Everything about Onondaga Geological Formation totally explained
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 isn't distinct west of Windsor, but begins to become noticible as a steep hill just northwest of
Leamington, as it forms a low ridge/escarpment along much of the
Lake Erie shoreline.
In several spots it's 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 don't 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.
Another smaller outcrop known as the
Portage Escarpment lies about 35 miles 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 a type of
chert (flint) 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 hyde scrapers. This variety of chert, which is of reasonably high-quality and which was highly-valued by First Nations peoples, is often the most common variety of chert recovered archaeologically, even from sites at a relatively great distance from the outcrops; for example, Onondaga-variety chert comprises 95% of all of the chert from some sites in
Milton, Ontario. 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 (Eley and von Bitter 1989; personal field research).
Age
Relative age dating of the Onondoga places its formation in the Eifelian to Givetian
stage of the
Middle Devonian period, or or 391.9 to 383.7
Ma. Radiometric dating of a sample from the bentonite at the top of the Onondoga placed it at 390 ± 0.5 Ma.
Further Information
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