DescriptionBig Blue Bentonite (Sentinel Butte Formation, Upper Paleocene; Little Missouri Badlands, North Dakota, USA) 20.jpg
English: The bluish-gray interval is a weathered bentonite bed (= volcanic ash horizon), interpreted as deposited in an ancient lake. Some of the silicate minerals in the bentonite have been altered to smectite clay (also known as montmorillonite). Smectite is famous for being a "swelling clay" - a significant volume expansion occurs when wet. After drying out, the clay shrinks.
Stratigraphy: Big Blue Bentonite (Big Blue Bed), Sentinel Butte Formation, Upper Paleocene
Locality: North Unit of Roosevelt National Park, Little Missouri Badlands, western North Dakota, USA
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