The park is named after pioneer Calvert Simson, whose descendants gifted the land to the province in 1983. Old logging roads, rock walls and the surviving orchard are reminders of the homestead that once proudly stood on the island.

This 461-hectare park features rolling, rocky hills and forested uplands backed by the impressive Spyglass Hill in the northeast of the island. Visitors can explore the rocky shoreline, pebble beaches and numerous bays.

The park is home to a diverse array of birdlife. Visitors should view the bird colonies with care and common sense, particularly during the nesting season.

Recreational activities include hiking the old logging trails, fishing and kayaking. Small boats, dinghies and kayaks may land at several coves on the eastern shoreline of South Thormanby Island. There are three wilderness/walk-in campsites, but no facilities are provided. The park is open year round when accessible.

Anchoring space for large cruising boats is limited, with rather poor protection in all but settled conditions. Access is from Welcome Passage, northwest of the Merry Island lightstation.