Formation of the Falls

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 bulldozer, 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 rearranged 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.

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.

The 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. 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 erosion 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 massive 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).1,2 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 the Horseshoe Falls. This volume is further halved at night, when most of the diversion to hydroelectric facilities occurs.