The database for the geologic map of the Frederick 30 x 60 quadrangle covers the distinct geologic provinces and sections of the central Appalachian region that are defined by unique bedrock and resulting landforms. From west to east, the provinces include the Great Valley section of the Valley and Ridge province, the Blue Ridge province, and the Piedmont province; in the extreme southeastern corner, a small part of the Coastal Plain province is present. The Piedmont province is divided into several sections; from west to east, they are the Frederick Valley synclinorium, the Culpeper and Gettysburg basins, the Sugarloaf Mountain anticlinorium, the Westminster terrane, and the Potomac terrane. The Blue Ridge province contains Mesoproterozoic (1 billion years old, or 1 Ga) paragneiss and granitic gneisses that are intruded by a swarm of Neoproterozoic (570 million years old, or 570 Ma) metadiabase and metarhyolite dikes. Unconformably overlying the gneisses are Neoproterozoic metasedimentary rocks and metavolcanic rocks associated with the dikes. The Mesoproterozoic gneisses were deformed and metamorphosed during the Grenville orogeny. Subsequently, the Neoproterozoic metasedimentary and metavolcanic rocks accumulated during a continental rifting event (Rankin, 1976). Clastic metasedimentary rocks of the newly formed continental margin were deposited paraconformably upon the Neoproterozoic rocks. To the east, Neoproterozoic and early Paleozoic metasedimentary and metavolcanic rocks were deposited on the margin of the rifted continent. These rocks underlie the Sugarloaf Mountain anticlinorium and Westminster and Potomac terranes. As the rifted continental margin stabilized and became a passive margin during the early Paleozoic, carbonate rocks were deposited on the broad continental shelf. Those carbonate rocks are now exposed in the Great Valley section and the Frederick Valley synclinorium. The early Paleozoic carbonate platform became unstable in response to the Ordovician Taconian orogeny. Deformation associated with this tectonic event is recorded in rocks of the Piedmont province to the east. These rocks, which are now part of the Potomac terrane, were thrust westward onto rocks of the Westminster terrane; next, rocks of the Westminster terrane were thrust onto rocks now exposed in the Sugarloaf Mountain anticlinorium and Frederick Valley synclinorium (Drake and others, 1989; Southworth, 1996). The early Paleozoic sea eventually closed up and disappeared during the continental collision of tectonic plates during the late Paleozoic Alleghanian orogeny. The Alleghanian orogeny transported all of the rocks within the map area westward along the North Mountain thrust fault, which is exposed immediately northwest of the quadrangle. The Alleghanian orogeny produced numerous thrust faults and folds in the rock and regional-scale folds that help define the geologic provinces. The Massanutten synclinorium underlies the Great Valley section, the Blue Ridge-South Mountain anticlinorium underlies the Blue Ridge province, and the Frederick Valley synclinorium and Sugarloaf Mountain anticlinorium underlie the western Piedmont province. Tens of millions of years after the Alleghanian orogeny, early Mesozoic continental rifting formed the Culpeper and Gettysburg basins, which once were connected to form a large down-faulted basin filled with sediments that eroded from the adjacent Blue Ridge and Piedmont highlands. Continued rifting resulted in igneous intrusions and extrusive volcanic rock at about 200 Ma, and eventually led to the opening of the Atlantic Ocean to the east. Sediments eroded from the Appalachian highlands were deposited by river systems and transgressing seas and now form the Coastal Plain province.