His letters, essays, and books of his adventures in nature, especially in the Sierra Nevada mountains of California, have been read by millions of people. Among his many notable journeys throughout the wilderness, Muir traveled from the Upper Mississippi Basin to the Gulf of Mexico and kept a journal of his travels called "'A Thousand-Mile Walk"'. Muir felt that the national parks and forests needed to be preserved and some resources which should be off-limits to industrial use. His activism helped to preserve the Yosemite Valley, Sequoia National Park and many other wilderness areas throughout the United States. The Sierra Club, which he co-founded, is now one of the most important conservation organizations in the United States.