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Location of the Onondaga limestone outcrop in New York State, USA and Ontario, Canada.

The Onondaga Formation is a group of hard limestones and dolostones of Devonian age that form an important geographic feature in the area of outcrop.

In upstate New York and southern Ontario the sedimentary rocks tend to slope slightly to the southward, and the Onondaga outcrops in a line that usually forms an escarpment (the steep face of a cuesta), because of its resistance to erosion. The outcrop can be traced from the Hudson River valley westward along the southern rim of the Mohawk River valley, passing just south of Syracuse, and along the northern heads of the major Finger Lakes to Buffalo, New York. From Fort Erie, Ontario it runs to Windsor just north of the Lake Erie shoreline, becoming less prominent as one travels westward. It is not distinct west of Windsor.

In several spots it is breached by geologically young streams and spectacular waterfalls are formed, such as at Chittenango Falls just east of Syracuse, Buttermilk Falls at Le Roy, New York and Indian Falls west of Batavia.

A few other breaches occur in older valleys, which likely once had waterfalls, but erosion eventually obliterated them. Such breaches occur at the Tully valley, the Genesee River valley near Avon, New York, and at Port Colborne, Ontario, where the old valley forms a harbor on Lake Erie.

The formation is broken by the only major fault line in western New York, the Linden Fault just east of Batavia, where the eastern side of the fault has dropped down and the ledge moved southward relative to the western side. On the western side of the fault in Genesee County the escarpment achieves its greatest prominence. The New York State Thruway has a rock cut at Batavia which clearly shows the fault and is a popular point for geology class field trips. The fault, which runs from Attica, New York northward to Lake Ontario, is still active and periodically causes minor earthquakes in the area.

The Onondaga Formation also can be found in other areas where rocks of the same age outcrop, such as in western Pennsylvania and Michigan but they do not form prominent geographic 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.

• Photo of Chittenango Falls