Formation
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 (i.e., 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.
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.)
Read more about this topic: Niagara Escarpment
Famous quotes containing the word formation:
“... the mass migrations now habitual in our nation are disastrous to the family and to the formation of individual character. It is impossible to create a stable society if something like a third of our people are constantly moving about. We cannot grow fine human beings, any more than we can grow fine trees, if they are constantly torn up by the roots and transplanted ...”
—Agnes E. Meyer (1887–1970)
“That for which Paul lived and died so gloriously; that for which Jesus gave himself to be crucified; the end that animated the thousand martyrs and heroes who have followed his steps, was to redeem us from a formal religion, and teach us to seek our well-being in the formation of the soul.”
—Ralph Waldo Emerson (1803–1882)
“The moral virtues, then, are produced in us neither by nature nor against nature. Nature, indeed, prepares in us the ground for their reception, but their complete formation is the product of habit.”
—Aristotle (384–322 B.C.)