28 Geologic Mapping in the West (Posters)

This poster session highlights geologic mapping in the western Cordillera and Rocky Mountains. We encourage submissions that explore bedrock and surficial geology, geophysics, hydrology, hazards, and more. Both professional and student submissions are welcome.

Tuesday, 19 May 2026
8:00 AM - 5:30 PM
Alvarado D/E

Landslide Map of the Historic Virginia City Area, Southwest Montana

Paper number: 28-5
Program: Technical Sessions
Time: 8:15 AM – 8:15 AM

Authors

Abstract

Virginia City was one of Montana's first major placer mining districts and is now a designated National Historic Landmark, drawing substantial tourism to the area. The district and surrounding area are underlain by widespread landslide deposits, which potentially pose a significant hazard to local infrastructure. Additionally, landslides may be an important geohydrologic control over a local spring used for Virginia City’s municipal water supply (Bobst and others, 2022). In 2025, the Montana Bureau of Mines and Geology (MBMG) conducted surficial mapping to further assess the extent and potential hazard of mass wasting in the region, building on work completed in 2017. Mapping efforts were supported by the STATEMAP component of the National Cooperative Geologic Mapping Program.

Virginia City is located in Madison County of southwest Montana, near a low mountain pass separating the Tobacco Root Mountains from the Greenhorn Range. The map area is underlain by Archean metamorphic rocks consisting mostly of quartzofeldspathic gneiss intercalated with amphibolite that is crosscut by Proterozoic diabase and pegmatitic dike swarms. The basement assemblages are nonconformably overlain by late Eocene to early Oligocene volcanogenic rocks composing the Virginia City volcanic group. The volcanic deposits are prone to mass wasting, and consequently, numerous landslides and debris-flow deposits rim the high-standing volcanic tablelands. Many of the landslides appear to fail within devitrified tuff intervals that are saturated with groundwater and locally emanate spring creeks.

New GIS-based landslide mapping identifies areas of gravity-driven movement using one-meter-resolution LiDAR data, geomorphic and topographic analyses, satellite imagery, and published geologic maps. Landslides are classified based on the relative recency of surface movement.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410561

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