ArrowPoint
Catoctin and South Mountain metarhyolite, photo 1
Photo: National Park Service, public domain, via Wikimedia CommonsLicense (opens in a new tab)Photo source (opens in a new tab)

Named stone

Catoctin and South Mountain metarhyolite

Also called Maryland rhyolite, Maryland metarhyolite, Catoctin metarhyolite, South Mountain rhyolite

A kind of rhyolite, felsite and metavolcanic rock

Catoctin and South Mountain metarhyoliteRhyolite, felsite and metavolcanic rockMid-Atlantic

Description

Catoctin metarhyolite is the dark, slightly metamorphosed volcanic rock of Catoctin Mountain and South Mountain in Frederick and Washington counties, Maryland. Most pieces are dark blue-gray to black and weather to lighter hues. Maryland archaeologists recognise several kinds: porphyritic stone with white rectangular feldspar crystals in a dark groundmass, uniform fine-grained aphanitic stone, banded stone with light and dark layers, mottled stone, and a rarely used red porphyritic type. The best porphyritic material has a chert-like texture. Middle Woodland knappers clearly favoured the porphyritic kind, especially for Fox Creek points. A mottled metarhyolite from Adams County, Pennsylvania, also turns up on Maryland sites.

How to identify it

Colour
Most commonly dark blue-gray to black, lighter when weathered (Maryland guide). The red porphyritic variety has a medium to light gray, pink or purple groundmass; a greenish metarhyolite occurs at the Jefferson site in Frederick County.
Texture
Aphanitic (too fine to see grains) in the plain variety; the best porphyritic stone has a chert-like texture; the red variety can have a phyllitic texture and sheen (Ebright).
Luster
Phyllitic sheen on the red variety only (Ebright).
Inclusions
Porphyritic stone has white rectangular feldspar crystals (phenocrysts) in a dark groundmass; banded stone has alternating light and dark layers, sometimes an almost oolitic look.
Cortex
Weathering produces lighter hues (Maryland guide).

Geology

Metamorphosed volcanic rock of the Catoctin Formation, which crops out on Catoctin Mountain and South Mountain in Frederick and Washington counties, Maryland. The rock called rhyolite in the eastern United States is technically metarhyolite, slightly metamorphosed rhyolite. Adams County, Pennsylvania, produces a mottled metarhyolite sometimes found on Maryland sites. Large metarhyolite outcrops also lie near Mount Rogers, Virginia, and in the Uwharrie Mountains, North Carolina.

Where it comes from

Mid-Atlantic

States

Quarries and outcrops

  • Catoctin Mountain, Frederick County, Maryland
  • South Mountain, Maryland
  • Adams County, Pennsylvania (mottled variety)

Used by

  • Middle Woodland: a distinct preference for porphyritic metarhyolite, especially for Fox Creek points (Ebright)
  • Found at (not a source): Jefferson site, Frederick County (greenish metarhyolite found in large quantities)

Look-alikes

Stones that are easy to confuse with Catoctin and South Mountain metarhyolite.

  • Mount Rogers rhyolite (Grayson County, Virginia): gray to purple with white or pink crystals
  • Uwharrie (Morrow Mountain) metavolcanic rock of North Carolina
  • Very fine-grained metarhyolite can resemble chert (Ebright, cited by the Maryland guide)

Sources

  1. Ebright, Carol. 1999 (updated 2023). Maryland Rhyolites. Virginia Department of Historic Resources, Lithics (Projectile Points and Lithics guide).dhr.virginia.gov (opens in a new tab)
  2. Maryland Archaeological Conservation Laboratory. Rhyolite. Diagnostic Artifacts in Maryland: Common Stone Types (Jefferson Patterson Park and Museum).apps.jefpat.maryland.gov (opens in a new tab)
  3. Tolley (after Rankin 1994). 1999. Mount Rogers Rhyolite. Virginia Department of Historic Resources, Lithics (Projectile Points and Lithics guide).dhr.virginia.gov (opens in a new tab)
  4. Ebright, Carol. 1999 (updated 2023). Maryland Rhyolites. Virginia Department of Historic Resources, Projectile Points and Lithics guide.dhr.virginia.gov (opens in a new tab)

Points made from this stone

All materials