Geology Reference
In-Depth Information
ma-ta unleashed a tremendous rainstorm. Another, more thoughtful god, Pu-keh-eh, put his
daughter in a hollowed-out log to save her from the monstrous current rushing down to
the sea. After the floodwaters receded, she crawled out from her improvised vessel and be-
came the mother of all humanity. The story shares the same broad outline as that of Noah's
Flood—only with a matriarchal figure.
Still soaking up the geologic story, I began climbing again toward the canyon rim. At the
far end of Indian Gardens the shale gave way to the Muav Limestone, a section of carbon-
ate rock sitting directly on top of the Bright Angel Shale. The sequence of sandstone, shale,
and limestone I had just walked told me that the water deepened over time. At the upper
boundary of the Muav Limestone, another unconformity lies at the base of a formidable
cliff. This gap in the rock record documents another hundred million years lost to erosion
above sea level.
What you find directly on top of the Muav Limestone depends on your location in the
canyon. From where I was, the trail snaked up a looming wall of red-stained rock rising
several hundred feet upward. Climbing through a slot in the dark cliff, the trail followed
a fractured zone in the aptly named Redwall Limestone. Along the trail I spotted clamlike
fossils, evidence that life in the rocks was starting to become more visible, more complex,
and more familiar. Yet another unconformity lies atop the Redwall, this one perforated by
caves and sinkholes that formed when percolating rainwater dissolved soluble limestone
and made a Swiss cheese-like landscape. This unconformity represents another 25 mil-
lion years between the deposition of the Redwall Limestone and the overlying rocks of the
Supai Group.
Once above the cliff, the trail crossed back and forth countless times from siltstone to
sandstone to siltstone and back to sandstone again. I was climbing a geological staircase
made of shale treads and sandstone risers. Each step recorded the rise and fall of an ancient
sea, with harder sandstone forming cliffs, and weaker shale forming gentle ledges. This
means that the ancient sediments turned to stone before the canyon carved down into them.
If the now solidified rock had been wet and loose when the canyon was cut, the canyon
walls would reflect the strength of loose sediment rather than that of the rock. The sand-
stone would not hold cliffs because loose sand can only support slopes of at most 30 to 40
degrees, as you can see for yourself in the produce section of any grocery store by pulling
