The cover story in our April issue focused on the work The Nature Conservancy does in Arizona — in particular, at Aravaipa Canyon Preserve southeast of Phoenix. That story spurred Craig Kent, a Tucson resident, to send us an email, which included this photo.

"One of my favorite yearly backpacking trips involves the Aravaipa Canyon," he wrote, "and I've enjoyed the shoutouts to this special place in recent issues of Arizona Highways (which I love and have been reading since the '70's). Every time I hike through the canyon, I am floored by this distinct 'doorway' in the face of the canyon wall and have to believe it is nature-made and not man-made (I pray). With the many resources you have at hand, have you ever heard of any explanation for this freakish wonder? I am just curious, that's all."

To find an answer to Craig's question, we reached out to Mark Haberstich, the director of the preserve. He offered his theory of how this "doorway" formed.

"The layer of rock that covers much of this part of the canyon is called a volcanic tuff," Haberstich said. "It is made up of ash that spewed out of a volcano erupting in this area millions of years ago. Somehow the ash settled against a rectangular rock of another material. The rectangular rock fell away more recently (in geologic time) as the canyon has eroded. This left the distinct 'doorway' impression that stands out from the other tuff around it."

Haberstich added that he's fairly certain the shape occurred naturally, "but I'm sure you could get a lot of interesting explanations."

We hope that answers Craig's question. If you've got a question about Arizona's history, terrain, etc., drop us a line and we'll see if we can help!