The San Juan River and its tributaries drain the region known as the Four Corners country—the area surrounding the point where New Mexico, Colorado, Utah, and Arizona meet in a common boundary at right angles. Rising high in the Rocky Mountains of Colorado, the San Juan flows southwestward to dip down into the northwestern corner of New Mexico; then it courses northwestward into Utah almost at the point of juncture of the four states. With many twists and curves, roaring through deep canyons and gulches, it proceeds generally westward to empty into the mighty Colorado River in the southeastern part of Utah. The San Juan Basin is the major drainage basin of the Four Corners country. As such, its lower reaches formed a formidable barrier to travel by migrant primitive groups and to early white settlers as well. Its upper portions, however, especially its tributaries, were easier of access and supplied that most important element of all for life in the desert: water—water for drinking, water for irrigation. The land between the tributaries is highly diversified; much is arid or semiarid with small streams running intermittently or with scattered springs that may be dry during parts of the year. Other areas are mountainous with swift-flowing streams. In places there are mesas, or large tablelands, which frequently are covered with forests of pine, juniper, and pinyon. It is a land of warm, often hot, summers and cool, sometimes very cold, winters; a land of sharp contrasts; a land that seems perpetual, yet never appears exactly the same on any two successive days. Into this area many hundreds of years ago, possibly even thousands, came small bands of wandering hunters. Gradually some of them learned how to adapt to the rigors of the land. Eventually two centers arose in which the local inhabitants successfully adjusted to their environment: one along the Chaco Wash in northwestern New Mexico, and the second in southwestern Colorado in many places on the La Plata, Mancos, and McElmo drainages. Chaco, the first of these cultural manifestations, takes its name from the best known and finest examples of such ruins in Chaco Canyon National Monument. The other is best known at, and named for, the area incorporated in Mesa Verde National Park. [Illustration: ] On the northern side of the San Juan, most of its tributaries are perennially flowing streams and rivers, with broad, fertile valleys and bountiful plant and animal resources nearby. On one of these streams, the Animas, there existed a series of prehistoric towns and villages which exemplify the successful blending of cultural influences from both the major centers of Chaco and Mesa Verde. This is the general story of the San Juan River area, of the people who lived there long ago, and in particular the story of the great ruined pueblo on the Animas River near the present-day town of Aztec, N. Mex. EARLY HUNTERS AND GATHERERS.