The summit of Càrn Mairg is steep sided, partly craggy and is made up of greyish quartzite stones. South west of the summit are steep slopes which fall to the head of Coire Chearcaill which is drained by the Allt Coire a' Chearcaill, a tributary of the Invervar Burn and contains an old stalkers bothy on its lower north bank. The northern slopes fall to the glen containing Loch Rannoch and Loch Tummel while to the north east is Gleann Mòr which separates the mountain from Schiehallion which is four km distant. All drainage from the mountain feeds the River Tay either going north and then via the River Tummel or south and via the River Lyon.