The Chesapeake Group is divided into three formations. The oldest, the Calvert Formation, is composed of diatomaceous earth and dark sandy clays and marl. It is overlain by the yellowish sand and greenish clay and marl of the Choptank formation. The youngest deposits belong to the St. Marys Formation, which, as exposed at Little Cove Point, consists of bluish sandy clay and fine sandstone. The silty and sandy content of the Miocene deposits in Maryland was derived from the erosion of older Coastal Plain deposits and crystalline rocks of the Piedmont region. The calcareous deposits are organic in origin.