Bayport chert

Bayport chert is the main local chert of Michigan's Saginaw Valley, from the Upper Mississippian Bayport Limestone, which crops out around the town of Bayport on Saginaw Bay and at several other spots in the region. It forms ellipsoidal nodules from about 1 by 2 cm to 15 by 20 cm, in two forms: dense gray chert and a white, sandy, porous kind. A second kind of chert in the same rock replaced algal mats. Saginaw Valley people used it almost exclusively in the Middle Archaic, and it dominated assemblages until near the end of the Late Archaic, when imports such as Wyandotte and Onondaga cherts became common. In late prehistoric times Bayport chert reached 50 sites in Michigan, northwestern Ohio and southwestern Ontario.

Chert in the Upper Mississippian Bayport Limestone occurs in two places in the rock: as chertified algal mats, which formed in intertidal and supratidal zones, and as ellipsoidal nodules in the nodular limestone of the sublittoral platform. The nodules range from about 1 by 2 cm to 15 by 20 cm. Bacon read the rock as a record of a tidal-flat setting like a modern sabkha; the source of the silica is not known. Reid places outcrops at the town of Bayport and several other locations in the region, including islands in Saginaw Bay.