HELDERBERGIAN STAGE

lowermost division of Lower Devonian rocks and Early Devonian time in North America (the Devonian Period began about 408 million years ago and lasted about 48 million years). It precedes the Deerparkian Stage and was named for exposures studied in the Helderberg Mountains in Albany County, N.Y. During Helderbergian time, the Appalachian Geosyncline (great downbuckle in the Earth's crust in the region of the present Appalachian Mountains) slowly subsided but did not receive much sediment from bordering lowlands. This resulted in a compact record, only about 90 metres (300 feet) thick, and complex facies relationships predominate. Helderbergian rocks consist primarily of limestones and calcareous shales and have long been studied in the northeast. The uppermost formation of the Helderbergian is the Port Ewen Limestone, a shaly limestone, which is followed, in descending order, by the Alsen Limestone Formation, a cherty limestone with a crinoid and brachiopod fauna; the Becraft Limestone, a compact, pinkish limestone rich in crinoid columnals and brachiopod-shell fragments; the New Scotland Limestone, a shaly limestone with a rich and well-preserved fauna of brachiopods, trilobites, and gastropods; the Kalkberg Limestone Formation, noted for its well-preserved silicified fossils and black chert nodules; the Coeymans Limestone Formation, a hard, massive, bluish-gray limestone with excellently preserved silicified fossils; and the Manlius Limestone Formation, formerly included in the Silurian Period but now recognized as lowermost Helderbergian, which contains the enigmatic fossil Tentaculites.