Taghonic Falls - Ithaca, NY
SEDIMENTARY ROCKS & STRATIGRAPHY OF NEW YORK STATE
New York preserves a record of over a billion years of geologic and biologic history in its rock record. Indeed, the variety of rocks and geologic history found in a trip through eastern New York to the border with Vermont or Massachusetts is as great as that found on a trip from Mount Rushmore in South Dakota to central Nevada. New York’s abundant fossils and the wealth of clues that helped in the reconstruct of earth history meant that the state’s geologic record has provided a standard for global time correlation of the older fossil-bearing rocks from the mid-1800s. The earlier history of the science of geology is prologue to modern interpretations that include the role of continental collisions, global rises and falls in sea level, the location of New York at tropical latitudes, and the opening of the Atlantic Ocean as factors in determining the types of rocks that make up New York.
A significant portion of the state of New York (on the order of 75%) is underlain by sedimentary rocks (i.e., rocks originally laid down as sediments by wind or water). Some of these sedimentary rocks have been altered by heat and pressure so that they are metamorphosed, as in the Taconic hills of easternmost New York, but their origin as sedimentary rocks is easily demonstrated. Unless you are in the Adirondack Mountains, the Hudson Highlands, or parts of the New York City metropolitan area, the underlying bedrock is composed of sedimentary rocks.
Sediments of various types become "lithified" over time to form rock. Lithification takes place by compression and attachment of sediment grains to each other after burial or through their being cemented by the precipitation of minerals from water. Different sedimentary rock types are classified as "clastic" (or "siliciclastic"), "biogenic", and "chemical,” depending on what they are composed of. Shales, siltstones, sandstones, and conglomerates are composed of the broken fragments of pre-existing rocks or minerals, and represent different clastic rocks. Originally these rocks were composed of muds, silts, sands, and gravel- to boulder-sized particles that were eroded from adjacent highlands, deposited in water, and "bonded" together over geologic time. Many of the siliciclastic rocks in New York record sea-level rises or the uplift of ancient highlands in the Appalachians, or, as in the Taconic hills, are the record of very deep marine environments. Biogenic rocks in New York largely reflect New York’s position near the equator at times in the geologic past. These include limestone (CaCO3) and dolostone (CaMg(CO3)2), which often consist of mineral grains that were once part of marine organisms with limy parts such as shells. Black, organic-rich sediments (such as coal and some black shale) often consist of fragmentary tissue of land, fresh-water, and marine plants and animals, and are biogenic rocks. A few cherts (fine-grained rocks made of silica) in the Taconic hills in eastern New York are made of the siliceous shells of one-celled organisms known as radiolaria, and are biogenic rocks. Those sedimentary rocks that formed through the chemical precipitation of inorganic materials ("chemical” rocks) include halite (rock salt), gypsum and anhydrite (calcium sulphate minerals), iron-rich sedimentary rocks as limonite and hematite, and some limestones, dolostones, and cherts. Sandstone, shale, and limestone comprise the most abundant sedimentary rocks in New York State (for more basic information see "A Primer on Sedimentary Rocks and Stratigraphy").
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