Onondaga Limestone is a hard Middle Devonian belt of limestone and dolomite that forms a prominent escarpment across parts of upstate New York and southern Ontario because it resists erosion. Its outcrop extends from the Hudson and Mohawk valleys west toward Buffalo, then from Fort Erie inland and north toward Lake Huron, with related rocks continuing into Michigan. Streams cutting through it have created notable waterfalls, and the escarpment is also broken by older valleys and the Linden Fault, which still causes minor earthquakes. Its chert was widely used by First Nations peoples for tools, and the stone was later quarried for major structures including the Brooklyn Bridge.