr B.P. (falling within the Late Precontact Period of regional archaeological prehistory) that reflect subsistence reliance on bison as a food source, (2) the importance of tributary alluvial fans proximal to canyon walls in preserving archaeological remains, and (3) the role of the meandering Marias River channel in destroying these deposits over a time period of <100 yr. Sediment deposition by overland flow higher up the canyon walls at the Sparrowhawk site preserves a somewhat older (710-830 yr B.P.) archaeological record suggestive of a broader spectrum of resource (food) processing spatially decoupled from an area of bison kill/death.

The Upper Cretaceous to Paleocene(?) Canaan Peak Formation of southwestern Utah consists of approximately 100 m of cobble conglomerate and subordinate sandstone. Coarse-grained lithofacies include massive to very crudely stratified pebble to cobble conglomerate (Gm) and trough cross-stratified conglomerate (Gt). Minor associated lithofacies include trough (St) and planar (Sp) cross-stratified sandstone and scour-fill sandstone (Ss). Gravel deposition occurred during high-discharge periods within a Scott/Donjek-type perennial braided fluvial system on longitudinal bars (Gm) and sinuous-crested transverse bars (Gt), and as a product of longitudinal bar-top and interbar channel scour filling (Gt). Sand accumulated under lower flow-velocity conditions through migration of interbar channel dunes and transverse bars (St/Sp) and development of scour and fill deposits (Ss). Clast imbrication and trough-axis orientation measurements indicate east to north-east paleoflow directions. Gravel-sized clasts are predominantly resistant lithologies including Upper Precambrian-Cambrian quartzite, fossiliferous Paleozoic chert, and some silicified middle Jurassic volcanic rocks; less resistant Paleozoic limestone clasts are only locally abundant. Canaan Peak Formation detritus was derived from erosion of highlands created by Cretaceous Sevier-style thrust-fault development to the west in southeastern Nevada and western Utah and distributed across an extensive gravel-dominated braidplain complex. However, it is not possible to determine whether Canaan Peak deposition records active uplift or is postorogenic. Sediment was transported eastward a minimum distance of 45 to 60 km and was subject to continuous high-energy reworking within the Canaan Peak fluvial system, resulting in destruction of all but the most stable clast lithologies.