Stress and Strain:
Squashing Rocks in the Muav Gorge
(Continued from page 1)
gently up a few degrees as you approach the river. Bedding on the other
side of the river mirrors this fold: rising from the opposite direction to meet the main
These faults and folds occur within a twenty-five mile stretch of the canyon
that begins near Fishtail and ends somewhere below Tuckup. The folds are exposed from
river level to perhaps two or three hundred feet up the walls. They are petite in
comparison to other folds within the canyon like the East Kaibab Monocline. Their
expression is restricted entirely to the Muav, and they are persistently aligned with the
canyon no matter which way the walls twist and turn. Since the faults and folds so
faithfully follow the river, they can only be reached by boat. Voila: three research
permits for four people to spend twenty-one days simply messing about in boats during
April, July, and October.
Pat Tierney rowed a boat on the first of those three research trips. At the
foot of the Bass Trail, Pat and I came around a corner and saw a naked old man standing
knee-deep in the river. As we pulled up, Pat whispered that I wasnt going to believe
this. The man- George- bellowed out: Have you got any women? Pat said no.
George, a true gentleman, was relieved not to embarrass anyone as he took his bath. George
was once an Indiana mason; since retirement he had tried his hand at rock-climbing,
sky-diving, kayaking (Pat had taught him on Westwater the year before), and most recently
hiking. He had been in the canyon for six months now, hiking for three weeks at a shot.
George rode with us the next morning. We spent the day dawdling in upper
Elves. Story by story, without self-consciousness, he outlined a life led in honor. At
days end, I was sorry to leave him but we had to push on. Five days later, working
at Olo, we watched a Georgie White flotilla blast by, boat after boat after boat. Georgie
drove the last triple rig, wearing that leopard-skin get-up. George stood next to her in
the motor well.
Geologists frequently get away with wildly waving their arms as they describe
tremendous stresses that cause rocks to fold and mountains to grow. Excuse me,
but how much stress? Hard to say, but certainly a lot. Larry Stevens once
treated us to his wonderful analogy of 69,000 elephants pouring through Crystal hole every
minute. He was putting his money where his oars were, not just waving his arms about
lots of water. Most geologists are loathe to be so precise.
Peter Huntoon had ascribed the presence of the Muav folds to the underlying
Bright Angel Shale being squeezed up like toothpaste by the weight of the adjacent canyon
walls. A nice analogy, but one with some shortcomings: its vertical orientation of force
could not explain the thrust faults and kink folds that are intimately associated with
other, more gentle folds found in this exposure of the Muav. I suspected that Huntoon was
on to something with his toothpaste metaphor, but that the driving force behind the folds
would turn out to be a bit more complicated.
The beauty of studying the Muav river-level folds was that the stress that
caused the folding was quantifiable: gravity, all 32-feet-per-second-squared of it.
Regional stresses of the arm-waving ilk can be discounted because the folding so perfectly
follows the river as it snakes southwest past Matkat, south toward Sinyala, and northwest
toward Ledges. I idealized that section of the canyon into a flat-bottomed gash with
vertical walls (not far from the truth); plugged in the specific gravity of the Muav,
Redwall, and Supai; and ground the numbers through a finite-element model in order to
relate force to deformation, stress to strain.
The mathematics suggested that the most significant stress on bedrock just
under the river would be horizontal, not vertical. This is because rock beneath the canyon
walls exerts a horizontally directed pressure on its neighbors as it tries to flatten in
response to gravity. Rock just beneath the river, on the other hand, is not vertically
compressed; the greatest stress it feels is the laterally directed force derived from
rocks beneath the canyon walls.
My father was on the second trip. At the outset, I had discouraged him from
bringing a transistor radio; I knew that he would just listen to the Albuquerque traffic
reports on KOB. At Nevills on night five, I cooked dinner and yelled to come get it.
Nothing happened. I couldnt find Dad, and Jimmy Hendrick was missing too. I yelled
again. Still nothing. Ten minutes later on that October night in 1977, Dad and Jimmy
erupted up from behind a boulder fifty yards away, whooping and hollering as Reggie
Jackson hit his third home run in that years World Series.
Folds that follow canyon walls are not common. The Meander Anticline in
Cataract does parallel the Colorado River, but it has no kink folds and can probably best
be explained by Huntoons toothpaste model. The underlying Paradox salts are squeezed
up by the differential loading of canyon walls, bending rock along the way, oozing out
along the crest as small salt volcanoes in Red Canyon. Beyond Cataract, examples of other
river-oriented folds become difficult to find. Even within the Grand Canyon, the folds are
expressed in one place (Fishtail to Tuckup) where the Muav is at river level, but not at
another (36-Mile to Saddle). Why are the folds so rare?
