You Don't Need a Weatherman to Know Which Way the Wind Blows
Bob Dylan famously sang, "You don't need a weatherman to know which way the wind blows." And while that might be true for a breezy summer afternoon, what if you want to know which way the wind blew 190 million years ago?
For that, you don't need a weatherman. You need a geologist.
If you've ever traveled through the American Southwest—particularly around the Zion National Park area or the Grand Staircase-Escalante National Monument—you've likely marveled at the towering, swirling cliffs of the Navajo Sandstone. These massive rock formations aren't just beautiful; they are ancient weather reports frozen in stone, courtesy of a geological phenomenon known as cross-bedding.

Reading the Rocks: What is Cross-Bedding?
When you look closely at the cliffs of the Navajo Sandstone, you won't just see flat, horizontal lines. Instead, as geologists Janice Gillespie and Christa Sadler note in their
Geology Road Guide to Cottonwood Canyon, you'll see "swooping cross-beds—steeply dipping, diagonal layers within the rock."
These elegant, sweeping lines are the fossilized remnants of ancient sand dunes. But how do shifting sands become a permanent record of the wind?
It all comes down to how a dune moves:
- The Build-Up: Wind blows loose sand up the gentle, windward backside of a dune.
- The Avalanche: Once the sand reaches the crest, it cascades down the steeper front side (the slip face).
- The Migration: As this process repeats over thousands of years, the entire dune slowly migrates in the direction of the wind, leaving behind layered, diagonal deposits.
Because these diagonal cross-beds always angle downward in the direction the sand cascaded, they act as permanent, petrified weather vanes. They mark the exact location of the dune's front face at various points in time, pointing directly downwind.

The Jurassic Forecast: Prevailing Winds to the South
When geologists look at the cross-beds in the Navajo Formation, a clear pattern emerges. Most of these steeply dipping layers tilt toward the south.
By simply reading the angles in the rock, geologists can definitively say that during the Jurassic period, the prevailing winds sweeping across this massive desert blew steadily southward.

The Jurassic Forecast: Prevailing Winds to the South
Perhaps the most mind-bending part of the Navajo Sandstone story isn't just how the wind blew, but what it was blowing. You might assume the sand came from local rocks breaking down over time, but recent geological studies have uncovered an incredible cross-country journey.
The sand grains that make up these spectacular western cliffs actually originated on the opposite side of the continent:
- The Source: The sand was eroded from the ancient Appalachian Mountains in the eastern United States.
- The River Trip: Massive ancient river systems located to the north transported this Appalachian sand thousands of miles westward.
- The Wind Shift: Once the rivers deposited the sand, those prevailing Jurassic winds picked it up and blew it relentlessly southward, accumulating into the colossal dune field we now know as the Navajo Sandstone.
So, the next time you find yourself staring up at the swirling, swooping lines of a sandstone cliff, remember: you are looking at the remnants of an ancient Appalachian mountain, carried by rivers, and sculpted by a Jurassic wind. You just needed a geologist to read the forecast.
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