McNabs comprises nine large drumlins overlaid on slate and granite bedrock. Drumlins are glacially formed deposits of rocks and earth, and are generally well drained and suited to farming.

The island was originally covered in forest, which the founders of Halifax were quick to exploit. Over the years, most of the island was cultivated and the wood lots harvested.

Today, the island’s forests are of various ages. Older forests date to the 1800s and comprise red maple, beech and red spruce with an understory of hayscented fern. Old abandoned fields have become colonized by white spruce.

In the 1880s, Frederick Perrin, who was a keen gardener, introduced several hundred plant species to his Victorian estate on the island (near the site of the former teahouse). Many of the original trees and shrubs are still standing.

The island’s shoreline varies from cobbled stone to fine sand, with salt marshes in a few sheltered coves. McNabs Cove became McNabs Pond with the construction of the causeway to the Maugers Beach Lighthouse. The causeway also caused the formation of the dune system on Maugers Beach.

Deer, rabbits, coyotes and other animals inhabit the island. The island is known to birders, who have documented 206 species of birds on the McNabs Island.