Near Los Alamos National Laboratory, a few miles northwest of Santa Fe, Frijoles Creek cuts a sunlit canyon through one-million-year-old tuffs (compacted volcanic ash) of the Pajarito Plateau. In these walls of relatively soft stone -- material ejected from Jemez volcano, remnants of which form Valles Caldera -- wind and water have eroded shallow cavities. For about 400 years from late 1100's through late 1500's, people of the Rio Grande Pueblo culture occupied this site. Some dwellings are enlarged natural hollows in cliff walls; others comprise stone-walled structures at the cliff face and in the valley bottom. When Spanish explorers entered this area in the late 1500's, the Bandelier site was already abandoned.