Petroglyph National Monument preserves a sacred landscape of vesicular basalt formed by volcanic flows from fissures marked by five volcanic spatter cones along Albuquerque’s western horizon, with the last eruption occurring approximately 150,000 years ago. The boulders on which the petroglyphs were made were originally light gray, but over time developed a dark surface coating called desert varnish; when this surface was pecked or abraded away, the lighter stone beneath created two-tone images. Petroglyphs are images carved, pecked, or incised directly into rock with stone tools, unlike pictographs, which are painted, and archaeological surveys have found more than 21,000 such images along 17 miles of escarpment within the monument. An estimated 90 percent were created by the ancestors of today’s Pueblo people, whose communities have lived in the Rio Grande valley since before 500 A.D., with many new settlements appearing after a late 13th-century drought; most of the petroglyphs are believed to date from 1300 to 1700 A.D., though some may reach back as far as 3,000 years ago and others were made in historic times by early Spanish settlers beginning in the 1700s. The Spanish word pueblo literally means village or city, and after Spanish explorers traveled up the Rio Grande in 1540 A.D. they applied the term to the Native American farmers they encountered and to their above-ground dwellings; today 19 Pueblos remain in New Mexico, each with its own culture and traditions. Many Pueblo people prefer the term Ancestral Puebloan rather than Anasazi for their ancestors. The petroglyphs were made for many reasons that are not fully understood outside Native American culture, and they are not simply rock art, hieroglyphics, or graffiti but powerful cultural symbols reflecting complex societies and religions. Their meanings depend on the context of each image, including its orientation to the horizon, nearby images, and the surrounding landscape, and today’s Pueblo people say their placement was not casual or random. Some images carry meanings known only to their makers, while others represent tribal, clan, kiva, or societal symbols, religious entities, or records of who came to the area and where they went; some retain contemporary meaning, and others are still respected as the legacy of earlier people.