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The remains of a deeply eroded Late Cretaceous stratovolcano centered near Sliderock Mountain dominate the map area. The volcano consists mostly of lahars, although basaltic andesite lava flows are also present. An extensively eroded, weakly mineralized hypabyssal intrusive complex, inferred to represent the volcano's solidified magma chamber, intrudes previously erupted lava flows and associated lahars. Cretaceous sandstone and shale and Paleozoic sedimentary rocks, principally limestone of the Madison Group, host the stratovolcano; remnants of these rocks are preserved as large xenoliths and roof pendants.
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Reconnaissance geologic map of the Sliderock Mountain area, Sweet Grass and Stillwater counties, Montana