AZTEC RUIN, an American ruin of a prehistoric building of the pueblo type, situated in the valley of the San Juan river near the town of Aztec, New Mexico. It is a large structure covering 4.6ac. and containing about 5oo rooms. Of this building the first storey is standing, and in 24 of the rooms the original ceilings have been preserved. Many second-storey rooms remain and also parts of third-storey rooms. Where still intact, the ceilings are upheld by large wooden beams which were cut and dressed with stone tools. The sandstone walls, still reasonably perpendicular, have dressed faces. These ceilings and walls are interesting ex amples of carpentry and masonry done in the Stone age. The Aztec ruin is the most striking and best preserved of a large group of ruins that serve as an index to the ancient cultures that flourished in the San Juan valley. Excavations have brought to light objects indicating that portions of the Aztec ruin are of the same age as the ruins of Chaco Canyon (q.v.). In 1923 the American Museum of Natural History, which has conducted sys tematic explorations of the building, presented the plot of land containing it to the United States, and immediately thereafter it was established as a national monument. The town of Aztec is reached by rail and is also on the route of the national park-to park highway. See NATIONAL PARKS, AND NATIONAL MONUMENTS.