NIAGARAN SERIES

second of the three major divisions of Silurian rocks and time (438 to 408 million years ago) in North America. It was named for exposures studied in Niagara county, N.Y. The characteristic exposures for the series are found in the gorge of the Niagara River below Niagara Falls; the gorge is rimmed by a resistant dolomite formation, the Lockport Dolomite, below which fossiliferous shales and thin limestones comprise the remainder of Niagaran rocks in the area. West of the Cincinnati Arch, Niagaran strata are largely calcareous, very widespread, and often attain a considerable thickness. In the Niagaran dolomites of Indiana, Illinois, and Wisconsin, fossil coral reefs are frequently encountered, some of which are about 10 km (6 miles) in diameter and attain a thickness of about 300 m (1,000 feet).