Late Cretaceous Inland Sea Geological Time Scale The Milk River Formation Deadhorse Coulee Member

Late Cretaceous Inland Sea

About 85 million years ago, a huge inland sea covered the middle of North America. The location of Writing-On-Stone, on the Milk River, would have been located on this very large and stormy sea. Sand was deposited on the shore which, over millions and millions of years, slowly compacted to become sandstone rock. This became part of the Milk River Formation.

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Geological Time Scale

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Era	Period	Formations	Events
Cenozoic 65 million years ago  to present	Quaternary		10-15 thousand year ago: last Ice Age ends 1 million years ago: North American Ice Ages begin
	Tertiary	Cypress Hills Ravenscrag Porcupine Hills Willow Creek	48 million years ago: formation of Sweet Grass Hills
Mesozoic 225 million year ago to 65 million years ago	Cretaceous	Whitemud St. Mary River Blood Reserve Bearpaw Foremost Milk River Alberta	Rocky Mountains uplifting   65 million years ago: last dinosaurs
	Jurassic Triassic		225 million years ago: first dinosaurs
Paleozoic 570 million years ago to 225 million years ago	Permian Pennsylvanian Mississippian Devonian Sulurian Ordovician Cambrian
Precambrian Prior to  570 million years ago			Formation of Earth's Crust 4,600 million years ago.

This is a synthesis of information from many different sources.
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The Milk River Formation
Over millions and millions of years, the beach on the Late Cretaceous Inland Sea became buried. The weight of the material covering this sand compacted it into hard sandstone rock. This sandstone is evident where the Milk River Formation is exposed.

The Milk River Formation has three main components:

The Virgelle Member of the Milk River Formation consists of magnificent sandstone cliffs. Upper Virgelle, being softer, is characterized by the presence of impressive hoodoos, as seen at the top of this picture. Lower Virgelle, bottom of the cliffs, is characterized by relatively dense (hard) on which there is native rock art.

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Deadhorse Coulee Member
One characteristic of the Deadhorse Coulee member is the presence of conspicuous mounds and badland landscapes. Both of these formations have a high bentonite clay content. Click on the two pictures below for a closer look.

Bentonite clay is a swelling-and-shrinking clay. When wet, the clay expands and is extremely slippery. When it dries, the clay shrinks back to its original size and can assume a popcorn-like structure. Often found on the surface of bentonite clays are gypsum crystals (CaSO4-H20), seen here just above camera lens. These crystals form as a result of repeated wet and dry periods.

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North America During the Last 150,000 Years
Terrain Sciences Division of Natural Resources Canada
Alberta Geography
Canadian Landforms (Cyberwall)
 

For more information:
• Beaty, Chester B., 1975. The Landscapes of Southern Alberta: A Regional Geomorphology. Lethbridge: University of Lethbridge Production Services, 95 pp.
• Leckie, Dale and R.J. Cheel, 1986. Tidal channel facies of the Virgelle Member (Cretaceous Milk River Formation), southern Alberta, Canada. In Current Research, Part B. Geological Survey of Canada, Paper 86-1B
• Leckie, Dale A., R.J. Cheel and L. Rosenthal, 1989. Sedimentology of the Upper Virgelle Member (Upper Cretaceous Milk River Formation), Writing-On-Stone Provincial Park, alberta and the Southern Alberta Transect. Calgary: Canadian Society of Petroleum Geologist. Unpublished manuscript.
• Meijer Drees, N.C. and D.W. Myhr, 1981. The Upper Cretaceous Milk River and Lea Park formations in southeastern Alberta. Bulletin of Canadian Petroleum Geology, v. 29, p. 42-72