The Frontenac Arch

Outdoors in the Land O'Lakes by LSteve Blight

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A typical stream flowing over a rocky ridge on the Frontenac Arch

I often feel privileged to be able to spend time in the rich forests, fields, rocky barrens and wetlands of the Land O’ Lakes area. Much of this unique region owes its beauty to its geological heritage. It is situated at the north end of a unique geological feature known as the Frontenac Arch, also known as the Frontenac Axis, a thin arm of the Canadian Shield that links the enormous expanse of ancient bedrock of the north with that of the Adirondack Mountains in New York State. At the south end in the Thousand Islands area the Frontenac Arch is only about 80 km. wide. As one travels northwest, it widens out until it begins to lose its identity as a separate geological feature as it merges with the rest of the Canadian Shield somewhere between Westport and Highway 7.

The geological story of the region began more than one billion years ago along the south eastern margin of an ancient continent known as Laurentia. Laurentia then lay at about the present position of the equator and eventually became the core of the North American continent. From about 1.3 to one billion years ago, another continental mass moved north-westerly against the southeast margin of Laurentia, crushing it like a giant vice. As a result of this collision the ancient Precambrian rocks that formed the edge of Laurentia were deformed by heat, pressure and partial melting and then thrust up to form the north-easterly-trending Laurentian Mountain Belt.

Over the next billion years, this mountain belt wore down, leaving us with the landforms and features that we have today, including the Frontenac Arch. The bedrock is mostly composed of erosion resistant igneous and metamorphic rock. This rock is exposed at the surface or covered with a thin layer of sand and rocky debris called till, which was deposited when the glaciers retreated at the end of the last ice age. There is also some more easily erodible marble bedrock, and the arrangement of tilted and alternating layers of erosion-resistant and erodible rocks has created the ridge and valley topography of the area.

The rocks of the Frontenac Arch are among the youngest parts of the Canadian Shield – although one billion years old is anything but young! To the east and west of the Frontenac Arch, younger sedimentary rock bumps up against and overlays the older Precambrian bedrock. The dominant surface rock to the west is limestone, and to the east, a combination of sandstone and limestone. On the edges of the Frontenac Arch, the flat sedimentary rock is pierced in places by outcrops of Precambrian stone.

Because of its thin rocky soils and extensive wetlands, a good portion of the lands on the Frontenac Arch are still relatively untamed. The soils in this area generally make for poor agriculture, and many farms have been abandoned and have returned to forest cover. Combined with its location at the crossroads of northern and southern forest regions, this has resulted in this area being one of the most biologically rich and ecologically important areas in Ontario. It is known for its high diversity of reptiles and amphibians, including salamanders, frogs and toads, turtles, snakes and Ontario's only lizard, the Five-Lined Skink. Other rare species that can be found here include the Stinkpot Turtle, the Black Ratsnake, birds such as the Least Bittern and the Cerulean Warbler, and a fern species known as Blunt-lobed Woodsia. In addition, the Frontenac Arch serves as an important corridor connecting the wilder regions to the north with the Adirondack wilderness to the south, providing for the movement and dispersal of wildlife through the largely agricultural landscape of Eastern Ontario. And to think that all of this is because of a few rocks moving around over a billion years ago!

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Please feel free to report any observations to Lorraine Julien at outdoor1@look.ca  or
Steve Blight at natureobservations@rogers.com