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Helderberg Group (geology)

The Helderberg Group is a geologic group that outcrops in the States of New York, Pennsylvania, Maryland, New Jersey, and West Virginia. It also is present subsurface in Ohio and the Canadian provinces of Ontario and Nova Scotia. It preserves fossils dating back to the Early Devonian and Late Silurian period. The name was coined by T.A. Conrad (1839) in the New York State Geological Survey Annual Report, named for the Helderberg Escarpment or Helderberg Mountains.

Stratigraphy
Manlius Limestone The Manlius Limestone was first noted by Vanuxem (1840, p. 372) as a "waterlime" (hydraulic limestone) near Manlius, New York. The Manlius is composed of limestone, grainstone, calcareous mudstone and bindstone. The Manlius is 419 - 411 Ma. The Thacher Member of the Manlius extends along eastern New York and down into parts of northern New Jersey. It is characterised as a "ribbon rock", meaning it contains very thin layers of alternating limestone and argillaceous rock. The color of the Thacher is dark blueish-black. The limestone is fine- to medium-grained. It averages thick. It makes up the upper part of the Manlius in New York State and makes up the entirety of the Manlius in northeastern New Jersey. Moving southwest, it become more argillaceous and arenaceous as it grades into Depue Limestone. The Thacher contains fossils of stromatoporoids, crinoids, rugose corals, tabulate corals, tranchiopods, tentaculitids, and ostracodes. == References ==
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