Soils and Bedrock
The predominantly clay soils of the Niagara River corridor are a legacy of the last Ice Age.
18,000 years ago, southern Ontario was covered by ice sheets 2-3 kilometers thick. As they bulldozed their way southward shearing off layers of rock, the ice sheets likely gouged and enlarged a pre-existing river system, creating the basins of the Great Lakes. Then as they melted northward for the last time they released vast quantities of meltwater into these basins. The Niagara Peninsula became free of ice about 12,500 years ago, but not of the glacial meltwaters. Clay soils were laid down in the huge lakes while sand was deposited at meltwater river outlets.
The Niagara River was born as part of the outlet for the glacial meltwaters, and Niagara Falls began as a spillway for the meltwaters at the escarpment at Queenston Heights. From here the falls eroded eleven km (seven mi) upstream to its present location. But the recession was not steady. Several geological events helped to set the stage for the highly specialized plant communities found along the Niagara River today.
From about 12,500 to 10,500 years ago the falls eroded upriver to the present site of the Niagara Glen. Here, through an interplay of geological effects, this process was interrupted and a much-reduced falls stalled in the area. Conditions remained like this for about 5,000 years.
Today Rim Tours are conducted at the top of the Niagara Glen along Wintergreen Flat, a 15-meter (50-foot) thick layer of hard limestone formed from coral sediments of tropical seas approximately 415 million years ago.
Approximately 12,500 years ago Wintergreen Flat was the bottom of the Niagara River and the falls cascaded over its north end. The Niagara Glen itself is a nature area - a world of giant boulders abandoned by the much-reduced Niagara River. It is a glimpse of what lies beneath the Falls today, and of what lies ahead for the American Falls. The American Falls carry only 7% of the river’s volume. As the Falls continue to erode upriver, the Canadian Falls will eventually carry all of the river’s flow, leaving the American Falls to turn into another area like the Glen.
About 5,500 years ago the vast glacial meltwaters were once again routed through southern Ontario, re-establishing the Falls. About 4,500 years ago the Falls breached and took over a more ancient river bed in the area of today’s Whirlpool.
In just a few days, the Falls, probably as a huge churning rapids, tore out the glacial debris that filled the older gorge, re-establishing as a Falls only when it reached the area of the Whirlpool Rapids Bridge. From here the Falls resumed its steady recession through bedrock to its present location.
As a young, freshwater system born of ice, the Niagara River is characterized as a post-glacial incised valley rather than as a river with a typical V-shaped valley. But there’s more than Ice Age history here. Exposed in the Niagara River Gorge are rock layers laid down as sediments in tropical, saltwater seas approximately 440 to 410 million years ago. In these strata, infused under the sun with buff, rust, lavender and green, are the epic stories of plate tectonics, mountain-building, and fossils of marine creatures once alive, now gone forever.
It is these sedimentary rocks of limestone, sandstone and shales that underlie the predominantly clay soils of the Niagara River corridor.
