A sequence of Cretaceous formations as much as 7,000 ft (2,134 m) thick crops out in the eastern and southeastern parts of Glacier National Park and in the adjoining area to the east. Recognition of the many persistent lithologic units in this Cretaceous sequence should be of value in interpreting the complex structure of the region.
The oldest rocks, the Kootenai Formation of Early Cretaceous age, consist chiefly of variegated mudstone, siltstone and sandstone of nonmarine origin. In the Park area the thickness of the Kootenai is probably greater than 1,000 ft (305 m). A conspicuous black chert conglomerate, the Cut Bank Sandstone Member, forms the base of the formation and rests unconformably on Upper Jurassic rocks.
The Blackleaf Formation, also of Early Cretaceous age, conformably overlies the Kootenai and is about 750 to 800 ft (229-244 m) thick. A marine unit consisting of dark-gray fissile shale and quartzose sandstone forms the basal part of the Blackleaf. Most of the formation is alternating beds of light-coloured, nonmarine clastics.
The marine Marias River Shale of Late Cretaceous age unconformably overlies the Blackleaf and is about 1,200 to 1,300 ft (366-396 m) thick. Most of the Marias River is dark-gray shale that contains calcareous and ferruginous concretions and is locally sandy and calcareous.
Conformably overlying the Marias River Shale is a marine transitional unit consisting of interbedded shale and sandstone, herein referred to as the Telegraph Creek Formation. The formation ranges in thickness from 120 to 170 ft (36-52 m) on the Kevin-Sunburst dome east of Glacier National Park.
The Virgelle Sandstone, which averages 160 ft (49 m) thick, is a massive light-brown sandstone conformably overlying the Telegraph Creek Formation.
Resting conformably on the Virgelle Sandstone is the Two Medicine Formation, a nonmarine unit that consists mostly of mudstone with minor lenticular sandstone. The formation is about 2,000 ft (610 m) thick in the Park area.
A dark-gray marine formation, the Bearpaw Shale, conformably overlies the Two Medicine Formation. The Bearpaw is nearly 400 ft (122 m) thick east of the Park, but thins to only 235 ft (72 m) south of the Park. The Bearpaw grades upward into a unit of alternating sandstone, siltstone and shale that is as much as 400 ft (122 m) thick.
The Horsethief Sandstone, about 90 ft (27 m) thick, is a massive cliff-forming sandstone that was deposited in a regressive marine environment above the Bearpaw.
Conformably overlying the Horsethief Sandstone is the St. Mary River Formation, a nonmarine sequence. The St. Mary River Formation is about 1,000 ft (305 m) thick near the eastern margin of the Disturbed belt.
Another nonmarine unit, the Willow Creek Formation, overlies the St. Mary River Formation. The Willow Creek consists of about 800 ft (244 m) of variegated clayey rocks and sandstone. The top of the Cretaceous is somewhere within this formation.