The Geology of the Niagara Falls

The Geology of the Niagara Falls

The history of Niagara Falls started 600 million years ago when what was to become the site of the Great Lakes was in the middle of a wide and shallow sea that covered most of North America. Under this sea was the Canadian Shield, the ancient bowl-shaped rock bed that is the continent’s foundation. Anyone going on a Niagara Falls tour will be interested to know what happened to create this great natural wonder.

The next 100 million years saw wind, rain and tides grinding these rocks into powder and this dust collected in layers over the sea bottom.

An earthquake formed the Appalachian Mountains, causing rivers to flow in new patterns, taking mud westwards, forming huge deltas from what is now Lake Ontario to the far shore of Lake Huron. Over time, this mud compacted to make the purple shale that comprises the Queenston Formation and the sandy ledges of Niagara Gorge.

The seas in this area were once tropical, with lots of coral reefs; when these reefs died they were ground into lime dust and settled on the sea floor, forming the Lockport Dolomite, the caprock of the Niagara Escarpments.

This sea was all but gone by 300 million years ago, leaving a three-mile-thick layer of sediment on top of the Canadian Shield. By 250 million years ago, huge rivers were etching paths in this sediment and eroding basins for Lakes Michigan, Huron, Erie and Ontario.

North America continued to drift ever northwards, bringing it under the chilly auspices of the northern climate and its four ice ages. To find out more about the ice ages, click here. The last ice age saw most of Canada under a mile-thick sheet of ice. This ice sheet slowly travelled, shifting anything moveable ahead of it and crushing anything that wouldn’t move. It eventually settled, spanning east and west from what are now the Niagara Falls.

Around 12,000 years ago the ice receded enough to reveal a low channel to the Hudson River and water from the western end of the Ontario basin started to travel through it.

The water levels fell still further, exposing the Niagara Escarpment. Lake Erie flowed over the top of the escarpment into a lower basin – Lake Iroquois, now Lake Ontario. This channel was the forerunner of the Niagara River.

The ice continued to recede as the earth’s temperature slowly rose and the leading edge of the glacier began to melt, releasing immense amounts of water and the rocks it had picked up on its journey. Finally the ice left the Thousand Islands and Lake Iroquois surged through its new channel into the Atlantic.

The glacial water flowed off the continent, mostly into the seas, while some evaporated and some stayed in the hollows left in the ground by the glacier. Once the ice and water had gone, the continent, free of the weight, tipped back. It tipped unevenly, however, towards the south, changing drainage patterns and starting the flow of what is now the Niagara River.