Mexican Water, Arizona
This route crosses the Nokaito Bench, a plain which lies between the San Juan River and Monument Upwarp on the west, and Casa Del Echo Mesa on the east.
Drive east out of Mexican Water. US 160 is on Navajo Sandstone. After about 2.5 miles, turn north at the intersection with US 191, just east of milepost 437.
Milepost 1. The short mountains toward the east, unnamed on maps, are composed of Jurassic rocks: Carmel Formation at the base, rising through the Entrada, Summerville, Bluff, and Morrison Formations.
Slow down; we have a lot to talk about at milepost 3. At the base of the mesa toward the east are some weirdly contorted beds of the Summerville. This formation was laid down in shallow, quiet water at the edge of a continental sea. Possible explanations of the contortions include  being deposited wet and later squeezed by the weight of overlying sediments,  loss of soluble components, such as gypsum, that allowed the partial collapse of the strata; and  being exposed to the air, drying out and crinkling before more sediment was laid down. The lower Summerville is contorted to some extent almost everywhere around Bluff, but this example is exceptional.
The sculpted rocks rising on the west of the road are erosional remnants of Entrada Sandstone. When you first see them as you travel the other direction, they appear to be a natural arch.
The hill on the west and spots along both sides of the road ahead are littered with rocks that look volcanic. Closer inspection reveals that they are pebble conglomerate grading into sandstone. A heavy coating of desert varnish gives them the look of basalt from a distance. Up close, the conglomerate resembles chunks of asphalt paving, but if you break a piece, the color is light tan on the fresh surface. The pebbles in the conglomerate range in size from chat, or pea gravel, up to roughly 3 inches. These rocks have weathered out of the Morrison Formation that caps nearby mesas.
At Milepost 5 you enter Utah, and the milepost numbers begin again at zero.
At mile 1.5, Boundary Butte, one of about 300 diatremes in the Four Corners, is briefly visible at 2:00. The name of this feature has nothing to do with its nearness to the state line; it marked the northeastern corner of the original Navajo Reservation of 1868. The Navajo name for it translates as "Rabbit Ears." The Mormon exploration party of 1879 passed by the base of Boundary Butte as they worked their way easterly to Montezuma Creek.
Milepost 3. Sleeping Ute Mountain is visible on the eastern horizon.
Milepost 4. You are travelling across Navajo Sandstone. Here it is light colored, nearly white, as it is around Bluff.
At the "Bluff 20 Miles" sign, you have a good view of Boundary Butte, but you will have to pull over and stop to look back at it. From this vantage you can see an associated dike that runs from the volcanic plug into the mesa on the south. The overall length of the dike is about 6 miles.
Milepost 5. Your vista here includes the Bears Ears at 9:30; the Abajo Mountains between 10:30 and 11:00; and Sleeping Ute Mountain at 1:30. The two peaks of the Bears Ears, at elevations of 8500 and more than 9000 feet are composed of Wingate Sandstone standing high on an anticline. In Navajo myth, they represent the head of Changing Bear Woman. In White Canyon, near the buttes, is found one of the earliest Navajo archaeological sites. In the 1860s some Navajos fled to the area of the buttes and the Bluff region to avoid being sent to Bosque Redondo.
Milepost 6. Looking toward the south you have a good view of Boundary Butte, but the dike is not easily seen from here.
Milepost 8. The road runs through an area of sand dunes that have been stabilized by vegetation. You'll notice, however, that some of them are active, probably because of disturbance by grazing animals. The sand is being recycled from dunes laid down in the Mesozoic.
Milepost 10. On the west you will see a rounded lump of rock rising up with a smeary red and white surface. This is the east side of the Mule Ear, a feature that is part of Comb Ridge. To the north it presents a sharply pointed face that is the source of its name. Presumably it rises so much higher than the rest of the ridge because it has been protected from erosion by the large diatreme just to the west of it. It is also possible that the sandstone was somewhat hardened by the heat of the diatreme eruption. Other features in the view here include the stepped mountain named Sugarloaf, just north of the Mule Ear in your view, and Cedar Mesa on the northwest horizon.
Milepost 12. Straight ahead you can see the near-white Navajo Sandstone that forms the sloping, sculpted eastern flank of Comb Ridge. Comb Ridge, the eastern boundary of the Monument Upwarp, emerges from the Abajo Mountains running south, and then swings toward the west to disappear at Kayenta. The San Juan River cuts through it about three miles north of the Mule Ear. Many canyons notch the ridge and contain Basketmaker and ancient Puebloan ruins and rock art.
Milepost 16. Spires in Monument Valley are visible at 9:00.
Milepost 18. The road curves around the bluffs of White Rock Point, where massive, mid-Jurassic Bluff sandstone at the top displays its characteristic appearance of frosted chocolate. Below the Bluff Formation is the Summerville lying over the Entrada. In the exposure nearest the road, the Entrada is silty and resembles the Beds at Baby Rocks described in Road Log 4.
As the road descends to the bridge across the San Juan, notice how invasive Russian olive and tamarisk trees are displacing the native cottonwoods and willows. The gray-leaved Russian olive is more prevalent here, but the tamarisk [a.k.a. salt cedar] is probably the greater threat across the west. One tamarisk may suck up as much as 300 gallons of water each day, which is then lost through respiration. On the Colorado River it is estimated that eradication of tamarisk would save up to 150 billion gallons of water each year. They were originally imported from the Middle East in the 1830s as ornamental plants and also for flood control. Two species hybridized in the new habitat and now this vigorous shrub or small tree, with no local enemies to control its spread, is proliferating across the West. Tamarisks have roots that can go down fifty feet, which hinders efforts to eradicate them.
Junction of US 191 and 163. Bluff is three miles east of this intersection.
New Mexico Geological Society. Monument Valley. 1973.
Linford, Laurance D. Navajo Places; history legend, landscape. Univ. of Utah Press, 2000.
Semken, Steven C. Field guide to a geologic excursion in the Northeastern Navajo Nation. Western Slope Intercollegiate Geologic Field Conference. Navajo Community College, Shiprock, NM, 1992.