Conococheague Formation

The Conococheague Formation is a mapped Appalachian bedrock unit of Cambrian age, consisting primarily of limestone and dolomite. It occurs in central Maryland, southern and central Pennsylvania, the Valley and Ridge of Virginia and easternmost West Virginia.^[3]

Conococheague Formation
Stratigraphic range: Cambrian
Etched section of rock from the Conococheague Formation from Washington County, Maryland (dolomite is lighter than the limestone)
Type	sedimentary
Sub-units	Big Spring Station Member
Underlies	Beekmantown Dolomite
Overlies	Elbrook Formation[1]
Lithology
Primary	Limestone
Other	dolomite, sandstone
Location
Region	Appalachian Basin of eastern North America
Extent	Maryland, Pennsylvania, Virginia, West Virginia
Type section
Named for	Conococheague Creek[2]
Named by	G. W. Stose, 1908

Description

Depositional environment

Outcrop along Antietam Creek, near Hagerstown.

Outcrop along I-70, Mile Marker 20.5

The depositional environment of the Conococheague was most likely shallow marine to tidal on a carbonate platform.

Notable exposures

The type section is located at Scotland in Franklin County, Pennsylvania (northeast of Chambersburg).^[2]

Accessible outcrops occur along various roadcuts and waterways in the area, including roadcuts near mile marker 20 of I-70 near Hagerstown, Maryland. The bedding at this site is nearly vertical, with the strike nearly perpendicular to the highway, and thus one can easily walk up and down the sections.

The Conococheague also forms dramatic, 100-foot cliffs along the Potomac River upstream of Shepherdstown, West Virginia (northwest of Harpers Ferry), where entrenched meanders expose mile-long sections of tilted Conococheague strata.^[4] These may be easily seen from the towpath trail of the Chesapeake and Ohio Canal National Historical Park along the Maryland bank of the Potomac. In Shepherdstown itself, the top of a Conococheague cliff provides the setting for the monument to steamboat-inventor James Rumsey.

Fossils

Conodonts are present in the Conococheague.^[5]

Thrombolites and the possible chiton Matthevia are present in the Boxley Blue Ridge Quarry in Bedford County, Virginia.^[6][7]

Age

Relative-age dating of the Conococheague places it in the Upper Cambrian period.