In
doing so it exposed old marine rocks that
are much older than the geologically recent
glaciation. Three major formations are
exposed in the gorge that was cut by the
Niagara River. The Horseshoe Falls, one
of the three Niagara Falls.When the newly
established river encountered the erosion-resistant
Lockport dolostone, the hard layer eroded
much more slowly than the underlying softer
rocks. The aerial photo clearly shows
the hard caprock, the Lockport Formation
(Middle Silurian), which underlies the
rapids above the falls and approximately
the upper third of the gorge wall. It
is composed of very dense, hard and very
strong limestone and dolostone. Immediately
below, comprising about two thirds of
the cliff is the weaker, softer and more
crumbly and sloping Rochester Formation
(Lower Silurian). It is mainly shale,
though it has some thin limestone layers,
and contains large quantities of fossils.
Because
it erodes more easily, the river has undercut
the hard cap rock and created the falls.
Submerged in the river in the lower valley,
hidden from view, is the Queenston Formation
(Upper Ordovician), which is composed
of shales and fine sandstones. All three
formations were laid down in an ancient
sea, and their differences of character
derive from changing conditions within
that sea. Niagara Falls from space, April
2001The original Niagara Falls were near
the sites of present-day Lewiston, New
York, and Queenston, Ontario, but erosion
of their crest has caused the waterfalls
to retreat several miles southward. Just
upstream from the Falls' current location,
Goat Island splits the course of the Niagara
River, resulting in the separation of
the Horseshoe Falls to the west from the
American and Bridal Veil Falls to the
east.
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