The Albuquerque 30' x 60' quadrangle spans the Rio Grande rift between the Colorado Plateau and Great Plains geologic provinces, and includes parts of the Basin and Range and Southern Rocky Mountain physiographic provinces. Geologic units exposed in the quadrangle range in age from Early Proterozoic schist and granite to modern river alluvium. The principal geologic features of the area, however, chiefly reflect contractional folding and thrusting of the Late Cretaceous Laramide orogeny and the Neogene extension of the Rio Grande rift. Significant parts of the history of the rift in this region are displayed and documented by the geology exposed in the Albuquerque quadrangle. Post-Laramide erosion, beginning at about 60 Ma, is recorded by the Diamond Tail and Galisteo Formations (upper Paleocene and Eocene) that are preserved in the Hagan Basin and around the uplifted margins of the younger Rio Grande rift. Intermediate volcaniclastic deposits of the Espinaso Formation (upper Eocene and Oligocene) were shed in and around the contemporaneous volcanic-intrusive complexes of the Ortiz porphyry belt in the northeastern part of the quadrangle. The earliest fluvial sediments attributed to extension in the Rio Grande rift in this area are the Tanos and Blackshare Formations (upper Oligocene and Miocene) in the Hagan Basin, which indicate extension was underway by 25 Ma. Farther west, the oldest rift-filling sediments are eolian sand and interdune silty deposits of the Zia Formation (lower to middle Miocene). Major extension occurred during the Miocene, but subsidence and sedimentation were highly irregular from place to place. Parts of three rift sub-basins are known within the Albuquerque quadrangle, each basin locally as deep as about 14,000 ft, separated by less-extended zones (structural horsts) where the rift fill is much thinner. The geometry of these early, deep rift sub-basins suggests the primary extension direction was oriented northeast-southwest. Significant local folding and uplift within the complex rift seems to have occurred in the late Miocene, accompanied by erosion and recycling of earlier rift-fill sediments. This deformation may reflect clockwise reorientation of the primary extension direction to its Pliocene and current east-west alignment. Late Miocene and early Pliocene uplift and erosion were widespread in the region, as indicated by channeled and local angular unconformities at the bases of all Pliocene units, especially prominent along basin margins. These Pliocene fluvial and alluvial deposits (Ceja and Ancha Formations and Tuerto Gravel) and the upper part of the Cochiti Formation are all conspicuously coarser grained than the Miocene beds they cover, particularly near source areas along the margins of the rift. These observations together indicate that the regional streams flowed at much greater discharge than the Miocene streams and that the Pliocene onset of cooler, wetter climate worldwide was the most likely cause. Despite these higher discharge conditions, it appears there was no Pliocene trunk stream through the rift valley because the youngest Pliocene beds in the basin center are largely fine grained sand, pebbly sand, and sandy silt. No Pliocene cobble-gravel deposits, or thick crossbed sets indicative of major stream discharge, have been documented in the basin center. Considerable evidence indicates significant erosion began in late Pliocene time, coincident with and following eruption of abundant basalt from several local centers at about 2.7-2.6 Ma. The onset of central valley erosion marks the initiation of the first through-flowing, high-energy trunk stream (the 'ancestral' Rio Grande), which most likely was caused by integration of drainage southward through the Socorro region. No upper Pliocene fluvial deposits have been identified in the valley center; rather, a significant unconformity separates beds with medial (or earliest late) Blancan fauna (older than about 2.2 Ma) from