Chert, Part III:

Devonian "Shriver Chert" from Central Pennsylvania, U.S.A.

Three samples are featured this month, a provenance study investigating the properties of chert from Snyder county and adjacent areas. All photomicrographs were taken in transmitted, plane-polarized light.

Snyder county chert [217 kb]

"Rock of the Month # 24, posted June 2003"

Sample 40 is from the Walnut Acres site, and represents the local cherts of Snyder county. Although the chert is too variable, both within and between outcrops, for any one sample to display every feature, this sample exhibits several characteristics common to cherts in the study area. The area extends roughly 40 km east-west and 20 km north-south, west of the Susquehanna river near Selinsgrove. This sample is a laminated chert, part-silicified dolostone, dark brownish-black in hue, displaying intense effervescence in dilute hydrochloric acid along hairline fractures lined with calcitic carbonate.

Dark laminae may preserve signs of soft-sediment deformation in areas of cherty replacement of the carbonate host rock. In common with many local cherts, this sample contains siliceous spicules up to 0.5 mm long and small organic tests (carbon spheres representing spores and other microfossils). In reflected light these spheres, up to 0.3 mm in diameter, are often highly reflective and graphitic, indicative of the depth of burial and metamorphism in this area of the Appalachians. Even though the thermal alteration limits their value as biostratigraphic markers, they are a useful and widespread characteristic of `Snyder county' chert. The bulk of the rock is relict primary carbonate and fine-grained silica.

80X magnification, long-axis field-of-view 1.4 mm.

Stony Ridge chert [194 kb]

Sample 85 is from the Stony Ridge locality in Carbon county, some 120 km east of the Snyder county study area. The material from this site is a distinctive laminated, calcareous siltstone, partially replaced by dark grey to black chert. The chert-to-host-rock transition is sharp, occurring within 1 mm. The sample displays very slow acid reaction, faster in areas of chert which are enriched in a secondary, calcitic carbonate.

This chert contains abundant angular, detrital quartz in a granular groundmass of fine silica and carbonate. The abundant silt or sand component imparts a finely granular appearance to the hand specimens, and is immediately obvious under the microscope.

80X magnification, long-axis field-of-view 1.4 mm.

Hyndman chert [202 kb]

Sample 77 is from the Hyndman locality in Bedford county, some 140 km southwest of the Snyder county study area. This is a medium-grey chert with a dull orangey exterior rind. It can be described as a partially silicified, spicule-rich limestone which displays very rapid effervescence in dilute acid.

The sample contains various bioclasts, including abundant siliceous spicules as large as 0.75 mm in length, plus minor carbon spheres and rare fragments of macrofossils. Rhombs of secondary carbonate are present, and less-frequent nests of fibrous chalcedony (not shown). A fibrous calcite veinlet with opaque (?) iron oxide rind cuts the host rock.

40X magnification, long-axis field-of-view 2.8 mm.

This comparison of three Shriver / Old Port cherts reveals something of the variability of the siliceous material in the region. The geological setting of the region is explained in a series of review papers in Shultz (1999). Chert is a major raw material for stone tools in Pennsylvania, with many sites and thousands of artefacts recovered from Snyder county alone (Bailey and Katz, 2003). The design and conduct of an archaeological provenance study on chert in Snyder county is described by Wilson et al. (2003a), while a more detailed evaluation of the results is presented elsewhere (Wilson et al., 2003b). In total, 102 polished thin sections were studied and well over 100 rock samples and artefacts were subjected to multi-element whole-rock and trace-element analyses.

The geological, mineralogical and geochemical study of chert outcrops and artefacts was conducted by staff of consultancy A.D. Marble and Company, Inc. and Graham Wilson of Turnstone Geological Services Ltd. in January 2002- June 2003, on behalf of the Pennsylvania Department of Transportation (PENNDOT), Engineering District 3-0.


BAILEY,DN and KATZ,GM (2003) Prehistoric chert extraction and utilization in Snyder county, Pennsylvania. Manuscript.

SHULTZ,CH (editor) (1999) The Geology of Pennsylvania. Pennsylvania Geological Survey / Pittsburgh Geological Society, Spec.Publ. 1, 888pp.

WILSON,GC, KATZ,GM, BAILEY,DN and DAY-LEWIS,A (2003a) Design and implementation of a multi-disciplinary provenance study of chert from Snyder County in central Pennsylvania. Manuscript.

WILSON,GC, BAILEY,DN and KATZ,GM (2003b) A detailed provenance study of chert from Snyder County in central Pennsylvania. Manuscript.

Graham Wilson, posted 29 June 2003

For background information, see the original "Rock of the Month" Chert!

and also a second description of "Snyder county" Chert

Visit the Turnstone "Rock of the Month" Archives!

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