Potsdam Sandstone

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.

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.

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.

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 160 metres (520 ft) thick. The formation reaches its maximum thickness of about 450 metres (1,480 ft) in the northern Champlain lowland.

Potsdam sandstone has been quarried commercially in Nepean, Ontario and Covey Hill, Quebec.

In the 19th century, Potsdam Sandstone was highly regarded as a building material. There was extensive quarrying for Potsdam Sandstone in the Potsdam area, beginning in 1809. Properties of the rock that give it value as a building material include high compressive strength, attractive reddish coloring, and resistance to weathering. The rock also was said to be "soft and easy to carve" when freshly quarried but "extremely hard" and "weather-resistant" after exposure to the air, but modern geologists suggest that this is a misconception.

Local sandstone was used for many buildings in Potsdam, as well as for purposes such as gravestones and sidewalks.