Shelters in Tafoni (those Curious Holes in the Rock)

Why do holes in a rock catch our eye? Is it because they’re unexpected? visually pleasing? photogenic? mysterious?

Tafoni all in a row, McInnis Canyons Recreation Area near Fruita, Colorado.

Tafoni shelter, resident unknown.

At Moonstone Beach (Cambria, California), tafoni are more artistically arranged.

Tafoni wonderland at the Honeycombs in the West Desert of Utah.

Not surprisingly, geologists have a term for these curious holes—tafoni (singular tafone). But that doesn’t mean they know how they formed. The possibilities are many: salt weathering, variable rock composition, microclimates, pebbles falling out, and more. I’ve blogged on the topic here, here and here, and there’s an entire website devoted to tafoni. But this post isn’t about how tafoni form. It’s about how they’re used, specifically for shelter.

Tafoni at Montaña de Oro State Park, California, with makers’ remains still in place.

In some cases, how tafoni formed is revealed by the inhabitants—shellfish snug in cavities they themselves excavated. But more often, occupants move in long after the holes are created, for example packrats—creatures that are especially fond of tafoni (these are native woodrats, not the invasive rats from Europe).
When a woodrat takes up residence, it begins to build a midden—a debris pile composed of fecal pellets richly augmented with plant fragments, bone, other animal dung, and anything else of interest (especially if it’s shiny!). The pile is cemented with urine, which hardens into protective crystalline amberat. Construction may continue for many generations, producing huge middens. In arid climates, packrat middens can last for tens of thousands of years with the material remaining easily identifiable, making them important records of changes in vegetation and climate.

On a visit to the east end of the Uinta Mountains last month, I found a woodrat apartment complex! Or maybe these were single family dwellings with outbuildings—a residential style common in the rural American West. In any case, tafoni were clustered in sandstone near the base of a steep hogback (escarpment side). Small middens were common, made mostly of shredded juniper bark and twigs, and occasional cactus fragments. The amount of amberat varied.

Above, tafoni occurred near the base of the small hogback left of the superpositioned canyon on the right (Irish Canyon in northeast Colorado). Closeup below.

Many of these cavities have small middens.

Click on photo to view cactus areole with several spines, near left wall of cavity.

Midden well-cemented with amberat.

A week later, at the Devil’s Kitchen in the Utah Black Rock Desert (not the better known burning-man BRD of Nevada), I came across tafoni in basalt in a lava flow—actually on a fault scarp in the flow. Long after the lava hardened, faulting created a wall 20 m high.In one large cavity about 15 feet above the ground, something had built a nest out of sturdy interwoven sticks—definitely not a packrat.
I soon concluded the nest belonged to a raven or crow (probably raven, as crows use smaller materials). Just ten feet away, in a nook in the wall, I was surprised by three large young birds crouched on the ground. They stared at me with spooky blue eyes and opened their mouths, silently demanding food. They were downright creepy, and their nook reeked! I didn’t stay long.

This lovely bird-child is about a foot long. Is it a raven or a crow? (if you know, please comment)

After I left the Devil’s Kitchen, I drove many miles on gravel and dirt roads toward a pale mountain looming in the mist and light rain.It was Crystal Peak, a tafoni wonderland! In keeping with the magical mood, the sun came out just as I arrived. But I had made a most regrettable error. One afternoon was far to little time to explore and experience a place as remarkable as Crystal Peak. On my next trip, I will spend several days there. And I will again go in spring, for Crystal Peak is a wonderland of plants as well as rocks.My plan was to walk directly to the tafoni, but as soon as I got out of the pinyon-juniper woodland and onto gently rolling rock at the base of the wall, I was waylaid by plants. There were only a few under the pinyons and junipers, yet on the seemingly harsh rock I found a diversity of wildflowers, and wonderful bonsai-like trees and shrubs … amazing!

Pale leaves and reddish flowers are wild buckwheat, Eriogonum sp.

One of our many paintbrushes, Castilleja sp.

Limber pine in a patch of mat spirea (Pinus flexilis, Petrophytum caespitosum).

The rock of Crystal Peak is volcanic—a rhyolitic ash-flow called the Tunnel Spring Tuff. Tafoni are abundant, and mostly elongated parallel to bedding planes (ash layers). They’re also surprisingly uniform in spacing, forcing geologists to scratch their heads (more on this later; Crystal Peak will have its own blog post in the near future).

The tafoni were extremely photogenic, but really tough to photograph. The white tuff confused my camera. Occasionally it got it right and “saw” the rock the way I did:

… but more often it added brown tones:Still, it was wonderful to look through the viewfinder—to observe scenes in detail and in new ways as I composed photographs. But my reverie ended when I realized I hadn’t seen my field assistant in awhile. Normally she’s always on the move, always investigating, always coming into view, never far away. I carefully scanned the white rock for a half-white dog. Finally I spotted her brown patches at the base of the wall. Among the tafoni, she had found one she liked—a perfect shelter. This post is dedicated to geologist and Twitter friend @RonSchott (#GigaRon) who recently passed away, much too early! He always encouraged my geo-blogging and geo-tweeting, and I will miss him a lot.

Ron Schott early in his career (photo courtesy of his sister).