Wallace Stegner was a writer, historian, and most importantly (in my opinion) an environmentalist. In 1960 he wrote the Wilderness Letter to the Outdoor Recreation Resources Review Commission. This essay later helped win the passage of the Wilderness Act of 1964. The full text of the letter can be found here. An introduction to the letter, written by Stegner in 1980, can be found here.
Wallace Stegner packing his horse, Vermont 1938
Wallace Stegner with Ansel Adams