NEW TROPICAL SEAS
Glacial melting early in the Silurian (438-418 mya) led to sea-level rise. Shorelines advanced eastward only gradually, as the state east of Syracuse remained uplifted after the Taconic orogeny. Eastern highlands that continued to be eroded are indicated by the gravels and coarse sandstones of the Upper Silurian in the Shawangunk Mountains in southeast New York State, (Mohonk Mountain House in Ulster Co., is nestled in this rock). Fossiliferous early Silurian rocks occur west of Syracuse and include the Rochester Shale, with its more than 200 species of marine fossils.
The resistant caprock of Niagara Galls as formed of coral reef-bearing limestones consisting originally of calcium carbonate, now largely changed to dolostone. They comprise the Lockport Group, visible in the Niagara Escarpment that extends from Hamilton, Ontario, to Medina (Orleans Co.). Continuing sea-level rise drove shorelines east to Newburgh (Orange Co.) and Hudson (Columbia Co.) by the end of the Silurian, but this rise was so slow that the seas became shallow and very salty. Thus, most late Silurian rocks in the state are sparsely fossiliferous. Significant deposits of salt (used primarily for road salt) and gypsum (used for wallboard and plaster) are Upper Silurian and were early mined south of Syracuse. Dating to the late 1880s, the Retsof mine in Livingston Co. was the largest underground mine in the world by 1994, when groundwater started to flood it following a roof collapse. Salt production continued in the unflooded parts of the mine until 1996, when it was closed. Salt, gypsum, and thin-bedded dolostone and shale mean that the Upper Silurian does not resist erosion and is poorly exposed west of Syracuse, where it forms a lowland crossed by the old Erie and modern New York State Barge Canals.