Stratigraphy of the Rocky Mountain Front

Quaternary alluvium - deposits of gravel, sand, silt, and clay, product of flowing streams, can be found in river valleys and deltas, mainly from local sources.

Cretaceous Blackleaf Formation (150-600 ft. thick, three members) – Greenish-gray and gray thin bedded mudstone, sandstone, and bentonitic shale, and even some thin beds of bentonite (absorbent aluminum phyllosilicate clay), in some areas there are thin conglomerates at the base of the sandstones, gray marine mudstone with many units of thin-bedded sandstone, locally crossbedded, and dark-gray fissile shale with metallic luster on bedding planes and thin-bedded sandstone units at the top and bottom.

Cretaceous Kootenai Formation (650-800 ft. thick) – Non-marine, maroon and gray-green mudstone with lenticular greenish-gray interbeds of sandstone, which are poorly sorted; contains some brown iron-stained nodules and locally lentils of sandy limestone.

Mississippian Castle Reef dolomite (formation of the Madison group, has two members, 0-475 ft. thick) – light-gray fine- to medium-crystalline dolomite, in some areas dolomitic tick-bedded limestone, has some nodules and lentils of smoky-gray chert, medium- to light-gray dolomite, calcitic dolomite, dolomitic limestone and magnesium limestone, thin to thick bedded.

Mississippian Allen Mountain limestone (formation of the Madison group, has three members, 150-350 ft. thick) – medium- to dark-gray limestone with some dolomitic limestone, locally even dolomitic magnesium limestone, in other areas with thin mudstones, dark-gray chert lenses and nodules can also be found in some locations.

Quaternary alluvium - deposits of gravel, sand, silt, and clay, product of flowing streams, can be found in river valleys and deltas, mainly from local sources.

Quaternary alluvium of youngest alluvial terrace – Terraces developed north of the Teton River, the youngest terrace lies approximately 10 ft. above the present Teton River flood plain

Quaternary glacial outwash deposit – form extensive gravel plains at elevations lower than the Quaternary/Tertiary older gravels (smaller areas closer to the Bynum Reservoir and around Choteau). The gravel that consists almost exclusively of limestone clasts derived from exposures in the Rocky Mountain Front. The largest of these deposits is Burton Bench north of Choteau that was deposited when meltwater flowed through Ralston Gap south of Bynum. Farther north the meltwater flowed along the valley now occupied by Muddy Creek and formed outwash deposit.

Quaternary glacial deposit – glacial drift, meaning material deposited by a glacier, can range from sediments to erratics.

Quaternary glacial till – deposited along the Teton River by mountain glaciers. Where glacial till overlies the Kevin Shale along the Teton River south of the New Rockport Colony, the upper few feet of the Kevin Shale are deformed and mixed with glacial erratics. Visit our page on landscapes to learn more about glacial features in this area.

Cretaceous Virgelle Formation (150-200 ft. thick) – forms buttes and mesas surrounded by sandstone cliffs where the erosion-resistant titaniferous magnetite bed at the top of this formation protects underlying more easily eroded sandstone. This formation is easily recognizable from far away by the brown-weathering titaniferous magnetite beds above sandstone that appears white from a distance whereas it is actually light-gray. It is massive and has some thin beds, some crossbedding and some iron-stained (oxidized) zones.

Cretaceous Telegraph Creek Formation (80-170 ft. thick) – forms lightly vegetated, relatively gentle slopes below the cliffs of the Virgelle Formation and above the grass-covered gentle topography of the Kevin Member (Marias River Fm). This formation contains light-gray calcareous poorly solidified beds of sandstone inter-bedded with gray calcareous sandy shale, dark-gray calcareous hard dense sandstone (with crossbeds and ripple marks in some locations), some mudstone, and dark-gray calcareous fine-grained sandstone and sandy shale in beds with some ripple marks, crossbedding and minute laminae.

Cretaceous Blackleaf Formation (150-600 ft. thick, three members) – Greenish-gray and gray thin bedded mudstone, sandstone, and bentonitic shale, and even some thin beds of bentonite (absorbent aluminum phyllosilicate clay), in some areas there are thin conglomerates at the base of the sandstones, gray marine mudstone with many units of thin-bedded sandstone, locally crossbedded, and dark-gray fissile shale with metallic luster on bedding planes and thin-bedded sandstone units at the top and bottom.

