Land Lines
4:00 am
Thu May 17, 2012

Land Lines-The East Kaibab Monocline

The cool pine-scented air of the Kaibab Plateau at 8,000 feet is a respite on a hot day. You might want to stop at Jacob Lake Lodge for one of the best chocolate milkshakes this side of Grand Canyon. Stretch your legs, take a last slurp of that ‘shake, then pedal on down the hill on Highway 89A.

The road’s fairly level at first, but heading east it plunges downhill past yellow-belly ponderosas, through pinyon and juniper, toward sagebrush below, dropping three thousand feet in a hurry. The reason for this impressive hill--the East Kaibab Monocline.

The monocline is a gigantic wrinkle that bends the surface of the Colorado Plateau. It looks for all the world like a humpbacked ocean wave rushing to the east. Fifteen miles from Jacob Lake, pull into the overlook above House Rock Valley for a textbook view of the fold. Rock layers caught in the wave start out flat up by Jacob Lake, tilt five or ten degrees along the ramp you’ve just driven down, then level out in the valley below. North to south, the fold stretches a hundred and fifty miles from Bryce Canyon up in Utah, toward Flagstaff. 

Kaibab Limestone dominates this panorama, a cream-colored fossil-ridden rock capping the fold and continuing out across House Rock Valley. On the horizon stands another monocline--the Echo Cliffs--also gently stepping down to the east. Marble Canyon lies just this side of those cliffs, where the Colorado River carves a narrow slit faintly visible from here. Downstream, the river leaves Marble and enters its grandest canyon.

The key to understanding the East Kaibab is found deep inside Grand Canyon. There, the Butte Fault is exposed directly beneath the monocline. Three-quarters of a billion years ago, really ancient rock layers were torn apart along the fault.  Afterward, it lay dormant as many thousands more feet of rock were deposited.

Then, sixty-five million years ago, compression rippled across the American West. The Rocky Mountains reared up and the Colorado Plateau was squeezed. The old Butte Fault was reactivated in a new reverse direction. Once again, it chewed up rocks many thousands of feet underground--but didn’t fracture those closer to the surface. Instead, those upper layers were bent like taffy, forming the East Kaibab Monocline.

Other monoclines rumple the Colorado Plateau--the Waterpocket Fold in Capitol Reef, Comb Ridge near Bluff, the Circle Cliffs, and the Defiance Uplift.  All were pushed up during the same spasm that created the Rockies, all trending roughly north-south.

The East Kaibab best shows how a fold is related to its underlying fault. Similar fractures probably lie beneath the other folds, but we just can’t see them. With so many in the neighborhood, you might think monoclines are common features on Earth.  But they're not. You'd have to travel to the Tibetan Plateau to find such a concentration.

The take-home message: rocks may not be as solid as we think. Sometimes they break, and sometimes they bend.