Old Port Formation

Old Port Formation Stratigraphic range: Early Devonian
Ridgeley Member of the Old Port Formation
Type	sedimentary
Unit of	Helderberg Group
Sub-units	Ridgeley, Shriver, Mandata, Corriganville, and New Creek Members
Underlies	Onondaga Formation
Overlies	Keyser Formation
Thickness	150 to 190 ft (Mifflintown Quadrangle in PA)
Lithology
Primary	limestone, sandstone
Other	chert, shale
Location
Region	Appalachian Mountains
Extent	Pennsylvania, Maryland, Virginia, West Virginia
Type section
Named by	Conlin and Hoskins, 1962

The Devonian Old Port Formation is a mapped bedrock unit in Pennsylvania, Maryland, and West Virginia, USA. Members of the unit are also mapped in New Jersey and Virginia as parts of other formations or groups.

The Old Port Formation consists of several members with varying lithologies, which are (in ascending stratigraphic order): New Creek Member (limestone), Corriganville Member (limestone), Mandata Member (shale), Shriver Member (cherty limestone), and Ridgeley Member (sandstone). Where the Shriver Chert does not occur it may be replaced by the Licking Creek Limestone. It was originally combined from the Helderberg Group and Oriskany Group by Conlin and Hoskins in 1962 to form a single Formation with the above members.

The Ridgeley Member was first described as a member of the Oriskany Formation by Swartz and others (1913), as a calcareous sandstone which passes into an arenaceous limestone. It is still mapped as part of the Oriskany Group in New Jersey.

The type locality is at the town of Ridgeley, Mineral County, West Virginia.

The sandstone of the Ridgeley Member has been extensively mined due to its very pure quartz suitable for glass. The glass derived from the sandstone was used for lenses on the Hubble Space Telescope.

A separate formation, the Oriskany Sandstone, is a lateral equivalent of the Ridgeley Member, but bounded above and below by unconformities.

The Shriver Member, or Shriver Chert, was first described as a member of the Oriskany Formation by Swartz and others (1913), as dark siliceous shale with much black impure chert in nodules or layers of nodules. It is named after Shriver Ridge (the type locality), Allegany County, Maryland, and was originally mapped as the basal unit of the Oriskany Formation.

In Maryland and West Virginia, the Shriver is mapped as part of the Helderberg Group.