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Earth Geology Rock Types Dolomite


Pitted black dolomite in a glade on Iron Mountain.
Gray dolomite with a white chert lens in a road cut near Mansfield.
The most common rock type in the Bryant watershed is dolomite. Dolomite is usually quite deeply weathered and soil-covered. You can find it making up much of the walls of the bluffs in the deeper valleys, capped by the harder sandstone. Dolomite also crops out from the thin soil of "glades" on hilltops and hillsides throughout the area. You'll recognize it there as a dark grey to black, distinctly pitted rock. 

Dolomite is composed of the mineral of the same name. The mineral dolomite is a calcium-magnesium carbonate, CaMg(CO3)2. It is seldom pure but contains variable amounts of sand, silt, clay and chert. These "contaminants" occur as scattered grains and clumps and as layers or lenses within the dolomite. 

Dolomite is the dominant rock type of the Jefferson City Formation, which forms the hilltops and higher elevations of the watershed. Dolomite layers make up about half of the bluff-forming Roubidoux Formation. Dolomite is also the rock type of the exposed top layers of the Gasconade Formation along the central part of the deepest valleys of the Bryant.