The historical roots of Niagara Falls lie in the Wisconsin glaciation， which ended some 10，000 years ago. Both the North American Great Lakes and the Niagara River are effects of this last continental ice sheet， an enormous glacier that crept across the area from eastern Canada. The glacier drove through the area like a giant bulldozerbulldozer n.推土机， grinding up rocks and soil， moving them around， and deepening some river channels to make lakes. It dammed others with debris， forcing these rivers to make new channels. It is thought that there is an old valley， buried by glacial drift， at the approximate location of the present Welland Canal.
After the ice melted back， drainage from the upper Great Lakes became the present day Niagara River， which could not follow the old filled valley， so it found the lowest outlet on the rearrangedrearrange vt.再排列， 重新整理 topography. In time the river cut a gorge across the Niagara escarpment， the north facing cliff or cuesta formed by erosion of the southwardly dipping (tilted) and resistant Lockport formation between Lake Erie and Lake Ontario. 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.
When the newly established river encountered the erosionresistant 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 underlays 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 crumblycrumbly adj.易碎的， 脆的 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 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 sandstonessandstone n.［地］ 沙岩. All three formations were laid down in an ancient sea， and their differences of character derive from changing conditions within that sea.
The original Niagara Falls were near the site of present day Lewiston， New York， 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 Horseshoe Falls to the west from American and Bridal Veil Falls to the east. Although erosion and recession have been slowed in this century by engineering， the falls will eventually recede far enough to drain most of Lake Erie， the bottom of which is higher than the bottom of the falls. Engineers are working to reduce the rate of erosionerosion n.腐蚀， 侵蚀 to retard this event as long as possible.
The Falls drop about 170 feet (52 m)， although the American Falls have a clear drop of only 70 feet (21 m) before reaching a jumble of fallen rocks which were deposited by a huge rock slide in 1954. The larger Canadian Falls are about 2，600 feet (792 m) wide， while the American Falls are 1，060 feet (323 m) wide. The volume of water approaching the Falls during peak flow season is 202，000 ft3/s (5，720 m3/s).During the summer months， when maximum diversion of water for hydroelectric power occurs， 100，000 ft3/s (2，832 m3/s) of water actually traverses the Falls， some 90% of which goes over Horseshoe Falls. This volume is further halved at night， when most of the diversion to hydroelectrichydroelectric adj.水力电气的 facilities occurs.
Seeing the Falls
Peak numbers of visitors occur in the summertime， when Niagara Falls are both a daytime and evening attraction. From the Canadian side， floodlights illuminate both sides of the Falls for several hours after dark.
From the American side， the American Falls can be viewed from walkways along Prospect Park， which also features an observation tower. Nearby， the Cave of the Winds trail leads hikers down some three hundred steps to a point beneath Bridal Veils Falls. The Niagara Scenic Trolley offers guided trips along the American Falls.
On the Canadian side， Queen Victoria Park features well manicured gardens， platforms offering a spectacular view of both American and Horseshoe Falls， and underground walkways leading into observation rooms which yield the illusion of being within the falling waters. The observation deck of nearby Skylon Tower offers the highest overhead view of the Falls， and in the opposite direction gives views as far as distant Toronto. With the Konica Minolta Tower， it is one of two towers in Canada with a view of the Falls. Along the Niagara River， the Niagara River Recreationalrecreational adj.休养的， 娱乐的Trail runs the 56km (35 miles) from Fort Erie to Fort George， and includes many historical sites from the War of 1812.
The Maid of the Mist cruises， named for an ancient Ongiara Indian mythical character， have carried passengers into the whirlpoolswhirlpool n.漩涡， 涡流， 混乱， 纷乱 beneath the Falls since 1846.The Spanish Aerocar， built in 1916 from a design by Spanish engineer Leonardo Torres y Quevedo， is a cable car which takes passengers over the whirlpool on the Canadian side， below the Falls.
