The Thousand Islands region of the St. Lawrence River is known for its scenery, cottage country, and summer boat tours. Less well known is the geology beneath the water. The islands are actually the exposed tops of an ancient mountain range, part of a ridge of billion-year-old rock known as the Frontenac Arch. This ridge connects the Canadian Shield to the Adirondack Mountains, forming a natural corridor that has allowed wildlife to move between the two regions for millennia.

The rock itself is a pink, metamorphic stone that formed when ancient continents collided, creating a mountain range that may have rivaled the Himalayas. Over hundreds of millions of years, erosion wore those mountains down to a nearly level plain. Later, glacial ice scraped the region further, and meltwater shaped the St. Lawrence River. The harder rock of the old mountain core resisted erosion better than the surrounding softer rock, leaving behind the chain of islands visible today.

Geologists refer to this formation as a resistant bedrock sill. There are 1,864 islands in the area, defined as pieces of land that stay above water year-round and support at least one tree. This geological feature also explains the region’s biodiversity. The arch acts as a bridge where northern and southern forest types meet, making it one of the most biodiverse areas in Canada. Species such as the pitch pine, gray ratsnake, and Blanding’s turtle thrive here at the edges of their ranges.

The importance of this corridor was demonstrated in 2000 when a radio-collared moose named Alice walked from the Adirondacks, swam across the St. Lawrence, crossed Highway 401, and continued on to Algonquin Provincial Park. Her journey, covering roughly 570 kilometers, confirmed that the arch functioned as a natural wildlife route. Her path inspired the creation of the Algonquin to Adirondack Collaborative, a conservation organization working to protect this connection.

The corridor faces a specific challenge. Highway 401, one of the busiest roads in North America, crosses directly through the narrowest part of the arch. Research suggests that tens of thousands of animals are killed on roads in this region each year. The highway splits the habitat, limiting movement between the northern and southern sections of the ecosystem.

A proposed expansion of Highway 401 between Brockville and Mallorytown would widen the road from four lanes to six, and eventually eight. According to engineers, highway infrastructure of this type is designed to last about 75 years, meaning the decisions made now will shape the region for generations.

Conservationists have identified a potential solution: wildlife overpasses, which are bridges of forest-covered land built across highways. These crossings have been used successfully in Banff National Park, where bears, wolves, and elk regularly use them to cross the Trans-Canada Highway, reducing roadkill significantly. Similar structures are being proposed for the Frontenac Arch, with a short list of locations where they could be built.

The Frontenac Arch is a natural feature that connects two major wilderness areas and supports a wide range of species. The current decision about the highway expansion will determine whether that connection remains intact or is broken.