The Prichard formation along with the Altyn formation are the two oldest formations in Glacier National Park. Prichard formation is only found on the West side of the park. It is composed of thin layers of black argillite and siltite that were deposited in deep ocean waters. Radioisotope labelling studies date it at 1.7 to 1.3 billion years. Argillite is metamorphic rock of sedimentary origin derived from recrystallized clay materials. Siltite is derived from very fine mineral grain deposits. All were formed in low-oxygen conditions. The rust-colored material on the weathered surface is from iron sulfide minerals being oxidized as it leaches from the formation and interacting with current atmosphere.
The Altyn formation (1.45-1.35 billion years ago) is composed mostly of limestone and dolomite that formed in a warm shallow sea. Limestone is calcium carbonate formed from shell and skeletons of aqueous organisms as well as wastes and precipitates. Dolomite is calcium carbonate complexed with magnesium [CaMg(C03)2] found in mud and in magnesium-rich water that interacts with limestone.
The Appekunny formation is primarily argillite and siltstone deposited seas of varying depth during the Proterozoic 1.4 to 1.2 billion years ago.
Laminated beds composed mostly of argillite in the Appekunny formation.
Contorted beds mostly of siltite in the Appekunny formation. Rapid deposition of thick layers of sediment on top of very soft mud caused the thick layers to sink into and deform the underlying mud. Billions of years of pressure and heat resulted in the contorted pattern in hard rock.
The Grinnell formation is aged around 1.4 billion years and formed in very shallow water and broad tidal flats with ebb and flow of tides, stream run off and turbulent local condition leading to complex morphology. These laminated layers of argillite.
Grinnell formation: Ripples from water movement in mudflats.
Grinnell formation: Mud cracks.
Grinnell formation: Mud balls, mudchip brecca and white quartzite derived from silica-rich sand.
Grinnell formation: Fine striations of quartzite in agrillite.
Grinnell formation: Large white bands of quartzite.
Empire formation is composed of argillite, siltite and dolomite.
The Helena formation (1.40-1.35 billion years) is complex. This area has interbedded tan argillite with gray layers of oolite which are granular concretions of calcium carbonate (aka limestone).
Stromatolytes in Helena Formation. These stromatolyte remnants resembling cabbage heads were formed by colonies of cyanobacteria, Conophyton. These organisms were the first major life form to produce large amounts of oxygen through photosynthesis and alter Earth's atmosphere so that it could support animal life.
Stromatolytes in Helena formation.
Igneous sill in Helena formation. A thick layer of diorite is an igneous intrusion that occurred in the sedimentary layer. Diorite is an igneous rock with characteristics intermediate between granite and gabbro with respect to silica, iron and magnesium concentration.
Detail of diorite in the sill.
Snowslip formation (1.35 to 1.17 billion years) composed of argillite, siltite and small amounts of quartzite.
![The Altyn formation (1.45-1.35 billion years ago) is composed mostly of limestone and dolomite that formed in a warm shallow sea. Limestone is calcium carbonate formed from shell and skeletons of aqueous organisms as well as wastes and precipitates. Dolomite is calcium carbonate complexed with magnesium [CaMg(C03)2] found in mud and in magnesium-rich water that interacts with limestone.](images/02_Altyn_P8075343_1600.jpg)
