We’re finally on the road to Alaska!  We spent the first night at Flaming Gorge Reservoir in northeastern Utah.  The Uinta Mountains are one of the very few east-west trending mountain ranges in the Lower 48.  Geologist-explorer John Wesley Powell led the first expedition through this gorge in May of 1869, the same month the first transcontinental railroad was completed at Promontory Point, Utah.  Powell named this gorge for the “flaming” red rock formations towering above him as he floated down the Green River (see photo).  He also developed his theory of "superposition" to understand why the Green River would flow across the hard Precambrian rocks in the core of the Unitas instead of around the range.  Today Utah provides great road signs along US 191 that identify each of these formations as you are driving up into the mountains, but down in geologic time.  From south to north, the highway runs from the Eocene Green River Formation (50 million years old) to the PreCambrian Uinta Formation (800 million years old).

 

 

From the Uinta Mountains, we crossed the desolate Green River Basin to the rugged Wind River Mountains.  Near the town of Pinedale is a 14,000 year-old terminal moraine that is famous to North American glacial geologists.  This moraine marks the greatest extent of the last advance of mountain glaciers in the Rocky Mountains, so this glacial time period is named the “Pinedale.”  In the U.S. Midwest and Canada, this same glacial period is called the “Wisconsin.”

Northwest of the Wind River Mountains is another northwest-trending range called the Gros Ventre Range.  The Gros Ventres are at the eastermnost front of the Wyoming Overthrust Belt, a north-south trending series of “thrust faults.”  Unlike strike-slip faults, thrust faults have nearly flat-lying or low-angle fault planes that resemble a new deck of playing cards.  If you split the deck in half and turn the two halves face up, you can see the cards in each half.  Place one half against a wall and push the other half against the first half.  Some of the slick second-half cards slide across the top of the stationary half and others slide between the stationary cards.  This analogy is that a thrust fault pushes one geologic formation over another.  Most of the overthrust belts in the Rocky Mountains, such as the foothills west of Calgary on the road to Banff, were pushed eastward by lhe island arcs crashing into the western margin of the North American Plate.  These forces pushed older rocks over younger rocks, the reverse of normal geologic sequencing.  Near our campground on Granite Creek, a tributary of the Hoback River, 150 million year-old Nugget Sandstones was pushed up over 60 million year-old Hoback Formation shales and sandstones (see photo of Flying Buttress Mountain taken from our campsite).  Great camping and interesting geology combined into a single visit to the mountains just outside Jackson, Wyoming.  You can’t beat that if you are a geologist!

Next time: Lunar camping—a day (and night) at Craters of the Moon National Monument.