The recipe for river-oriented folds first calls for steep walls (lie on your
back at Upset Hotel: youre looking two thousand feet straight up to the Esplanade).
Add a limestone with just the right consistency: solid beds separated by thin silt layers.
The silt, if wet, would allow the limestone layers to slide over one another, much like
the pages in a bending telephone book. Imagine trying to flex a single piece of
three-inch-thick wood. Maybe Dan Dierker could do it if Brian were holding the other end;
I sure cant. But anyone can easily bend a telephone book of the same thickness
because the pages are free to slide past one another. Similarly, the Muav limestone (in
lots of layers separated by silt) is much more easily bent than a massive limestone like
the Redwall. The direction of force acting upon the rock is critical. If the force is
perpendicular to bedding, it will tend to lock up the layers, and prevent them from
sliding relative to one another while bending. But if the force is directed
not-quite-parallel to bedding, then the layers dont lock up and the limestone is
more likely to bend.
Back in the office, I fussed and doodled, crumpling a lot of paper along the
way. Engineering equations gradually emerged that related stress to strain.
I remember a time during graduate school when I lived, breathed, and ate
those sort of equations. I once looked groggily out the back seat of a car at snow
swirling past the window. For a millisecond, I clearly visualized the snows movement
in terms of the differential equations with which I had been struggling in a dream. Then
the circuit breakers blew, smoke poured out my ears, and I fell back asleep. It is
fascinating to reflect on those days from this vantage point fifteen years down the road.
Did all those variables really make sense to me? Did I speak such a different language
then than I do now?
To complete the recipe for these folds, add a dash of raw imagination. Basalt flows
plugged up the canyon in the vicinity of Lava Falls about a million years ago, to depths
of at least seven hundred feet. Could this lake have had anything to do with formation of
the Muav folds? The river, with its depth measured in tens of feet, is able to wet only
superficial horizons of bedrock. Pressures at the bottom of the lake would have been much
higher, capable of driving water much deeper into the pores of the Muav. The lake would
have been much deeper at Kanab Canyon than back upstream at Buck Farm. This difference in
depth and pore pressure might explain the presence of folds in one place and not the
The added pore pressure may have been enough to adequately moisten the Muav,
and to allow the layers of limestone to glide over one another in response to laterally
compressive stresses transmitted from the weight of the nearby canyon walls. In geologic
terms, the folding could have occurred in a relatively short time; the mathematics and
field observations both point to an elastic, not plastic, type of
deformation which theoretically occurs at a rate independent of time. According to this
lake hypothesis, the folding happened a million years ago, before the river
eroded down to its current level; subsequent erosion has exposed the folds where we now
see them in the canyon walls.
Hugh Rieck and I each rowed a boat on my third research trip. I had already
written the thesis, but my research trip. I had already written the thesis, but my advisor
hadnt gotten around to reviewing it before our departure from the Ferry. I was
resigned to field checking the work without his comments. Two motor boats pulled alongside
near Bedrock, to gab and to ogle the two ladies who were working with us. One of the
boatmen was distracted from his ogling by a small plane flying at Redwall level (legal if
ill-advised in those days). His distraction turned to downright orneriness when the plane
returned and skimmed by at river level. As it passed, a package popped out and a parachute
blossomed. I rowed over and fetched the tupperware container: my advisor approved. I
jumped up and down, yelling incomprehensibly something that sounded like, I have a
masters degree! I have a masters in Science!
Wesley Smith, using a bright flashlight and an even brighter imagination,
once treated us to a remarkably pornographic shadow show, projected onto the wall across
from Ledges. The passengers howled. That same wall brought many of my thoughts about the
Muav folds into sharper focus. My equations had predicted that the limestone ought to fold
when units about a meter thick were able to slide past one another. Look at that wall next
time you drive by: the Kanab Canyon Member of the Muav does indeed seem to have parted
into slabs separated by silty planes about a meter apart. Well, about a meter. Whose
imagination was more fertile, mine or Wesleys?
Equations do not define the Muav folds; they are not a substitute. Instead, I
think of them as a light held inside the rock, an illumination that allowed me to inspect
the edges, ponder the details, and conjecture about the origin of one tiny aspect of the