USGS Logo Geological Survey Professional Paper 294—K
The Rocks and Fossils of Glacier National Park: The Story of Their Origin and History

APPENDIX A: Age and character of the rock units of Glacier National Park and vicinity
Geologic age Group Formation or rock unit Character Thickness (feet)
Modern alluvium, etc. Sand, silt, and gravel in stream bottoms, and hill wash and landslide debris on slopes. Variable, generally thin.
Glacial deposits (deposits of several glacial stages present). Gravel, poorly sorted and rounded, and sand and silt; in part in moraines and other features whose forms show their glacial origin. Variable.
Terrace alluvium (products of several erosion episodes). Gravel, sand. Variable.
Late Eocene and younger, probably includes Pleistocene.
Old alluvium and associated deposits. Sand, clay, limy deposits, cemented gravel, lignite; all compacted and more or less cemented; commonly mantled with younger material; not distinguished on maps. At least several hundred.
Cretaceous. Montana. St. Mary River formation. Clay and sandstone, light-gray, fresh and brackish water types, and some red and varigated shale. Somewhat less than 1,000.
Horsethief sandstone, Bearpaw shale and Two Medicine formation distinguished locally only. Eagle sandstone and Virgelle sandstone present in small areas. Sandstone and shale, largely gray and green; marine. 3,000±.
Colorado shale. Shale, mostly dark-colored, and some sandy shale and sandstone. Not measured; total may be 1,800+.
Kootenai formation. Sandstone, principally red-purple, red, gray, and green, and some sandy and carbonaceous shale and firmly cemented gravel; laid down on the continent rather than in seas. Variable; maximum may be 1,500±.
Jurassic. Ellis. Two or more formations present but not separately mapped. Shale, largely soft, black, and some sandy limestone and sandstone; marine. 400.
Hannan limestone. Limestone, thick- and thin-bedded, in part dolomitic, and a little shale; marine. 1,500±.
Probably 2 or 3 formations present but not named or separately mapped. Limestone, largely dark, in part fetid, marine, and some calcareous shale. 1,500±.
Formations not distinguished in the region immediately south of Glacier National Park. Farther south, Switchback shale, Steamboat limestone, Pentagon shale, Pagoda limestone, Dearborn limestone, Damnation limestone, Gordon shale, and Flathead sandstone, in descending order, are distinguished. Limestone, much of which contains blebs of clay, in part shaly; more or less micaceous shale, in part carbonaceous at several horizons; quartzite and a little conglomerate at base. Variable—1,900±.
Precambrian (Belt series). Missoula. Probably more than 7 formations, but mostly not named and only in part distinguished on maps. Argillite, red-purple and green, and calcareous and sandy argillite interspersed, a little conglomerate and some quartzite, lenses of limestone, some of which are thick, are present locally; Purcell basalt, lava flows up to 200 feet thick, near base of group; contains algal remains. Possibly as much as 20,000 originally, only remnants now present within the park.

Piegan. Siyeh limestone. Limestone, dark-bluish-gray, somewhat magnesian, locally oolitic acid argillaceous; "molar tooth" markings are common; contains several zones of algal remains. 1,800—5,000; may exceed 5,000 locally.
Ravalli. Grinnell argillite. At top: argillite, grayish-blue-greens, calcareous. Main body: argillite, red-purple, red, and green, locally calcareous, and some light-colored quartzite. 1,000—4,000+; probably near 3,000 in most places.
Appekunny argillite. Argillite, dark-gray and greenish, siliceous, locally limy; quartzite prominent locally; subordinate reddish beds in places. 2,000—5,000+
Altyn limestone. Limestone, dark, impure magnesian, with sandy beds; contains algal remains. 2,000±, with the base not exposed.

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Last Updated: 08-Jul-2008