Cretaceous Kootenai Formation (650-800 ft. thick) – Non-marine, maroon and gray-green mudstone with lenticular greenish-gray interbeds of sandstone, which are poorly sorted; contains some brown iron-stained nodules and locally lentils of sandy limestone.

Jurassic Morrison (200-550 ft. thick) – Differs by location: Eastern facies – tuffaceous gray to olive-drab mudstone with some thin sandstone beds; western facies – poorly sorted crossbedded conglomeratic gray sandstone and red-brown mudstone.

Jurassic Ellis Group (three formations) - Swift Formation (22-97 ft.) – The sandstone member is gray to gray-brown with fine to very fine grains with thin beds and some crossbedding, ripple marks and even wood and clay fragments. Its shale member is dark-gray to olive-drab sandy claystone with numerous thin beds of sandstone, some areas even have water-worn fossils. The Swift Formation is separated by an unconformity from the Rierdon Formation (120-350 ft.) – dark-gray to brownish-gray calcareous mudstone with many thin clay containing limestone beds, the upper part often has small barite nodules. The next formation might be separated by another unconformity, though Mudge (1972) placed a question mark next to it in his description. The Sawtooth Formation (0-85 ft.) has a calcareous siltstone member which is grayish-brown to yellowish-brown and thin-bedded, a fissile shale member dark-gray in color (locally with thin beds of sandstone and conglomerate) and a non-calcareous sandstone member which is gray and fine-grained and in some areas it has dark-gray thin-bedded shale. The basal bed of the sandstone member is a conglomerate of round pebbles and cobbles of limestone and chert.

Mississippian Madison Group – Castle Reef Dolomite (two members, 0-475 ft. thick) – light-gray fine- to medium-crystalline dolomite, in some areas dolomitic tick-bedded limestone, has some nodules and lentils of smoky-gray chert, medium- to light-gray dolomite, calcitic dolomite, dolomitic limestone and magnesium limestone, thin to thick bedded. Allen Mountain Limestone (three members, 150-350 ft. thick) – medium- to dark-gray limestone with some dolomitic limestone, locally even dolomitic magnesium limestone, in other areas with thin mudstones, dark-gray chert lenses and nodules can also be found in some locations.

Devonian - Three Forks Formation (50-589 ft.) – Intraformational breccia that in places is overlain by grayish-brown limestone; locally upper part contains siltstone, carbonaceous shale, and green mudstone, Jefferson Dolomite(150-650 ft., two members) - light-grayish-brown dolomite, calcitic dolomite, and limestone, some parts have thin pinch-and-swell-type bedding, others have a fetid odor (in some areas like oil), and Maywood Formation (26-229 ft., two members) – dark-gray to gray brown thinly bedded dolomite, calcitic dolomite, and limestone with distinctive yellowish-gray mottling and greenish-gray mudstone with some reddish-gray mudstone beds in western part; some thin beds of yellowish-gray dolomite.

Cretaceous Virgelle Formation (150-200 ft. thick) – forms buttes and mesas surrounded by sandstone cliffs where the erosion-resistant titaniferous magnetite bed at the top of this formation protects underlying more easily eroded sandstone. This formation is easily recognizable from far away by the brown-weathering titaniferous magnetite beds above sandstone that appears white from a distance whereas it is actually light-gray. It is massive and has some thin beds, some crossbedding and some iron-stained (oxidized) zones.

Cretaceous Telegraph Creek Formation (80-170 ft. thick) – forms lightly vegetated, relatively gentle slopes below the cliffs of the Virgelle Formation and above the grass-covered gentle topography of the Kevin Member (Marias River Fm). This formation contains light-gray calcareous poorly indurated beds of sandstone interbedded with gray calcareous sandy shale, dark-gray calcareous hard dense sandstone (with crossbeds and ripple marks in some locations), some mudstone, and dark-gray calcareous fine-grained sandstone and sandy shale in beds with some ripple marks, crossbedding and minute laminae.

