DEVELOPMENT OF FLUVIAL SURFACES IN ARID CANYONS OF SW NEVADA AND THEIR AFFECT ON CULTURAL SITE PRESERVATION

Surficial features of alluvium in selected dry washes in southwestern Nevada were studied to determine their maturity and potential for preserving cultural sites. Different processes dominate these deposits at different stages of development, each resulting in unique surficial features. Thus, careful observation of surface features can determine relative stability for preservation of cultural sites. The most immature and unstable surfaces are active washes composed of fair to poorly sorted gravel bars and sand-filled channels. There is no soil development, desert pavement, Av horizon, desert varnish, or eolian deposits. The age of active wash surfaces is measured in years and cultural artifacts located here have been reworked. Recently abandoned wash surfaces are characterized by relict gravel bars and channels 10 to 15 cm above the active wash. Thin overbank flood deposits of silt and sand sparsely covered by cryptobiotic soil lack desert pavement or Av horizons. Desert varnish is absent but half-buried cobbles exhibit thin coatings of pedogenic carbonate. The age of recently abandoned wash surfaces is measured in decades and cultural artifacts found here are disturbed. Older abandoned wash surfaces lie 15 to 60 cm above the active wash and are characterized by thicker eolian and overbank deposits of silt and sand partially burying relict bars and channels. Fairly well developed bioturbated soils exhibit mature cryptobiotic soils and juvenile desert pavement with incipient Av horizons. Desert varnish is absent but coatings of pedogenic carbonate on half-buried cobbles is common. The age of older abandoned wash surfaces is measured in centuries and younger cultural sites are intact, though partially buried. Terraces have the oldest and most mature surfaces, occur several meters above active washes, and are characterized by very mature cryptobiotic soil, mature desert pavement with a well developed Av horizon, burrow mounds of silt and sand, and desert varnish coating cobbles. The age of terrace surfaces is measured in millennia and often predates local human habitation; cultural sites located on these surfaces are stable and undisturbed.