Customarily considered among the youngest rocks in the Cambrian System, the Potosi Dolomite is chiefly comprised of fine- to medium-grained dolomite. The rocks also contain some slender interbeds of shale or siltstone, as well as intermittent grains of sand, which are most prevalent in the northern parts of Indiana. In some areas, erratic inclusions of chert and glauconite also occur in the Potosi. The color of the rocks ranges from light gray and brown to much darker shades of the same colors.