Glass Mountain in Lower Cathedral Valley of Capitol Reef National Park is an unusual geologic formation: a low mound of large selenite crystals, the transparent form of gypsum, rising about 15 feet above the surrounding ground. It began about 165 million years ago, when evaporating seawater left gypsum deposits that were later buried by sediment. Over time, the gypsum pushed upward through fractures; elsewhere this usually formed layers, but here it created a rare dome-like plug. It is the only exposed selenite plug in the park, and because another nearby example was collected away to ground level, it remains an especially important survivor.