Banded slate (glacial)

Also called Ohio banded slate, Huronian banded slate, glacial banded slate

A kind of argillite, hornfels, slate and siltstone

Banded slate (glacial)Argillite, hornfels, slate and siltstoneMidwest/Ohio Valley

Description

Banded slate is the fine-grained, striped stone of most birdstones, many gorgets, bar amulets and pendants found around the southern Great Lakes and the Ohio Valley. It is not a chipping stone: it was sawn, ground, drilled and polished. The favoured kind is a greenish-gray banded slate often called Huronian. In Ohio it occurs as glacial erratics, boulders and cobbles of very old Canadian rock pushed south by the ice sheets; geologists note banded slate or argillite of glacial-lake origin among the Precambrian erratics of northern Ohio, often beside Gowganda tillite boulders, which come from the north shore of Lake Huron. Shaped slate ornaments became common around 3,500 years ago in the Late and Terminal Archaic and were made into the Woodland period. Birdstones date mostly to about 1500-500 BCE and are tied to Glacial Kame and Red Ochre burials.

How to identify it

Colour: Greenish-gray with darker and lighter bands (the 'Huronian' variety favoured for birdstones, per Wheeler 2018); a banded slate gorget from Ontario ranges from light gray to dark gray (Ellis 2002).
Texture: Fine-grained and soft enough to saw, grind and polish; too soft for a working knife edge (Funkhouser and Webb 1928).
Luster: Takes a high polish on finished pieces.

Geology

Precambrian (Huronian) banded slate or argillite, probably laid down in glacial lakes, found in Ohio as glacial erratics in Wisconsinan and Illinoian drift. It travels with Gowganda Formation tillite erratics (2.2-2.4 billion years old) whose bedrock lies between Sault Ste. Marie and Sudbury, Ontario, along the north shore of Lake Huron. The exact bedrock source of the artifact slate was not established in what was opened.

Where it comes from

Midwest/Ohio Valley, Northeast

States

Quarries and outcrops

Glacial drift of northern and glaciated Ohio (banded slate erratics; e.g. with tillite erratics in Richland County)
Ultimate bedrock source: Huronian rocks north of Lake Huron, Ontario (inferred from the tillite erratics; not proven for artifact slate)

Used by

Late/Terminal Archaic onward: shaped ground slate ornaments common from about 3,500 years ago into the Late Woodland (Ellis 2002)
Birdstones: Late Archaic and Early Woodland, about 1500-500 BCE, including Glacial Kame and Red Ochre (Wheeler 2018)
Early to Middle Woodland gorgets (Ellis 2002)

Look-alikes

Stones that are easy to confuse with Banded slate (glacial).

Plain gray or black slate and shale, also used for gorgets and bar amulets
Gowganda tillite erratics, which collectors often call 'porphyry' (Murphy 1973); tillite was used for celts, axes and hammerstones

Sources

Wheeler, Ryan. 2018. Birdstones. Robert S. Peabody Institute of Archaeology, Phillips Academy, Andover (blog post, March 30, 2018).peabody.andover.edu (opens in a new tab)
Funkhouser, W. D., and W. S. Webb. 1928. Ancient Life in Kentucky. Kentucky Geological Survey, Series 6, Vol. 34. Chapter XIV, Miscellaneous Artifacts.uky.edu (opens in a new tab)
Murphy, James L. 1973. A Large Tillite Erratic, Originally Described by J. E. Hyde, in Cuyahoga County, Ohio. Ohio Journal of Science 73(3):180-184.kb.osu.edu (opens in a new tab)
Angle, Mike. 2016. Gowganda Tillites in Ohio. Ohio Department of Natural Resources, Division of Geological Survey (Ohio Geology EXTRA).dam.assets.ohio.gov (opens in a new tab)
Ellis, Christopher. 2002. An Unusual Slate Gorget Fragment from Oxford County, Ontario. Ontario Archaeology 74:22-.ontarioarchaeology.org (opens in a new tab)

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