Location maps. Southwestern Montana – the initial study area - with selected valleys identified on a digital relief image of southwestern Montana (Montana State Library, NRIS data bank, 2001). The valleys of southwestern Montana contain Cenozoic rocks and sediment.  The strata are largely of fluvial origin, although volcanic and eolian deposits also are a part of the Cenozoic rock record.  The valleys are typically segmented into individual basins, with some basin-fill attaining thicknesses of approximately 5000 meters.