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Cedar Valley Group cherts, photo 1
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Named stone

Cedar Valley Group cherts

Also called Grand Meadow chert, Rapid chert, Cedar Valley chert, Cedar Valley jasperoid, Root River chert

A kind of chert (and flint)

Cedar Valley Group chertsChert (and flint)Midwest/Ohio Valley

Description

Grand Meadow chert, the best-known chert of the Devonian Cedar Valley Group, is a distinctive fine-textured chert from southeastern Minnesota. It occurs as small egg-shaped or cylindrical nodules, typically about 8 cm long, in lag clay at the edge of the Driftless Area near Grand Meadow, where the quarry was used for at least 8,000 years. Fresh stone is mottled olive-gray to yellowish gray and dull, with a thin yellow-gray carbonate rind. Sponge spicules, bryozoans and brachiopod fragments are nearly always visible under a loupe, and fenestrate bryozoans are diagnostic. Heating is not needed but turns it pinkish and raises the luster. Rapid chert is the Iowa cousin from the Rapid Member, and the group also yields yellowish-brown opaque chert in Fillmore County.

How to identify it

Colour
Light to medium gray, mottled, olive-gray in the unweathered interior (Munsell 5Y 4/1 to 5Y 6/1); transmitted light has a golden tint; pieces patinate yellowish gray to light gray.
Texture
Medium fine, about 0.5 to 1.5 mm; fine (Bakken).
Luster
Dull, becoming satiny with use-wear polish or heat.
Inclusions
Sponge spicules, bryozoans (fenestrate and bifoliate), brachiopod fragments, crinoids, horn corals, the trilobite Proetus and Tentaculites; fossil casts are sometimes filled with blue chalcedony or druzy quartz.
Cortex
A thin yellow-gray (Munsell 5Y 8/1) carbonate cortex, reddish brown or yellowish orange if iron-stained, over a lighter gray subcortical rind; stream gravel pieces lose the cortex and gain a deep reddish brown patina.
Heat treatment
Not necessary for working the stone; 204 to 260 degrees C (400 to 500 F) reddens the hue from 5Y 7/2 to 5YR 7/2 and raises the luster; higher temperatures craze and pot-lid it (Wendt and Trow).

Geology

Grand Meadow chert probably comes from the Bassett Member (formerly the Rapid Member) of the Little Cedar Formation, Cedar Valley Group (Devonian), although no bedrock outcrop has been found in Minnesota. It is concentrated as lag clay just above the unconformity between the Bassett Member and the older Wapsipinicon Group at the Grand Meadow quarry (site 21MW8). Similar gray chert occurs in stream and terrace gravels in Fillmore, Mower and Freeborn counties, and in a bedrock quarry at Lime Springs, Iowa. The Cedar Valley chert family also includes Rapid, Wapsipinicon and Shell Rock cherts; in Minnesota the name Cedar Valley chert is kept for a different, opaque or translucent chert in Fillmore County.

Where it comes from

Midwest/Ohio Valley

States

Quarries and outcrops

  • Grand Meadow Chert Quarry (site 21MW8) near Grand Meadow, Mower County, Minnesota (National Register 1994)
  • Lime Springs, Iowa (modern bedrock quarry with similar chert)
  • Spring Valley, Minnesota (lag deposits, near the Mundfrom/Till site)
  • Fillmore County, Minnesota (Cedar Valley chert quarry sites)

Used by

  • Quarried for at least 8,000 years at Grand Meadow (Wendt and Trow)
  • Found at archaeological sites in 52 Minnesota counties and in Iowa, South Dakota and Wisconsin (Minnesota DNR)

Look-alikes

Stones that are easy to confuse with Cedar Valley Group cherts.

  • Galena chert (usually coarser, more opaque, with small white fossil flecks)
  • Burlington chert and Knife River flint, which circulated more widely (Bakken)
  • Other Devonian cherts of the family: Wapsipinicon, Shell Rock, and Lime Creek Quarry chert in Iowa

Sources

  1. Wendt, Dan, and Tom Trow. 2020. Grand Meadow Chert: A Distinctive and High-Quality Chert in Southeastern Minnesota. The Minnesota Archaeologist 77.mowercountyhistory.org (opens in a new tab)
  2. Bakken, Kent E. (written about 1995; PDF issued later). Chipped Stone Raw Materials. Chapter 6 in Stone Tools of Minnesota. Minnesota Office of the State Archaeologist, St. Paul.mn.gov (opens in a new tab)
  3. Wagen, Sommer. 2026 (January-February). Minnesota Profile: Grand Meadow Chert. Minnesota Conservation Volunteer, Minnesota Department of Natural Resources.dnr.state.mn.us (opens in a new tab)

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