This is about a curious geological phenomenon found at places in the Waterpocket Fold. It may also occur elsewhere in our folding desert country, although I don’t know of other specific examples.

One day several years ago, Kline Barney, Ken Goodearl and I were driving down the Bullfrog road south of Notom, looking for interesting things to do. Ken spotted a canyon cut into the Fold which looked from the distance like it had impressive Wingate walls. So we stopped at a dry wash on the watercourse out of ‘Ken’s Canyon’ and started walking.

It led into a wonderful scenic little canyon with few obstacles. The climax of this gem was the boxed–off end, where the canyon opened up into a huge magnificent cathedral. The salmon Wingate walls were painted with long vertical streaks simulating the pillars of a gothic edifice. We were astounded.

After arriving home, I developed some ideas about how this phenomenon might arise, wherein a canyon, narrow at its mouth, abruptly widens, when you walk upstream, before being boxed in. Its image on a topo map is that of a pear or tear drop. That happens, it turns out, at several places in the Waterpocket Fold. The prototypical example of a tear drop, and apparently the largest one, is Miller’s Creek south of the Hall Creek narrows. Its topographic image is truly striking. In the case of Ken’s Canyon, Miller Creek, and the others I’ll list below, they are like someone had taken a pear–shaped cookie cutter and excised a piece from the Navajo and Wingate layers. The removed piece leaves a hole which is narrow at the downstream end but becomes dramatically wider upstream. This is a contrast to most of the canyons cut into the Fold, which either remain narrow slots as far as they exist, or are cut all the way through the Fold from east to west.

Here is a likely explanation of the tear–drop phenomenon. First, let me explain that there are two relatively hard sandstone layers at the top of the Fold, the Navajo and Wingate, resting on softer layers (most notably the Chinle formation) below. The folding of the earth’s crust, when the aforementioned layers tilted and rose out of the desert sands, created the present Waterpocket Fold. This emergence occurred roughly on a north–south line, the east edge of the Fold. The lifted set of layers ruptured at the western edge of the fold, but we will not be concernd with that; the teardrop phenomenon occurs only in the eastern part, where the layers have remained intact except for the uplifting action and erosion by running water.

When the Fold was younger, watercourses were being etched into the Navajo sandstone layer uppermost in the Fold by rainwater erosion. Slots were eventually formed, extending deep into the Navajo and Wingate layers. Narrow slots of this type are in fact abundant in this country. In this case, the slots carried water generally in an eastern direction, and became steadily deeper as the water erosion progressed. Some slots were eventually cut all the way down to the much softer Chinle formation, which is below the Wingate. At the eastern edge, where the Fold emerges from the flat desert, we assume the water from the canyon simply seeps into the sand (this is usually correct) and therefore ceases its erosive action.

For any given location along the length of any given slot, the elevation of its bottom will never be lower than that of the mouth of that slot, which is in the Navajo. Therefore if the slot reaches the Chinle at all, it does so at a location some distance away from its mouth.

If and when the underlying softer material is reached, a different kind of canyon–widening process would have started. The soft material would be, at any given point in time, at the base of a vertical Wingate cliff. I’ll call the region where the soft layers are exposed the ‘soft–bottomed region.’ It does not extend all the way east to the mouth of any slot. So during the new process, we have vertical Wingate cliffs resting on a bed of exposed softer material. Due to its softness, the latter cannot sustain all that much pressure formed by the weight of the harder layers on top of it; the shale is simply squeezed out from under the heavy Wingate/Navajo cliffs. That undercutting at the edge of the soft–bottomed region causes cantilever bending stresses in the vicinity of the edge in the Wingate and Navajo layers. Those formations are therefore prone to break off in vertical columnar pieces at the edge of the soft–bottomed region. This process widens the soft–bottomed region which had been previously created by water erosion. That cantilever/flaking effect happens not only at the upper end of the region, where water is flowing in (after a rain), but just as readily on the sides as well. Thus the canyon widens due to this action, in every place where the Chinle is exposed below a cliff. The body of the tear drop would correspond to the area of this exposure, i.e. to the soft–bottomed region.

We found that to be the case in Ken’s Canyon; Chinle was indeed exposed in the upper ‘cathedral.’ I also observed it to be the case in Miller’s Creek, which I would consider to be the prototypical tear–drop cavity. In fact, geological formations below the Chinle also appeared.

So we have, in addition to the usual canyon–widening and deepening erosion processes, the flaking process due to cantilever stress. The latter, where it can occur, will probably proceed at a faster rate than the former. This differential rate is probably the reason that the tear–drop cavities are blunt at their upstream end.

Finally, we have to account for the fact that the shape of the teardrop tapers off to a narrow passageway at its lower end. The canyon, including the soft–bottomed area, collects water during a rain, and since that water is always flowing out of the area through its lower end, it is funneled through the mouth of the teardrop, which is in the harder formations. But cuts through the hard layers with no soft material around are narrow, generally slots. This explains the tapering.

The main ingredients allowing the formation of tear drops, then, would be (1) hard flakable layers overlying a soft foundation, and (2) a folding, hence tilt, of these layers. Thinking there must be more of these tear drops, I had a look at topo maps of the entire Waterpocket Fold and found the following other candidates, listed from north to south:

• South Coleman (south of Oak) (North Coleman looks like
      an aneurysm, which no doubt is a related phenomenon.
      I haven’t been there.)
• Sandy Draw
• Red Canyon
• Maybe the canyon north of Ken’s
• Ken’s Canyon (unnamed on the maps)
• The two canyons south of Ken’s
• Miller’s Creek in the southern fold (prototype)
• The two canyons a half mile and a mile south of Miller’s.

© 2007 Paul Fife