The geological formations that get the most press in southern Utah are the sandstone spires, buttes, and columns of Monument Valley (and to a lesser extent Valley of the Gods). But equally impressive are the volcanic plugs (necks) and dikes that are spread throughout the northern portion of the Navajo Nation (southern Utah, northern Arizona, and northwestern New Mexico). These remains of ancient volcanoes have names like Shiprock, Agathla, Alhambra, Church Rock, and Mules Ear. But the lesser-known lava formations are equally impressive. And most are easily observed and appreciated from roadside.
The Navajo Volcanic Field covers about 30,000 square km in the Four Corners region. The plugs (necks) and dikes are what remains of volcanoes that erupted between 25 and 30 million years ago. Since that time, erosion has lowered the ground surface hundreds of meters, exposing the deeper levels of these extinct volcanoes. Typically, volcanic plugs tend to be more resistant to erosion than the enclosing rock formations. Thus, after the volcano becomes inactive and deeply eroded, the exhumed plug stands out as an irregular columnar structure.
Shiprock, in northwestern New Mexico, is a classic example of a volcanic plug or neck. It projects upward 483 meters above ground level, the approximate height of the Empire State Building (including towers). Shiprock has three major dikes which resemble gigantic volcanic fences (or fortress walls) which radiate out from the plug. In many ways the dikes are more visually impressive than the actual plug. From both the ground and the air the dikes are easily observable.
There is an excellent grouping of plugs and dikes around the Navajo community of Kayenta in northern Arizona. Included here are Agathla (north of Kayenta about 5 miles and south of Monument Valley) and Church Rock (east of Kayenta about 5 miles). But there are also several smaller plugs and dikes in the area.
About 12 miles south of Bluff UT on highway 191 (near the UT/AZ border) is Boundary Butte and Arch. Its name has nothing to do with its nearness to the state line; the butte marked the northeast corner of the original Navajo Reservation of 1868. About 2 miles north of the butte there is a turnoff (with cattle guard) which heads toward the southeast. To see the hole or arch in the formation, you need to travel a short distance down this sand-and-gravel road. From a vantage point along this side road, you can see an associated dike that runs from the volcanic plug into the mesa on the south.
Just south of Mexican Hat UT is a formation known as Alhambra Rock. Typical of other volcanic formations in the Four-Corners area, erosion has left the resistant neck standing as a lone and prominent sentinel. Alhambra Rock can seen in the distance from the overlook at Goosenecks of the San Juan State Park.
Note: Shiprock is considered by the Navajo people to be a sacred mountain. According to a 2006 press release:
Reports of the Navajo Parks and Recreation Department allowing rock climbing are false. Yet several websites have postings on how to evade Navajo Nation regulations and proceed with dangerous and illegal rock climbs. Even more serious than the possible physical harm illegal climbs could pose is the religious damage done to the Navajo people by these non-Navajo visitors.
So please respect all requisite Navajo regulations.