Slieve Gallion is a volcanic plug. The mountain is formed from volcanic-derived igneous rock, and also granite on the southern slopes. It is steep, and has a flat top. There is a telecommunications mast on the northeastern summit. To the west, the landscape consists of blanket bog and coniferous forest; most of the trees in the Slieve Gallion area are Sitka spruce. There are also lodgepole pine trees, as well as oak towards the south of the area. Animal species living in the area include the Irish hare (a sub-species of the mountain hare), the curlew, and the red grouse. The otter can also be found around nearby rivers. Slieve Gallion is classed by the Northern Ireland Planning Service as an area of High Scenic Value.