Cambrian period [Lat. Cambria = Wales], first period of the Paleozoic geologic era (see Geologic Timescale, table) extending from approximately 570 to 505 million years ago. It was named by the 19th-century English geologist Adam Sedgwick, who first studied the great sequence of rocks characteristic of the period near Cambria, Wales. During the Cambrian, the continents and seas differed from present day configurations. Four major continents, Gondwanaland, Angara, and the two sections of Euramerica, were inundated with a rising sea level, accumulating thick sedimentary deposits (see sediment). This sedimentary rock, i.e., conglomerate, sandstone, shale, and limestone, was formed in shallow seas that covered large areas of present-day North America, Europe, and Asia. In the United States, Lower Cambrian formations are found in the Appalachian; the sandstones near Waucoba Springs, S Calif.; and the thick layers of conglomerates and sandstones in Georgia, Tennessee, and North Carolina. Middle Cambrian rocks are found in New Brunswick, near Braintree, Mass. Upper Cambrian formations include the St. Croix sandstone of Wisconsin and the upper Mississippi valley, parts of the Arbuckle limestone of Oklahoma, and the Potsdam sandstone in New York's Adirondacks. In Russia, the Cambrian beds are remarkable in that they comprise mostly undisturbed and unconsolidated sand and clay despite their great age. The Cambrian rocks are the first rock layers to contain many easily recognizable fossils. The known Cambrian fauna—all marine—includes every phylum of invertebrates; the possibility that vertebrate fossils may be found cannot be excluded. The dominant animal was the trilobite, along with sea snails, brachiopods, sponges, and archaeocyathids. The ages of the various rock layers are distinguished according to the different genera of fossils they contain. The sudden appearance of highly developed and diversified fauna in Cambrian rock is best explained by the assumption that more primitive forms flourished during a missing stratigraphic interval between the close of the Precambrian and the beginning of the Cambrian. Remnants of these early organisms were either destroyed by erosion or their soft bodies easily decayed in a short period of time. In addition, at the beginning of the Cambrian, numerous animals eventually developed skeletons, or hard parts, capable of leaving behind fossil remains.