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

The Ennis 1:100,000 quadrangle lies within both the Laramide (Late
Cretaceous to early Tertiary) foreland province of southwestern
Montana and the northeastern margin of the middle to late Tertiary
Basin and Range province.
The oldest rocks in the quadrangle are Archean high-grade gneiss, and
granitic to ultramafic intrusive rocks that are as old as about 3.0
Ga. The gneiss includes a supracrustal assemblage of quartz-feldspar
gneiss, amphibolite, quartzite, and biotite schist and gneiss. The
basement rocks are overlain by a platform sequence of sedimentary
rocks as old as Cambrian Flathead Quartzite and as young as Upper
Cretaceous Livingston Group sandstones, shales, and volcanic rocks.
The Archean crystalline rocks crop out in the cores of large basement
uplifts, most notably the "Madison-Gravelly arch" that includes parts
of the present Tobacco Root Mountains and the Gravelly, Madison, and
Gallatin Ranges. 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.