Geologic map of the Ennis 30' X 60' quadrangle, Madison and Gallatin Counties, Montana, and Park County, Wyoming

These basement uplifts or blocks were thrust westward during the Laramide orogeny over rocks as young as Upper Cretaceous. The thrusts are now exposed in the quadrangle along the western flanks of the Gravelly and Madison Ranges (the Greenhorn thrust and the Hilgard fault system, respectively). Simultaneous with the west-directed thrusting, northwest-striking, northeast-side-up reverse faults formed a parallel set across southwestern Montana; the largest of these is the Spanish Peaks fault, which cuts prominently across the Ennis quadrangle.

Beginning in late Eocene time, extensive volcanism of the Absorka Volcanic Supergroup covered large parts of the area; large remnants of the volcanic field remain in the eastern part of the quadrangle. The volcanism was concurrent with, and followed by, middle Tertiary extension. During this time, the axial zone of the "Madison-Gravelly arch," a large Laramide uplift, collapsed, forming the Madison Valley, structurally a complex down-to-the-east half graben.

Basin deposits as thick as 4,500 m filled the graben. Pleistocene glaciers sculpted the high peaks of the mountain ranges and formed the present rugged topography. Compilation scale is 1:100,000.

Geology mapped between 1988 and 1995. Compilation completed 1997. Review and revision completed 1997.

Archive files prepared 1998-02.