 Rùm is mainly igneous in origin , and its mountains have been eroded by Pleistocene glaciation . It is now an important study site for research in ecology , especially of red deer , and is the site of a successful reintroduction programme for the white @-@ tailed sea eagle . Its economy is entirely dependent on Scottish Natural Heritage , a public body that now manages the island , and there have been calls for a greater diversity of housing provision . A Caledonian MacBrayne ferry links the island with the mainland town of Mallaig . 
