Figure 12. Simplified geologic map of the Eocene paleovalley near Cornwall Mountain (modified from Coash, 1967). Basal conglomerate with rounded clasts up to 4 x 6 m is overlain by the zoned, 45 Ma tuff (⁴⁰Ar/³⁹Ar date from this study). An upper, probably lacustrine sequence of tuffaceous siltstone and shale has numerous interbedded breccia lenses containing clasts up to 4 m diameter of the upper, less silicic part of the 45 Ma tuff.