The Potsdam Sandstone, more formally known as the Potsdam Group, is a geologic unit of mid-to-late Cambrian age found in Northern New York and northern Vermont and Quebec and Ontario. A well-cemented sandstone of nearly pure quartz, in the 19th century it was widely used in construction and in refractory linings for iron furnaces. Name and type locality The Potsdam Sandstone is named for its type locality in Potsdam, New York, where in 1838 Ebenezer Emmons described it in outcrops along the Raquette River. Stratigraphic setting and lithology The Potsdam Sandstone lies unconformably on a surface of Precambrian metamorphic rock. It is the earliest unit in the marine-transgressive sedimentary rock sequence deposited during the early Paleozoic as sea level rose to gradually inundate the craton of the paleocontinent of Laurentia. The rock, which is formed from sediments eroded off unvegetated terrestrial landscapes and deposited in near-shore coastal environments, consists almost entirely of sand-size quartz grains held together by quartz cement. It ranges in color from gray to tan, yellow, and red, with the colors imparted by small amounts of the red iron oxide mineral hematite, Fe2O3, and the yellow iron oxide mineral goethite, FeO(OH). As sea level rose in the depositional environment, increasing amounts of carbonate minerals were deposited in the sediment, with the result that the unit grades upward into dolomitic sandstone in the upper Potsdam and then to sandy dolomite at the base of the overlying Theresa Formation. Paleontology Fossil remains of whole animals are rare in the Potsdam Sandstone, but there are some significant occurrences of trace fossils. Trace fossils in the unit include both vertical burrows, such as Diplocraterion and Skolithos and horizontal trackways, such as Diplichnites, Protichnites, and Climactichnites. In 1903, a 20-ton (18-tonne) slab of Potsdam Sandstone from Clinton County, New York, displaying tracks attributed to trilobites, was placed in the New York State Museum. Fossil impressions of the whole bodies of jellyfish have also been found in the Potsdam. Geographic occurrence In New York state, the Potsdam is found primarily north and west of the Adirondack Mountains. Outcrop exposures of the Potsdam Sandstone occur throughout the Saint Lawrence lowlands, western Lake Champlain Valley, and northern Mohawk Valley. Ausable Chasm, near Plattsburgh, has a continuous exposure of a section more than thick. The formation reaches its maximum thickness of about in the northern Champlain lowland. Uses in construction and industry In the 19th century,