Presqu'ile -- French for "almost an island" -- is a boomerang-shaped spit of sand and limestone that juts sharply into Lake Ontario. The land formation is a tombolo, created as wind and waves piled sand and gravel from the lake bottom between a limestone island and the mainland. The action of the water and wind reinforces the peninsula with sand and causes the beach to grow by as much as 2 metres each year! Richly varied, the park's natural landscape features sand dunes, sand and pebbled beaches, limestone outcroppings, forests, reedy marshes and coastal wet meadows called pannes which are of international significance.

The park is a haven for migratory birds. About 126 species of birds nest here, and at least 321 species have been sighted enroute to destinations such as South America, the Arctic, Europe and Asia. Each autumn, the park is a gathering point for monarch butterflies as they begin their migratory trek to Mexico.