Within Four Corners of the United States, which unit of the National Park Service has ruins of the indigenous tribes and is within the boundaries of the Navajo Nation?
Four Corners:  In addition to the monument, commonly visited areas within Four Corners include Monument Valley, Mesa Verde National Park, Chaco Canyon, and Canyon de Chelly National Monument. Canyon de Chelly National Monument:  Reflecting one of the longest continuously inhabited landscapes of North America, it preserves ruins of the indigenous tribes that lived in the area, from the Ancestral Puebloans (formerly known as Anasazi) to the Navajo.
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