The Helderberg Group is a geologic group that outcrops in the State New York, Pennsylvania, Maryland, New Jersey, and West Virginia. It also is present subsurface in Ohio and the Canadian Providence of Ontario 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.^[1]

The upper portion of the Helderberg, or the Kalkberg Formation is host to the Bald Hill ash bed, dated to 417.6 million years ago.^[2]

The Helderberg is composed chiefly of limestone and dolomite.^[3]

In Maryland and southern Pennsylvania, the Helderberg is divided into three formations. These are the New Creek Limestone, the Corriganville Limestone, and the Mandata Shale. The total thickness is about 60 feet. The formations weather easily and are poorly exposed except in cuts and quarries.^[4]

The Manlius Limestone was first noted by Vanuxem (1840, p. 372) as a "waterlime" (hydraulic limestone) near Manlius, New York.^[5] The Manlius is composed of limestone, grainstone, calcareous mudstone and bindstone. The Manlius is 419 - 411 Ma.^[6]

The Thacher Member member of the Manlius along eastern New York and down into parts of northern New Jersey. It is characterised as a "ribbon rock", meaning that 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 51 feet thick. It makes up the upper part of the Manlius in New York and makes up the entirety of the Manlius in northeastern New Jersey. Moving southwest it become more argillaceous and arenaceous, as it grades into the Depue Limestone.^{[citation needed]}

The Thacher contains fossils of stromatoporoids, crinoids, rugose corals, tabulate corals, tranchiopods, tentaculitids, and ostracodes.