| Niagara Escarpment ： ウィキペディア英語版|
The Niagara Escarpment is a long escarpment, or cuesta, in the United States and Canada that runs predominantly east/west from New York State, through Ontario, Michigan, Wisconsin and Illinois. It is composed of the Lockport geological formation of Silurian age, and is similar to the Onondaga geological formation, which runs parallel to it and just to the south, through western New York and southern Ontario. The escarpment is most famous as the cliff over which the Niagara River plunges at Niagara Falls, for which it is named.
The Niagara 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,〔(Lake Ontario Waterkeeper ). Waterkeeper.ca (2010-12-03). Retrieved on 2013-07-26.〕 the escarpment shapes in part the individual basins and landforms of Lakes Ontario, Huron and Michigan. In Rochester, New York, there are three waterfalls over the escarpment 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. In southern Ontario it spans the Niagara Peninsula, closely following the Lake Ontario shore through the cities of St. Catharines, Hamilton and Dundas, where it takes a sharp turn north in the town of Milton toward Georgian Bay. It then follows the Georgian Bay shore northwestwards to form the spine of the Bruce Peninsula and Manitoulin Island, as well as several smaller islands located in northern Lake Huron where it turns westwards into the Upper Peninsula of northern Michigan, south of Sault Ste. Marie. It then extends southwards into Wisconsin〔John Luczaj, ("Geology of the Niagara Escarpment in Wisconsin" )〕 following the Door Peninsula through the Bayshore Blufflands and then more inland from the western coast of Lake Michigan and Milwaukee, ending northwest of Chicago near the Wisconsin-Illinois border.
Study of rock exposures and drillholes demonstrates that there is no displacement of the rock layers at the escarpment: this is not a fault line but the result of unequal erosion. The escarpment's caprock is dolomitic limestone ("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 conceal the Niagara Escarpment, such as north of Georgetown, Ontario, where it actually continues under glacial till and reappears farther north.
The dolostone 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 essentially represents 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 (dolomitic) sedimentary strata in the same fashion. Worldwide sea levels were at their all-time maximum in the Ordovician; as the sea retreated, erosion inevitably began.〔(The Niagara Escarpment )〕
This dolostone basin contains Lakes Michigan, Huron and Erie. (The geological formations under Lakes Superior and Ontario were formed from volcanic rifts. Soft basalt line its canyon walls, bottoming at 1300 feet below sea level.)
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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