Iron Mountain is a mountain in the San Gabriel Mountains of Los Angeles County, California. Sometimes referenced as Iron Mountain #1 or Big Iron, the 8,007 ft (2,441 m) peak was originally called Sheep Mountain by the early miners in San Gabriel Canyon because of the large bands of bighorn sheep that roamed the slopes. The United States Geological Survey, which mapped the San Gabriels in the 1890s, ignored the local designation and gave it its present name. According to Will Thrall, there was once a USFS plan to change the name back to Sheep Mountain in 1940. The mountain is at the heart of a federally designated wilderness area, called the "Sheep Mountain Wilderness" in reference to the original name.
While this mountain is far lower in elevation than other Southern California summits, it is the most difficult mountain to claim in the entire region. There are no water sources on the mountain, unless one finds snow. There are no maintained trails, the summit is seldom visited, and the climb is of about 6000 vertical feet, much of it at a 30 to 50 degree angle on loose soil, decomposed rock, or through brush. Some approaches to the summit require high degrees of rock-climbing skill. One route over the ridgeline from nearby Mt. San Antonio is long, difficult, and dangerous. The south mountain slopes in summer are directly exposed to the sun. Some climbers stay at the ruins of the Allison Mine about 3900 vertical feet below the summit. There are no facilities of any kind on the mountain.