page_title: Niagara Escarpment
section_title: Formation
hierarchical_section_title: Niagara Escarpment / Formation
caption_attribution_description: English: An image of the en:Niagara Escarpment from en:Highway 55 (Wisconsin) just south of County Highway F, approximately 5 miles south of en:Stockbridge, Wisconsin. The camera was roughly held level. This image was taken on September 26, 2006 by User:Royalbroil.
context_page_description: The Niagara Escarpment is a long escarpment, or cuesta, in the United States and Canada that runs predominantly east/west from New York, through Ontario, Michigan, Wisconsin, and Illinois. The escarpment is most famous as the cliff over which the Niagara River plunges at Niagara Falls, for which it is named.
  The Escarpment is a UNESCO World Biosphere Reserve. It has the oldest forest ecosystem and trees in eastern North America.
  The Escarpment is composed of the Lockport Formation of Silurian age, and is similar to the Onondaga Formation, which runs parallel to it and just to the south, through western New York and southern Ontario. The Escarpment is the most prominent of several escarpments formed in the bedrock of the Great Lakes Basin. From its easternmost point near Watertown, New York, the escarpment shapes in part the individual basins and landforms of Lakes Ontario, Huron, and Michigan.
  In Rochester, New York, three waterfalls over the escarpment are where the Genesee River flows through the city. The escarpment thence runs westward to the Niagara River, forming a deep gorge north of Niagara Falls, which itself cascades over the escarpment.
context_section_description: Study of rock exposures and drillholes demonstrates that no displacement of the rock layers occurs at the escarpment: this is not a fault line but the result of unequal erosion. The escarpment's caprock is dolomitic limestone, also known as ("dolostone"), which is more resistant and overlies weaker, more easily eroded shale as a weathering-resistant "cap". The escarpment thus formed over millions of years through a process of differential erosion of rocks of different hardnesses. Through time the soft rocks weather away or erode by the action of streams. The gradual removal of the soft rocks undercuts the resistant caprock, leaving a cliff or escarpment. The erosional process is most readily seen at Niagara Falls, where the river has quickened the process. It can also be seen at the three waterfalls of the Genesee River at Rochester (additional resistant rock layers make more than one escarpment in some places). Also, in some places thick glacial deposits, such as the Oak Ridges Moraine, conceal the Niagara Escarpment, such as north of Georgetown, Ontario, where it actually continues under glacial till and reappears farther north.
  The dolomite cap was laid down as sediment on the floor of a marine environment. In Michigan, behind (southwest of) the escarpment, the cuesta capstone slopes gently to form a wide basin, the floor of an Ordovician-Silurian-age tropical sea. (The escarpment is essentially the remnant shoreline of that sea.) There the constant deposition of minute shells and fragments of biologically-generated calcium carbonate, mixed with sediment washed in by erosion of the virtually lifeless landmasses, eventually formed a limestone layer. During the Silurian period, some magnesium substituted for some of the calcium in the carbonates, slowly forming harder dolomite layers in the same fashion. Worldwide sea levels were at their all-time maximum in the Ordovician; as the sea retreated, erosion inevitably began.
  This dolomite basin contains Lakes Michigan, Huron, and Erie. (The geological formations under Lakes Superior and Ontario were formed from volcanic rifts. Soft basalt lines their canyon walls, bottoming at 1300 feet below sea level.)
caption: Calumet County, Wisconsin
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