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.