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Barnes Butte, Phoenix Arizona

Last Updated: 9th Jul 2017

By Dave Crosby

A curious feature of most of the caves in the Papago Buttes of Phoenix Arizona recently caught my attention.
I've been trying to figure out how the caves were formed ever since.

I seriously doubt it was by millions of years of wind blown sand as reported in the Papago Park literature.

The above photo is of McDowell Butte, across the street (McDowell road) from Barnes Butte.
The "hanging tongues" visibly protruding from many of the caves don't look like the wind could have produced them.

Unfortunately I don't live in the area, so I had to resort to Google Earth to research them.

A Google aerial view of Barnes Butte. North is at the top. The separate pink and purple layers are clearly visible.

All of the Papago Buttes were capped with a pinkish "Barnes Butte Breccia" - Reddish-brown, course·grained, poorly sorted and stratified fluvial conglomerate. Clasts are 70 to 85% granite and 10 to 20% meta-rhyolite. They are angular to subangular, 1/4 in. to 15 ft (0.7 to 4.5 m) in diameter, in a matrix of rock fragments and ferrugenous cement.

The next layer down is a purplish "Zoo Breccia" - Purple to reddish·brown, coarse~grained, unsorted, poorly stratified, massive fluvial conglomerate. Clasts are 30 to 100% meta-rhyolite, 0 to 35% granite, and a small amount of a distinct dacite porphyry. They are angular, 1/4 in, to 2 ft (0.7 to 60 cm) in diameter. The Matrix is a mixture of silt, clay, and ferrugenous cement.

Note the difference between the two layers:
The pink is mostly large chunks of granite and pink cement.
The purple is mostly smaller pieces of meta-rhyolite, clay and purple cement.

The caves appear to have formed everywhere on the contact zones between the two layers.

A McDowell Road view if the southwest corner of Barnes Butte showing many caves with "hanging tongues."

A closer Google aerial view makes the tongues more visible.

Here I have numbered some of the tongues.

Viewing the Barnes Butte West side. Note that the further north we look, the more the pink layer (Barnes Butte Breccia?) has been weathered away, but the caves and tongue features remain in the purple layer.

View of the northwest corner of Barnes Butte. A new view of the surviving tongues with the pink layer removed.

Moving out from the northwest corner view of Barnes Butte. Notice the long purple tongues in the upper left hand side.

A Google view of the north side of Barnes Butte where the pink layer apparently slid away long ago.

I believe that a fluvial landslide covered the base granite buttes with the purple conglomerate, probably at the end of an early Holocene ice age. A later Holocene fluvial landslide deposited the pink layers on top. These layers are listed as being Tertiary, but the cement is too soft and crumbly to have survived exposed for that long.

Then I suspect slippage by the saturated clay of the purple layers poured out with some of the overlying pink layers to cause the creation of the caves with the Zoo Member tongues hanging out over the Barnes Member.

Does anyone know of this geological oddity occurring elsewhere in the world?

Article has been viewed at least 692 times.


Papago Buttes are interesting. When it cools down I would like to look at them closer.

You mentioned that "The caves appear to have formed everywhere on the contact zones between the two layers." The photos suggest otherwise. It appears that the formations and therefore the contacts are steeply dipping especially in the first photo. The caves however are nearly all horizontal to subhorizontal and occur over a wide area not just at contacts.

I found an interesting article by Arizona State geology professor, Julia Johnson, that suggests how the caves were formed by water that pooled. She said, “The holes are thought to have started as small features, such as an indention, where one of the large, angular clasts fell out,” she says. “Those indentions then collect water, which causes those rocks to weather away more rapidly, and so a smaller hole becomes larger with time.” See: "Why is there a hole in the rock at Papago Park?" from the Arizona Republic April 16, 2016 found at http://www.azcentral.com/story/news/local/phoenix-contributor/2016/04/19/hole-rock-papago-park/83217684/

I note that standing slightly acidic rainwater will disolve the calcite cement in the rock, allowing wind to blow away the loose sediments thus forming horizontal caves.


John Christian
14th Jul 2017 6:01am

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