DEVONIAN CHERTS OF NEW YORK

To most regional geologists and archeologists, New York’s Devonian strata feature silica-rich rock/lithic material sometimes identified as “Onondaga”, “Esopus” and “Kalkberg” cherts. However, chert occurs within at least 15 different Devonian formations in New York. These include (low to high) the Lower Devonian Kalkberg, New Scotland, Becraft, Alsen, Port Ewen, Port Jervis, coeval Oriskany and Glenerie, Esopus, and coeval Schoharie and Bois Blanc formations; and the Middle Devonian Onondaga, Union Springs, Skaneateles (Stafford Mbr.) and Moscow (Menteth Mbr.) formations. Abundance in the units varies from rare to extremely common, even locally dominant. Most of the chert-bearing units occur in eastern New York (Otsego to Orange cos.). Across central to western New York, only the Onondaga Fm. features chert of any significance. Abundance and character of chert varies not only between units, but also vertically and laterally within same unit. Differences in character affecting the usuability of chert for knapping by prehistoric peoples include the size of individual masses, impurities (e.g., dolomite) and structural fractures/joints. Color also varies some, but most New York Devonian chert falls within a range of dark gray shades. Most of the chert formed during diagenesis via silica replacement of lime sediment and/or limestone rock (“nodular” type chert). Some formed via microscopic silica precipitation that cemented together siliciclastic muds to silts (“bedded” type, i.e., Esopus Fm., and to some degree, Port Ewen Fm.), to form cherts of varying purity. Environmental conditions (e.g., water depth) in part controlled the formation of chert. However, in at least some strata (e.g., Kalkberg-New Scotland, Esopus and Onondaga +/- Glenerie, Oriskany, Port Ewen fms.), chert occurs between or below clay-rich beds of altered airfall volcanic tephra. Post-depositional alteration of volcanic ash/glass to smectitic clays would have released abundant silica to waters in adjacent to subadjacent sediments, leading to chert precipitation. In summary, chert occurs in many formations in New York. Their character varies widely. Some yield workable chert for knapping. Much of the chert occurs in shades of dark gray. Identification of source units can be very difficult. based solely on visual appearance.