FIELD TRIP GUIDEBOOKS, VOL. T120, PP. 83-87, 1989
Day 6, Wednesday, July 5: Gallup to Grants, New Mexico (143 mi, 229 km)
We begin by driving west on I-40 across the Gallup sag in continental Upper Cretaceous Crevasse Canyon Formation. The Gallup
sag is a Laramide-age graben-like structure bounded on the east by the Nutria monocline and on the west by a monocline along
the eastern edge of the Defiance uplift (pl. 3). It is not known if these monoclines are underlain by faults but that seems
probable. As we proceed west along the Puerco River Valley, we pass Twin Buttes, an Oligocene-age minette intrusive being
quarried for road metal. Just over a mile (2 km) west of town, the Gallup Sandstone is exposed on the east limb of the Torrivio
anticline, the type section of the Torrivio Member of the Gallup Sandstone. We pass through the anticline; its axis is well
exhibited on the north side of the highway. Beyond the anticline, coal and carbonaceous shale in the Dilco are exposed in
roadcuts. The Dakota Sandstone and underlying Jurassic rocks are brought back up on the eastern limb of the Defiance uplift.
The cliffs on either side of the highway are capped by Dakota Sandstone underlain by the eolian and fluvial sandstones of
the Recapture, and eolian sandstone of the Cow Springs and Entrada at the base. We exit at Lupton, pass under the highway,
and go east about 2 mi (3 km) on the frontage road.
Citation: Huffman, A. C., Jr.,