White Cliffs of the Missouri Field Trip

Before the Ice Ages the Missouri River flowed around the north side of the Bear's Paw Mountains through Big Sandy, Havre, and Chinook. As the glacier retreated to Canada, the river cut a new gorge on the south side of the Bear's Paw Mountains, exposing picturesque and impressive geological features. The white cliffs of this area are one such feature. About 75 million years ago volcanic ash fell as sand was being deposited as off-shore sandbars. The ash subsequently altered to white kaolinite clay and now forms white cliffs of clay-semented sandstone. Beds of black shale, rich in montmorillonite, also known as bentonite, form striking color contrasts.

Under the Bear's Paw Mountains are up-faulted blocks of basement rock, which formed a large, gentle fold in the overlying sedimentary and volcanic rocks. The structure is similar to that under the Little Belt Mountains and the Big Snowy Mountains, although not as pronounced. The south flank of the Bear's Paw Mountains is only tilted 3 degrees to the south. However, this gentle tilt was sufficient to cause slow slippage on a very slippery bed of bentonite clay. The overlying sedimentary rocks slowly slid downslope, breaking into large plate-shaped blocks, with upslope blocks overriding downslope blocks. Many tilted fault blocks are seen on the trip. The beds were intruded by dikes, stocks, and plugs of shonkinite magma to form dark igneous rocks. Diatremes were also emplaced. The igneous activity occurred between 52 and 48 million years ago. Differential erosion has produced dramatic landscapes.

4 Day Canoe Trip Through the White Cliffs of the Missouri

Route: Virgelle-Pilot Rock-Eye of the Needle-Eagle Buttes-Citadel Rock-Hole in the Wall-Judith Landing

Cretaceous stratigraphy, ash deposits, coal beds, bentonite beds, landslides, landslump, gravity slide blocks and thrust faults, igneous dike swarms, internal structure of a diatreme.

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Last Updated December 1, 1998 by David Baker