The Thousand Islands Parkway is a scenic route along the St. Lawrence River between Gananoque and Brockville through the rugged terrain of the Frontenac Arch, a protrusion of the Canadian Shield southward into New York state. In this area, the soil is underlain by layers of Paleozoic limestone and a granite bedrock. The granite often extends above the ground surface as large rock outcroppings. The Thousand Islands Parkway was part of the original alignment of Highway401. However, because of the residential properties and the scenic nature of the route, a new inland route was constructed through the mid-1960s. A recreational trail follows the right-of-way of the westbound carriageway, which was never completed.