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Abstract The area discussed here is in San Juan County, New Mexico, approximately 30 mi (48 km) south of Farmington and 20 mi(32 km) northwest of Chaco Canyon National Monument (Fig. 1). The small settlement of Bisti is most conveniently reached by traveling south on New Mexico 371 from Farmington or north on New Mexico 57 and New Mexico 371 from Thoreau, which is about 73 mi (117 km) east of Gallup on 1-40. Both of these routesare paved to within a few miles of Bisti. From New Mexico 44 atHuerfano an improved dirt road (County Road 15) runs southwest about 25 mi (40 km) to intersect New Mexico 371 about 9 mi (14 km) south of Bisti. All of the stops indicated on Figure 1 are accessible by two-wheel drive vehicles, but many of the side roads into the badlands and arroyo areas require four-wheel drive. Viewing of areas off the main roads is best done on foot. The nearest gasoline, food, and water are in Farmington and several stores along New Mexico 44. Most of the area under consideration is within recently established federal wilderness areas, managed by the U.S. Bureau of Land Management (BLM). A few sections are owned by the State of New Mexico or the Navajo Nation, but no part of the area is within the Navajo Reservation. Permission is not required to examine exposures on federal or state land (a permit from the BLMis required to collect fossils), but permission should be obtained from the few

Abstract 1–40 follows Tijeras Canyon through the Precambrian coreand overlying Pennsylvanian-Lower Permian sequence of the south end of the Sandia Mountains, beginning immediately eastof the Albuquerque city limits. Geologic features of this area are well displayed in road cuts and exposures near 1–40 and are summarized in this text for an 8.6-mi (13.8-km) interval of 1–40, beginning at the eastbound Carnuel (locally pronounced “carnway”) exit (Fig. 1). This section of 1–40 was constructed with wide shoulders, and there are numerous places where, with reasonable care, it is safe to stop along the highway to examine road cuts. Site descriptions, below, progress from west to east(from Precambrian to Permian), but the traveler should be aware that most of the road cuts referred to are on the north side of the interstate highway, along the west-bound lane. To examine them closely, if proceeding eastward, it is necessary to carefully cross the interstate or to drive to a nearby exit and backtrack. Short hikes off the highway, particularly in Precambrian terranes, can be most informative, and old U.S. 66, which parallels 1–40 between Carnuel and Tijeras, provides the safest access for a “hands-on” look at the Precambrian geology. Recent geologic maps of the area considered here were published by Kelley and Northrop (1975); Myers and McKay (1976) and Connolly (1982).