The Western Isles were affected by glaciaton, which had a strong influence on the geomorphology. The Atlantic Ocean has a direct influence on the coastline, causing erosion and deposition in various locations, and controls the climate of the Western Isles. Dunes require a ready supply of sand in order to form, which for the machair comes predominantly from broken shells that are washed up on the coast.

The management of the machair also has a strong influence on the physical landscape by producing the mosaic of habitats.