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The Secret Life of River Stones

Zion National Park

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The Secret Life of River Stones

Hiking Wildcat Canyon to Right Fork of North Creek, my ranger friend, Jon, raises a finger to his lips signaling silence. Stopping abruptly, I scan the broad expanse of Navajo Sandstone, dotted with fist-sized iron concretions like a red-spotted toad’s raised nubs. Following Jon’s line of sight, I search for the anticipated animal: mountain lion, rare bird, perhaps bobcat? “If you’re very quiet and they think you aren’t looking,” whispers Jon, “you can see them moving around and hear them talking to each other.” Humored, I realize Jon’s mystery animal was not a rare species, but the abundant tribe of round iron concretions dotting the landscape before us. I could easily imagine them, carrying on in a bustle of activity until our human presence rendered them silent and still.

All stones, although not alive, are a foundation of life. Aldo Leopold wrote in A Sand County Almanac, “Land then, is not merely soil; it is a fountain of energy flowing through a circuit of soils, plants, and animals.” Stones lining Zion’s Virgin River remind me of this fountain of energy and the circuit of life created by their many minerals. Along the Virgin, point bars and shoreline deposits reveal stones ranging from tiny pebbles to bowling ball-sized cobbles. All these stones share some of life’s building blocks, mainly oxides and minerals, whose elements bond together in unique combinations.

Concretions form when water dissolves and precipitates minerals from Navajo Sandstone, coating the inner surface of air pockets, resulting in an iron marble impregnated in stone. Over time, less-resistant sandstone erodes, leaving a carpet of loose, round, dark nodules. Erosion transports many concretions and other loose stones down slopes, into tributaries, and, finally, on a ride down the Virgin River.

Stone combing, like beach combing, is a discovery tour where sandstones, conglomerates, limestones, and metamorphic rocks splash the riverbed with diverse color, pattern, and texture. Mineral compositions provide a glimpse of geologic, climatic, and life forces in action when each rock was deposited. Rambling the Virgin River at Big Bend, looking at the sea of stones, it’s easy to enter a hidden doorway to the past, like coming across a centuries-old photo album buried in the attic, and spending an afternoon immersed in lives of another time.

At Zion, another time is exactly what stones reveal. From the Mesozoic‘s 240-millionyear- old Moenkopi Formation to recent Cretaceous lava flows, Virgin River stones derive from a smorgasbord of paleoenvironments. I imagine the original setting of each stone; a wide shallow sea bustling with fossil oysters; a vast desert with hot sand fiercely blown; an oasis of tiny plants and massive dinosaurs on the verge of extinction; a forest of early evergreens destined for petrification; or lava, glowing red as steam jets hiss.

During creation of Zion’s Lava Point, vents and cinder cones spewed lava at more than 2,000 degrees Fahrenheit. Eroded downhill over thousands of years, most Virgin River lava stones look like black sponges—full of pits and holes—evidence of hot gases escaping as hot rock met cool air. Black lava rocks, more resistant to the river’s erosional forces than weaker sandstones, are abundant in the riverscape, even though the massive Navajo Sandstone far outweighs lava in the surrounding landscape.

When sandstone chunks enter the river-rock tumbler from overhanging cliffs, chemical and physical processes dissolve the glue— primarily iron oxide and calcium carbonate— that fill intersticies between sand grains. The river’s tumbling action smoothes jagged rocks to rounded stones. As stones wear, minerals are released, beginning again the cycle of creating soil, and the circuit of energy continues. If you stop along the Virgin’s stonescape and are silent a moment, you might hear particles of newly emerging soil giggling with joy.

Barb Graves

Description

Unlock the hidden geologic mysteries of river stones with Park Ranger Robin Hampton as she reads an article written by Park Ranger Barb Graves for the park's Nature Notes.