Chert Type Key

Blanding Chert

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Note that there is a general overlap between Blanding, Elwood-Joliet, Burlington, and Glacial Till chert in all five of the classificatory properties. Correct assignment of small items of debitage to these types must remain somewhat problematic.

TEXTURE:
The texture is medium to medium fine.

LUSTER:
Luster tends to be dull.

COLOR:
Color ranges from white to gray. The penetration of ground water into bedding laminations has resulted in iron oxide deposits that produce pale yellowish, brownish, or orangish streaks and faint banding. Fossils fragments are frequently in these contrasting colors. There is occasional mottling. The mottled regions will have comparatively finer texture, brighter luster and greater translucency.

FOSSILS:
Fossil inclusions are sponge spicules, brachiopods, corals, and crinoid columnals. Rarely do they have lengths greater than 0.5 mm. Sponge spicules of a distinct white color will be common (Figure 1).

Figure 1. At magnification of 14X, one can observe sponge spicules and oolites.

STRUCTURAL CHARACTERISTICS:
Oolites are frequently encountered (Figure 1).

HEAT TREATMENT:
The effects of thermal alteration on color and luster are as would be expected with color moving to tones of pink and luster becoming more satin-like.

SOURCE:
The illustrated specimens were generously provided by Peter J. Geraci of Illinois Transporation Archaeological Research Program. The items came from an outcrop of Silurian System materials exposed in the right of way of US Highway 20, some five kilometers west of Elizabeth, Jo Daviess County, Illinois. Curiously, Google Earth/Street View (Figure 2) provides a photograph (Reference Number: 2774 U.S. 20) of the roadway and formation. Note the light colored chert bearing horizons.

Figure 2. The Blanding specimens were recovered from the imaged outcrop of Silurian System bedrock.

Figure 3. Blanding Chert. Click on image for full view.