Have you ever camped in a sandy place? Was the wind blowing and grit collecting in your ears, hair, tent and food? Despite being annoying at times, sand and its “parent,” sandstone, are fascinating to contemplate.
One very distinctive sandstone layer in the Flagstaff area is the Coconino. Wonderfully exposed in the walls of Walnut Canyon National Monument, Oak Creek Canyon and, of course, Grand Canyon, it generally forms buff to tan-colored cliffs hundreds of feet high. These often display dramatic steep-angled crossbeds — diagonal lines that represent ancient sand dune migration driven by prevailing winds. This vast Coconino desert covered much of today’s northern Arizona and New Mexico from about 275 to 272 million years ago.
Tiny quartz and occasional feldspar grains were tumbled and rounded by the wind. After topping a dune crest, these grains came to rest on the leeward side. As more and more sand collected in layers on the lee, the cross-bed pattern was produced.
In time, the Coconino dunes were buried beneath thousands of feet of other sediments. Compaction and cementation by minerals like silica, precipitated from groundwater lithified the grains together, forming sandstone.
Millions of years later the Coconino Sandstone was exposed as forces of uplift and erosion removed the younger formations and carved canyons through the Permian layers.
Early 20th century geologists noticed the crossbeds and wondered about their origin -- were they eolian (wind) or fluvial (flowing water)? They also occasionally found fossilized tracks on the bedding planes. Some were tiny: spiders, scorpions and millipedes. But larger tracks showed claw marks, perhaps made by early rodentlike reptiles or synapsids.
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There was something curious about the larger tracks. Almost all travelled up the crossbed planes (evidenced by small sand mounds at the rear of each footprint, formed as the animal pushed off and upward), not down. Wild ideas emerged: the vertebrates had wings: they climbed the dune and flew or glided off. Or the creatures migrated in one direction and never returned. Perhaps the crossbeds formed underwater: animals swam to the shallow dune crest, walked up it, then swam off.
But Eddie McKee, one of the first Grand Canyon park service naturalists, decided to investigate. In a dune field east of the Canyon, he attempted to dig a trench. The sand kept collapsing so he hired some Navajos to haul water in and soak the sand. Then Eddie got out his shovel and went to work. Once the trench was finished, he could easily see that the sand was deposited in sloping crossbeds just like those in the Coconino. One mystery solved. Now about those tracks.
Eddie performed some experiments, placing lizards on dunes and watching. When the dunes were dry, larger lizards made distinctive tracks going uphill; but when they came down, avalanching usually destroyed the tracks. Tracks that eventually became fossils were probably moistened by mists or fogs drifting in from the ocean west of the Coconino Desert before they were covered with more sand. The Nambian Desert in southwestern Africa is a modern analog of this setting.
From his early interest in sand and sandstone, Eddie spent much of his geologic career studying eolian desert dunes, coastal dunes, ergs (sand seas), and tidal flats around the globe, earning the nickname “Mr. Sand Dunes.” He continued digging trenches including a 40-foot deep one at White Sands, New Mexico. His lifelong research culminated in a global sand seas monograph featuring NASA Landsat satellite photos. Eddie’s contributions to geology — that the Coconino was an eolian sand sea, and that Coconino tracks only go uphill because descending tracks were not preserved—are universally accepted today within the geologic community.
So the next time the wind is howling and little sand dunes are accumulating on your eyelids, take solace in remembering the remarkable stories told by each individual grain of sand and by a crossbedded sandstone cliff.
If this article has piqued your interest, you might enjoy reading Grand Canyon Geology, Second Edition edited by Stanley Beus and Michael Morales, Oxford University Press, 2003, and The Grand Canyon: Monument to an Ancient Earth edited by Carol Hill, Gregg Davidson, Tim Heble and Wayne Ranney, Kregel Publications, 2016.