U.S. DEPARTMENT OF THE INTERIORBUREAU OF LAND MANAGEMENT
Geology of the Upper Missouri River Breaks National Monument
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.
|Last updated: 09-17-2014|
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