Helderberg cherts

Also called Kalkberg chert, New Scotland chert, Becraft chert, Alsen chert, Helderberg Group chert

Helderberg chertsChert (and flint)Northeast

Description

Helderberg chert is a family of cherts from the Lower Devonian Helderberg Group, whose limestones form the Helderberg escarpment west of the Hudson River in eastern New York. Four members carry chert: Kalkberg, New Scotland, Becraft and Alsen. In New York the stone is black to blue-black, smooth and waxy, with white dendritic scaling along the bedding, and it weathers to a thin gray-white coat. Kalkberg chert is the most uniform and litters the talus at the foot of the scarp. A Maryland sample from the same group is paler, cream to bluish-gray with crinoid and brachiopod fossils. Hudson Valley Paleoindians preferred Normanskill chert, so Helderberg stone is uncommon in their sites.

How to identify it

Colour: Black or very dark blue-black in New York (Munsell N2 to 5BG 1); cream through bluish-gray with light mottling in the Maryland sample described by Ebright.
Texture: Smooth and waxy in New York; Kalkberg is very fine grained and uniform, a silicified limestone with 30 to 55 percent carbonate in thin section (Funk). Dull to waxy in Maryland (Ebright).
Luster: Dull to waxy; Kalkberg is relatively dull surfaced.
Inclusions: White dendritic scaling parallel to bedding and scattered pockmarks (New York). Fossils are common: corals, bryozoans and brachiopods (Alsen), trilobites, brachiopods and corals (New Scotland), crinoids, brachiopods and conodonts (Maryland sample). Dolomite crystals and pyrite occur in thin section.
Cortex: Weathers to a thin gray-to-white coating (Funk); the Maryland sample has a dull porous crust with calcite micro-crystals.

Geology

Lower Devonian Helderberg Group, which in Greene County, New York, stacks Coeymans/Manlius, Kalkberg, New Scotland, Becraft and Alsen limestones on the Rondout Formation. Chert occurs in the Kalkberg, New Scotland, Becraft and Alsen members. In Maryland the Virginia guide ties a light bedded variety to the New Scotland Member and reports black chert from the Licking Creek Limestone member. Weathering of the escarpment leaves blocks of chert in the talus.

Where it comes from

Northeast, Mid-Atlantic

States

Quarries and outcrops

Helderberg escarpment, Greene County, New York (Kalkberg chert blocks in the talus)
Flintstone, Allegany County, Maryland (sample location in the Virginia DHR guide)

Used by

Paleoindian: Kalkberg chert is a minor stone at the West Athens Hill quarry-workshop (Funk)

Look-alikes

Stones that are easy to confuse with Helderberg cherts.

Normanskill chert: dark varieties are easily confused with dark Kalkberg (Funk); trace-element profiles are very similar
Dark eastern Onondaga chert

Sources

Funk, Robert E. 2004. An Ice Age Quarry-Workshop: The West Athens Hill Site Revisited. New York State Museum Bulletin 504. University of the State of New York, Albany.exhibitions.nysm.nysed.gov (opens in a new tab)
Ebright, Carol. 1999 (updated 2023). Helderberg Chert. Virginia Department of Historic Resources, Lithics.dhr.virginia.gov (opens in a new tab)
U.S. Geological Survey, National Geologic Map Database. Geolex: Helderberg.ngmdb.usgs.gov (opens in a new tab)

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