Cretaceous Marias River Shale (four members, the youngest is the more common one in this area) – Kevin Shale Member (600 – 1050 ft. thick): dark-gray calcareous mudstone and shale beds with lesser gray-weathering limestone formed by concretion, very fine-grained sandstone, ironstone concretions which weather reddish, and abundant thin bentonite beds in the lower part. Ferdig Shale Member (200-350 ft.) – gray non-calcareous mudstone with many iron-stained lenses of silt- and sandstone, further to the west the thin sandstone beds of this member has nodules. Cone Calcareous Member (~100 ft.) – gray shale with platy calcareous clay- and siltstone containing a thick bentonite bed and abundant fossils of Inoceramus labiatus, a mollusk. Floweree Shale Member (~30 ft.) – dark-gray non-calcareous fissile shale with thin beds of siltstone in lower part containing one or more beds of chert-pebble conglomerate.

Cretaceous Blackleaf Formation (150-600 ft. thick, three members) – First member is a greenish-gray and gray thin bedded mudstone, sandstone, and bentonitic shale, and even some thin beds of bentonite (absorbent aluminum phyllosilicate clay), in some areas there are thin conglomerates at the base of the sandstones, second member is a gray marine mudstone with many units of thin-bedded sandstone, locally crossbedded, and lastly, dark-gray fissile shale with metallic luster on bedding planes and thin-bedded sandstone units at the top and bottom.

Cretaceous Kootenai Formation (650-800 ft. thick) – Non-marine, maroon and gray-green mudstone with lenticular greenish-gray interbeds of sandstone, which are poorly sorted; contains some brown iron-stained nodules and locally lentils of sandy limestone.

Jurassic Ellis Group (three formations) - Swift Formation (22-97 ft.) – The sandstone member is gray to gray-brown with fine to very fine grains with thin beds and some crossbedding, ripple marks and even wood and clay fragments. Its shale member is dark-gray to olive-drab sandy claystone with numerous thin beds of sandstone, some areas even have water-worn fossils. The Swift Formation is separated by an unconformity from the Rierdon Formation (120-350 ft.) – dark-gray to brownish-gray calcareous mudstone with many thin clay containing limestone beds, the upper part often has small barite nodules. The next formation might be separated by another unconformity, though Mudge (1972) placed a question mark next to it in his description. The Sawtooth Formation (0-85 ft.) has a calcareous siltstone member which is grayish-brown to yellowish-brown and thin-bedded, a fissile shale member dark-gray in color (locally with thin beds of sandstone and conglomerate) and a non-calcareous sandstone member which is gray and fine-grained and in some areas it has dark-gray thin-bedded shale. The basal bed of the sandstone member is a conglomerate of round pebbles and cobbles of limestone and chert.

Mississippian Madison Group – Castle Reef Dolomite (two members, 0-475 ft. thick) – light-gray fine- to medium-crystalline dolomite, in some areas dolomitic tick-bedded limestone, has some nodules and lentils of smoky-gray chert, medium- to light-gray dolomite, calcitic dolomite, dolomitic limestone and magnesium limestone, thin to thick bedded. Allen Mountain Limestone (three members, 150-350 ft. thick) – medium- to dark-gray limestone with some dolomitic limestone, locally even dolomitic magnesium limestone, in other areas with thin mudstones, dark-gray chert lenses and nodules can also be found in some locations.

Devonian Jefferson Dolomite (150-650 ft., two members) - light-grayish-brown dolomite, calcitic dolomite, and limestone, some parts have thin pinch-and-swell-type bedding, others have a fetid odor (in some areas like oil).

Devonian Maywood Formation (26-229 ft., two members) – dark-gray to gray brown thinly bedded dolomite, calcitic dolomite, and limestone with distinctive yellowish-gray mottling and greenish-gray mudstone with some reddish-gray mudstone beds in western part; some thin beds of yellowish-gray dolomite.

Cambrian Devils Glen Dolomite (100-400 ft.) – very light gray massive dolomite with fine to medium sized crystals.

Cambrian Switchback Shale (70-253 ft.) – mostly non-calcareous clayey shale, greenish-gray, thinly laminated, with some thin beds of dolomite, sandstone, and conglomeratic dolomite in upper part. In some areas it has maroonish-gray beds.

Cambrian Steamboat limestone (229-239 ft.) – mainly thin-bedded limestone and dolomite with some interbedded greenish-gray dolomitic mudstone; gray-brown, distinctively mottled yellowish-gray to gray-orange.