FLINT RIDGE
Flint Ridge, Ohio is
world-famous for having beds of extremely colorful & complexly-patterned
chert. “Flint” used to be an academic geology term that, according to one
early defintion, referred to dark-colored chert (= cryptocrystalline quartzose
sedimentary rock). Nowadays, flint is supposed to be an archaeological
term only, referring to worked chert artifacts. British geologists still
use “flint” as a geology term, especially when referring to very dark-colored
chert nodules in the Cretaceous-aged chalk beds of southern Britain.
For flint knappers and
mineral collectors, “flint” is perceived to be high-quality and high-purity,
while “chert” is low-quality and low-purity. These are arbitrary,
imprecise, not well-demarcated definitions. “Chert” is the proper term
for cryptocrystalline quartzose sedimentary rock.
“Flint”does appear to be
used (grudgingly) as a valid academic geology term when referring to the chert
beds of Flint Ridge, Ohio. Flint Ridge is an ~east-west trending ridge in
southeastern Licking County and western Muskingum County in east-central Ohio,
USA. Other Pennsylvanian-aged chert horizons in Ohio and surrounding
parts of the Appalachian Basin have also been called "flint" in the
geologic literature.
The “Flint Ridge Flint” is
properly known as the Vanport Flint (lower Allegheny Group,
Desmoinesian, upper Middle Pennsylvanian). The Vanport Flint consists of
laterally discontinuous meganodules of secondarily chertified limestone.
Original limestone textures and clasts are visible in some samples (for
example, oolites, oncolites, and fossils).
Vanport Flint - this is the largest
flint-collecting pit at Nethers Flint Quarries near the eastern end of Flint
Ridge, Ohio (as it looked in mid-April 2008). This site is intensely
collected. High-quality, variably colored and patterned flint is abundant
here, as are quartz crystal masses from irregularly-shaped geodes. Turgite
is encountered here.
Vanport Flint - loose flint pieces at
Nethers Flint Quarries, Flint Ridge, Ohio, USA.
Vanport Flint - in-situ flint
outcrop in main pit at Nethers Flint Quarries, Flint Ridge, Ohio. The
flint color here is off-white. The yellowish-brownish limonite staining
is developed along fractures/joints.
Vanport Flint - megaquartz-lined cavity
in flint at Nethers Flint Quarries, Flint Ridge, Ohio. Irregularly-shaped
geodes are moderately common in this unit. Individual cavity-lining
quartz crystals vary in size from nearly microscopic to a couple centimeters in
size. Many are clear (“rock crystal”), but some are grayish to blackish
(“smoky quartz”), and some are a nice honey-yellow color (approaching “citrine”).
Flint Ridge flint occurs in
just about any color of the rainbow. Specimens are sometimes
finely-banded (“pinstripe flint”). Flint that's been brecciated and
fractured & healed with chalcedonic quartz veins is common. Oolitic
flint is somewhat common as well.
Vanport Flint (6.8 cm across)
Vanport Flint (5.9 cm across along the
bottom)
Vanport Flint (4.3 cm across along the
bottom) - gray pinstripe flint with whitish-gray chalcedony-filled fracture.
Vanport Flint (4.2 cm across along the
bottom) - multicolored banded chert with whitish-gray chalcedony- &
megaquartz-filled fractures. The brownish, rough-textured areas at the
top of the specimen is from extreme weathering. Chert is a very
weathering-resistant rock, but it can break down under Earth-surface
conditions. Rough-textured, extremely weathered chert is called rottenstone
or tripoli. This sample has just begun rottenstone development.
Vanport Flint (5.0 cm across along the
bottom) - complex, multicolored specimen of brecciated pinstripe flint with
angular clasts cemented by bluish-gray chalcedony.
Vanport Flint (4.6 cm across along the
bottom) - the coloring agent of this deep reddish-colored flint specimen is
disseminated hematite (iron oxide). “Jasper” has been used to refer to
reddish-colored cherts.
Vanport Flint (6.1 cm across along the
bottom) - oolitic pinstripe flint with two small reverse faults (see left side
of specimen).
Vanport Flint (11.4 cm across at its
widest)
Vanport Flint (4.8 cm across along the
bottom) with prominent bluish-gray chalcedony-filled fractures.
Vanport Flint (3.5 cm across along the bottom)
with pale bluish-gray chalcedony-filled fractures.
Vanport Flint (5.6 cm across along the
bottom) - pinstripe flint with a whitish to pale bluish-gray, chalcedony-filled
fracture that has offset some of the bands.
Vanport Flint (left: 4.2 cm across
at its widest; right: 3.1 cm across along the bottom)
Vanport Flint (left: 1.8 cm across
along the bottom; center: 1.7 cm across along the bottom; right:
1.8 cm across along the bottom)
Vanport Flint (4.0 cm across along the
bottom) - pinstripe flint with much of the pinstripe banding secondarily
bleached out.
Vanport Flint (6.0 cm across along the
bottom) - flint with some megaquartz-lined and megaquartz-filled
cavities. The dark reddish, dark brownish, and dark bluish-gray are
various iron oxides coating a fracture surface (= the surface facing the
viewer). These iron oxides include hematite, turgite, and (apparently) goethite.
Vanport Flint (9.4 cm across at its
widest)
Vanport Flint (field of view: 2.7 cm
across) - oolitic flint. Oolites are sand-sized, concentrically-layered,
calcium carbonate (CaCO3) grains that form by back-and-forth wave
action in shallow water settings. This fabric indicates that the rock was
originally an oolitic limestone and has been secondarily chertified.
These oolites are now filled with brownish-colored iron oxides.
Vanport Flint (field of view: 3.0 cm
across) - oolitic flint. The oolites here are small-scale weathering
centers, resulting in a rough appearance and secondary porosity - the beginning
of rottenstone, or tripoli.
Vanport Flint (field of view: 4.5 cm
across) - oolitic pinstripe flint. The oolites have been preferentially
weathered and eroded, resulting in secondary oolitic porosity in the chert.
Vanport Flint (10.5 mm across) (above
& below) - fusulinid foraminifera in flint. Fusulinids are one of
several groups of fossils observed in the Vanport Flint at Flint Ridge,
Ohio. This is despite the extensive chertification of the original
limestone. Fusulinid foraminifera are a group of large, unicellular,
non-photosynthetic eucaryotic organisms that made a calcium carbonate (CaCO3)
shell in the shape of a rice grain or football. In transverse
cross-section view, the fusulinid tests have a subcircular, coiling shape.
Some info. from:
DeLong (1972) - Bedrock
geology of the Flint Ridge area, Licking and Muskingum Counties, Ohio. Ohio
Geological Survey Report of Investigations 84.