Geology FAQ

irregular holes ("tafoni") in Entrada sandstone
Irregular holes ("tafoni") in Entrada sandstone

NPS / N Scarborough


How old are the canyons?

The past three million years are likely when the canyons we see today became recognizable. The most recent uplift and erosion occured within the last three to 50 million years. The rock layers that make up the canyons formed across billions of years.

What are the vertical, dark streaks on the canyon walls?

Desert varnish. It's where tiny bacteria have oxidized (basically, rust) the rock. This means they've taken chemicals from air and water and caused a chemical reaction with the host rock, changing its color. Zion National Park's two-minute video provides an overview about desert varnish.

Did the Colorado River carve the canyons?

Not exactly. The ancestral Colorado River carved the shape of the Grand Valley. However, erosion from the highlands (modern Pinyon Mesa and the larger Uncompahgre Plateau) carved these canyons.

What are the holes in the rocks?

Tafoni. These rounded holes are where pockets of softer minerals eroded more quickly than the host rock layer. They can most easily be seen in the Entrada sandstone layer, such as along Alcove Nature Trail. You can also find them in the Wingate sandstone, near the clifftops.

Are there arches or slot canyons in the Monument?

Not really. There are very few arch-like formations and slot-like canyons in the Monument. Look for both along trails that travel the Entrada or Wingate sandstones. Alcove Nature Trail provides a short walk (0.5-mile) to a slot-like canyon. Larger formations are found in nearby Utah, such as Arches and Canyonlands National Parks.

Why are there rock towers ("monoliths") here?

Erosion of widening canyons, plus fault lines that run northwest-southeast, created the monoliths. As the strip of land separating two canyons gets thinner, narrow walls of rock called "fins" are the last pieces of land standing. Erosion occurs more quickly where there are cracks and weaknesses in rocks, such as along fault lines. So, rocks will wear away faster at the junction of a fault and a fin.

Look at a topographic map of the Monument, and you'll notice that almost all the monoliths occur along this northwest-southeast line. They also stand near the arm of a canyon, revealing how they used to connect to the rest of the canyon walls. Pipe Organ, Independence Monument, and Kissing Couple stand along this fault line. Independence Monument is the last section of the rock fin that used to completely separate Wedding and Monument Canyons.

What caused the tilted rock formations?

Geologic uplift. This area went through several periods of uplift, meaning the entire landscape was pushed up. Softer rock layers were able to bend during the incredibly slow movement, only breaking in certain areas. There are both normal and reverse faults here, where land moved up and down at different times. When you visit Window Rock, you can see the deep bend in the Wingate Sandstone on the feature known as the Island. When you stop at Redlands View overlook, you can also see this same bending along the Monument's boundary.

Who named the rock formations?

It's likely that Indigenous peoples have their own names for many of the landscape features of the Monument. People living in the Grand Valley, including John Otto, named many of the rock formations that we're familiar with today. The US Board on Geographic Names maintains "official" records of place names nationwide. The rocks don't seem to care what you call them though.

Last updated: September 8, 2022

Park footer

Contact Info

Mailing Address:

1750 Rim Rock Drive
Fruita, CO 81521


970 858-2800

Contact Us