To many, the Canadian Shield is the quintessential Canadian landscape - the rugged North, and a land of forests and lakes. But many haven't realized that a portion of the Shield extends southward though Ontario, and into the U.S. The Frontenac Arch, as this hourglass-shaped outlier of the Canadian Shield is known, is the ancient backbone of North America. In Mohawk tradition, this massive landform is The Bones of the Mother.
Even to the most casual observer, the Frontenac Arch is an entirely different landscape than the remainder of southern Ontario and upper New York State. To someone driving across the land, an otherwise flat and plain countryside is suddenly interrupted by rolling hills and rugged cliffs, topped with windswept pines, and valleyed with sinewy creeks and tranquil lakes and wetlands. But about a billion years ago, this was a far more dramatic landscape. The Frontenac Arch, and the Canadian Shield, was a massive range of towering mountains. It took hundreds of millions of years for these mountains to weather down to their roots - and those basement rocks of the mountains are what we experience in this landscape today.
It happens that the lowest elevation along the Frontenac Arch lies at the east end of Lake Ontario. In fact Lake Ontario owes its existence to the Arch, which nearly impounded the lake. Through the last ice ages, glaciers bulldozed the basins for the Great Lakes and as each of the lakes filled to their brims, they overflowed on their race to the sea. A young Lake Ontario first flowed out the Oswego River. As the land rose further, released from the enormity of glacial ice, the lake tipped gently towards the east. The lake's waters finally rose up the flanks of the Frontenac Arch, far enough to spill between the granite hilltops. A thousand hilltops, actually the roots of the ancient mountains, became the Thousand Islands in this flooded landscape.
And so, the greatest natural intersection on the continent came into being at the end of the last ice age: the intersection of the Frontenac Arch and the St. Lawrence River Valley. The Arch connects the Canadian Shield and the boreal forest to the forests of the Adirondack and Appalachian Mountains - a south to north/north to south migration route. The river valley is a route from the Great Lakes forest heartland of the continent to the forests of the Atlantic Coast. The Thousand Islands are at the very centre of that intersection. Here, the five great forest regions of the eastern continent meet and intermingle, with many species at range limits, and with many as remnant populations from forests altered by millennia of climate change and evolving landscapes. This region has, arguably, Canada's greatest diversity of plant and animal communities.