features. A similar and more prominent outcrop known as the Niagara Escarpment runs parallel and about 25 miles (40 kilometers) to the north through upstate New York, but curves northwestward in southern Ontario toward Lake Huron and eventually into Michigan's Upper Peninsula and Wisconsin's Door Peninsula. Another smaller outcrop known as the Portage Escarpment lies about to the south, running along the southern ends of the Finger Lakes and forming Cascadilla, Ithaca and Buttermilk Falls in Ithaca. The Onondaga Escarpment contains significant outcrops of flint (a type of chert) which bears the escarpment's name. This variety of chert was of great importance to First Nations peoples throughout Southern Ontario, who used it to make stone tools (lithics) such as projectile points and hide scrapers. This variety of chert, which is of reasonably high-quality and which was highly valued by First Nations peoples, is often a common variety of chert recovered archaeologically from sites relatively adjacent to outcrops; for example, Onondaga-variety chert comprises 95% of all of the flint material from some sites in Milton, Ontario. The material has also been found as well at some distance from its original source; Onondaga chert has been recovered at the late archaic Duck Lake archaeological site in northern Michigan, circa 400 kilometers from the nearest outcropping of the material. This wide distribution implies either a very large seasonal migration of ancient peoples or long-distance trade routes, with both likely being the case at different times throughout the prehistory of the Great Lakes region. Description The Onondaga Limestone is composed of four main subunits. In descending order: Seneca Member Moorehouse Member Nedrow Member Edgecliff Member The Seneca and Moorehouse members are sharply divided by the Tioga-B bentonite layer, which was formed in a very short time period as the result of a large volcanic eruption in what is now the state of Virginia. Relative age dating of the Onondaga places its formation in the Eifelian to Givetian stage of the Middle Devonian period, or 391.9 to 383.7 Ma. Radiometric dating of a sample from the bentonite at the top of the Onondaga placed it at 390 ± 0.5 Ma. The formation is time equivalent with the Floresta Formation of the Altiplano Cundiboyacense, Colombia. Onondaga limestone dimension stone Onondaga limestone was quarried as dimension stone for construction of limestone buildings. The following buildings contain structural Onondaga limestone: Brooklyn Bridge (cable anchorages and towers below the