Sea levels have risen and fallen throughout geologic history. When they rise they encroach on the lowlying areas of continents, called basins, and create shallow seas. New York, as well as Pennsylvania, Ohio, and West Virginia, were all covered by a shallow sea around 390 million years ago, and this sea deposited Marcellus Shale and many other
rock units. Rivers and streams fed into this basin, and brought with them small quantities of clay-like sediments. Algae and other planktonic organisms inhabited the surface waters of this sea, and, as they died, sank to the bottom and mixed with
the sediment. The Marcellus Shale’s characteristic dark color and natural gas abundance is due to the high concentration of organic matter trapped with the sediment that eventually became rock