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Geology of the Upper Missouri River Breaks National Monument

Today's river traveler sees many widely contrasting scenes. The wide, fertile valley below Fort Benton differs considerably from the scenic white cliffs down river from Coal Banks Landing. The stark, rugged badlands below Judith Landing present still another vista.

The valley of the Upper Missouri is a living museum, the product of many events over time. The land was originally laid down in horizontal layers, the sediments and shorelines of a great inland sea that once covered most of the Great Plains. These layers have since been folded, faulted, uplifted, modified by volcanic activity and sculpted by glaciers. Erosion then added to the variety seen along the river today, a landform known as the Breaks.

Erosion has cut through the layers deposited by the great inland sea which covered the area for about ten million years (starting some 80 million years ago). The shoreline of the sea migrated back and forth across the area in response to climatic changes and shifts in the earth's crust. Marine deposits, materials that settled out of the water to the bottom of the sea, resulted in beds of shale. Just like the oceans of today, sandstone layers were deposited along shorelines and river deltas. The river's downcutting through this "layer-cake" of sandstone and shale has exposed some ten million years of geologic history.

Erosion also washed away the soft sediments from around the harder volcanic materials that were extruded into cracks in the shales and sandstones. Consequently, walls or "dikes" stand out from the surrounding bottom lands and valley slopes. At places, large intrusive plugs capture the traveler's attention. The black color of these volcanic features contrasts sharply with the lighter colored shales and sandstones.

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About the Monument

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Contact Information

  • Lewistown Field Office: 406-538-1900
  • Interpretive Center: 406-622-4000
  • Upper Missouri River Breaks National Monument
    920 Northeast Main
    Lewistown, MT 59457

Last updated: 06-22-2015