This document lists 4 sources cited in a research paper: a printed book by Russlel D. Butcher from 1997 about national parks in the UK and Ireland, a webpage from the National Park Service website from 2009 about frequently asked questions of the Grand Canyon, an entry from the Grolier Online encyclopedia from 2010 about Grand Canyon National Park, and a printed book from 1998 by Greer L. Prince about the stories and histories behind national park landscapes.