Rock Talk By Larry Larason
The Gallup Sag.
What is the Gallup Sag? Well, it's not a new dance step. And it's got nothing to do with the local economy between Ceremonial and the balloon rally. Remember, we're talking geology here.
The Gallup Sag is a geologic term for the mild structural depression that most of Gallup sits in. In other words, it's a syncline. The sag lies between the Zuni Mountains and the Defiance Plateau-Chuska Mountains. At Gallup it extends from the base of the hogback [a.k.a. the Nutria Monocline] west to Lupton, Arizona. North-south it covers an area from Zuni to near Newcomb, where it merges with the San Juan Basin. It's up to 28 miles wide on the south side and about 70 miles long north to south.
Why not just call the sag the Gallup Syncline? Well, geologists used to be down to earth people [pun intended!] who used English instead of high-fallutin' Latin or Greek words to describe the Earth. The sag was named early on in the last century. Were it just now discovered, I'm sure that they would come up with something more jargonistic than "sag". And maybe "sag" was chosen because it's not just a simple syncline: it is wrinkled with anticlines and smaller synclines, as well.
What are all these "clines" I'm talking about? Consider the word "incline." One meaning of incline is "to lean." An anticline is a geologic structure with two sides opposing, or leaning against, each other. In other words, it is an arch. A syncline is the opposite; it dips, bowl-like, toward a center. And a monocline? Think of it as half an anticline, that is the rocks are bent but remain flat on both sides of the wrinkle. As an example of a monocline, look at the hogback [Nutria Monocline] on the east side of Gallup. The Crevasse Canyon Formation lies only slightly wrinkled on the west side, and just beyond the jutting rocks you can see the Morrison Formation in the White Cliffs. Although the Morrison displays some dip into the monocline, it is flat toward the east.
The sag was formed during the Laramide Orogeny, which began about eighty million years ago and continued for possibly forty million years. This was a period when mountains were pushing up all across the western part of North America. But as mountains and highlands rose, other areas subsided to become basins. The major one in this region is the San Juan Basin.
At the surface of the sag erosion has sculpted buttes and mesas, so there isn't much indication of the sagging bed rock. However, the fairly flat bottom dips at about sixty feet per mile into the San Juan Basin. So the sag might be described as a bay off the San Juan Basin. Because the basin and the sag were both low areas, they remained swampy after the Cretaceous sea departed and vegetation that grew in the wetlands was buried to become coal. Nearly all the coal mined around Gallup came from the sag.
Although the sag itself is not apparent at the surface, you can see evidence of the wrinkles. Interstate 40 crosses three of these short anticlines.
The Gallup Anticline has a very gentle eastern slope, which rises from the base of the west side of the hogback. The sharper angled western flank of the Gallup Anticline goes beneath the surface east of Ninth Street, just behind the Flea Market. As you drive north into town on Second Street there are a couple of places where you can look across the Interstate to glimpse the tilted rock of the western flank. You can also see them from a vantage point somewhere high on the south side of town. Tilted rocks along NM 602 around the Nizhoni intersection might be part of the Gallup Anticline but, they are more likely part of the Allison Syncline, which defines the lowest part of the sag.
The Torrivio Anticline is the most visible of the three crossed by I-40. The Rio Puerco has cut a water gap through it, which makes the structure apparent. The water gap, which is used by both the railroad tracks and I-40, is just about dead center in the anticline's north-south axis. The rocks in the Torrivio Anticline are Gallup Sandstone overlying Mancos Shale. The Gallup Sandstone, as a formation, includes both the sandstone for which it is named as well as shale and conglomerate beds. There is a short roadcut at Milepost 13 on I-40 that shows the angle of the Gallup Sandstone beds on the western flank of the anticline. Formations that would have been atop the Gallup Sandstone originally, including the Crevasse Canyon and Menefee Formations, have been eroded away.
Speaking of Gallup Sandstone, it is the same formation that forms the western face of the hogback. As the Cretaceous sea moved eastward leaving behind deposits of Mancos Shale, the Gallup Sandstone was laid down on beaches, or in lagoons and deltas. The sandy portions of the formation resemble Dakota Sandstone. Although this formation was named for Gallup, it is not the most prominent stone in our area, but it does occur in a fairly wide region of western New Mexico. Other outcrops of Gallup Sandstone may be seen along Second Street.
The Pinon Springs Anticline is the largest of the three crossed by I-40, extending from Zuni to north of the highway. However, where the interstate crosses it, the angles in the rocks are pretty subtle and barely noticeable. It's hard to watch them at seventy-five miles per hour.
In case you are wondering, there are two other sags associated with the San Juan Basin. The nearest one is the Acoma Sag. The other one is the San Juan Sag, which extends off the northeast quadrant of the basin and lies almost entirely in Colorado.
I have another, personal, use for the term "Gallup Sag". It describes what I feel as I take the exit at Gallup after two hours of dodging tractor-trailer rigs on I-40 while driving home from Albuquerque. When I slow down for the exit I feel a sense of relief and relaxation. It's always good to get home to the sag!