You know. A canyon with only one way to get in or out. That’s where the posse traditionally traps the bad guys or where the rustlers corral the missing herd in old westerns.
Do they exist? If so, how were they formed? I always thought canyons were cut by flowing rivers or streams. Surely, all canyons should have at least two ways of getting in and out.
Box canyons can be formed by a stream that falls over a cliff, with the erosion forming the canyon. Once the canyon is formed the downstream end is easy to get in and out of but the upstream end isn’t so. If there were no abuprt change in ground level a canyon wouldn’t be formed in the first place.
Waterfalls? Works for me. Does anybody know how common such canyons are and where I might find one in the Southwest? I’m not a rustler, I swear.
Box canyons…in San Diego they are everywhere. Go out to the Anza-Borrego badlands and there’s a million. If you want an endless maze of them, try Arroyo Tapiado off the I-8 freeway on Highway S-2 out of Ocatillo. At least for this area, the book “Afoot and Afield in San Diego” by Jerry Schad lists 90% of the hikes in and around the county, a good number of which are box canyons. I’d suggest the one to Cedar Creek Falls as one of my personal favorites…
Just to clarify: you don’t need a waterfall per se. An intermittent trickle will do. It’s just that all the water falling in that area will run over the canyon lip and collect in the bottom on its way out. The little trickles will cause some erosion and the collected water will do significantly more. As the canyon is cut deeper and deeper the sides (and the back–the “box” end) collapse into the canyon and are washed away.
In fact, my guess is that a steady stream of water is more likely to cause a “non-box” canyon because the action of the stream will notch the back of the canyon further and further upstream until it’s no longer box-like, whereas the intermittent flow is more likely to steepen and collapse the sides and back of the canyon. As noted, this is just a guess.
A bona fide waterfall is usually a function of the relative strength of the rocks in the riverbed above and below the falls. A hard layer of rock (eroding slowly) followed by a soft layer (eroding quickly) leads to a differential in the river bottom at their juncture. The water then falls with some force on the softer layer, further hastening its erosion, in a self-accelerating process. Actually any hard obstacle will do to start a waterfall and the waterfall will continue until the hard obstacle is finally washed away and the river flattens out again.
It may be that something similar happens in areas where box canyons form – a hard surface layer overlying softer rock or soil. Running water doesn’t make much dent in the top layer but quickly erodes the softer layer beneath.
“I used to think the brain was the most important organ in the body, until I realized who was telling me that.”
Emo Phillips
Box cañons abound in Arizona along the Mogollon Rim, as well as in various places along the San Juan River in SW Colorado, SE Utah, etc.
I believe a true box cañon has a relatively flat back, a vertical drop, if you will, making the effect be that of a box, though I admit of no proof for that other than reading several westerns as a youth. 
Aloha PB… To check out a couple, you can head out on the I-10 to Joshua Tree Nat’l Park - to the lesser-used southern entrance right on the I-10 called “Cottonwood”. If you take that road south about 10 minutes [as opposed to into the park], you’ll come across some. One is even named “Box Canyon”, on “Box Canyon Road”, famed amongst SoCal reptile hunters. There are some in the park, too, if you’re willing to hike a little ways [off-road driving not allowed inside the park].
O le mea a tamaali’i fa’asala, a o le mea a tufanua fa’alumaina.