Geoarchaeological investigations were conducted at 41PS800, a Late Prehistoric archaeological site within the Redford Bolson portion of the Presidio Basin. Cultural materials at 41PS800 were present in the fill of a late Holocene terrace of the Rio Grande River. The terrace, which has been heavily impacted by stream channel migration and highway construction, measures approximately 75 m long and 6 to 8 m wide, and is comprised of fine sand and silt overbank deposits. The terrace tread is situated 4 to 5 m above the average low-water level of the modern Rio Grande channel and 1.5 to 2 m above the higher of two active Rio Grande floodplain surfaces. The geoarchaeological investigations focused on four key aspects of geological and geomorphic processes at 41PS800:(1) the physical composition of stratigraphic units within the terrace fill; (2) the geochronology of local sediment aggradation; (3) characteristics of former on-site surface topography; and (4) natural site formation processes and their potential impact on systemic contex.