Question

the deer that feeds on lichens grown on rocks

Answer 1

Caribou or reindeer (Rangifer tarandus), feed mainly on lichen during the winter months. Caribou are found in North America, where winters are very cold and snow is common.

 Lichen is made up of algae and fungi which grow symbiotically. The algae photosynthesise, providing carbohydrates to the fungus, while the fungus forms a framework for the algae to grow on.  

See the attached picture of a male Caribou by Dean Biggins (U.S. Fish and Wildlife Service) - US FWS, DIVISION OF PUBLIC AFFAIRS, WO3772-023, Public Domain, https://commons.wikimedia.org/w/index.php?curid=1214764