A moonless, star-splattered night sky rimmed by massive cliffs of the Virgin River Gorge and a campfire that could probably be seen from the international space station were the last things I remembered as I drifted off to sleep at Cedar Pockets campground, the growling of eighteen-wheelers grinding their gears on nearby Interstate 15 a constant background drone.
The stars had disappeared with the morning light but the roar of traffic remained. We drank our cowboy coffee, ate our instant oatmeal, packed up our gear, and wedged ourselves cozily into our three rented vehicles for the second day of the Southern Utah University Geology Symposium field trip. We left the trailer at the campground where we would retrieve it on our way home
We drove south beyond Mesquite, Nevada to Whitney Pocket, an area near Lake Mead rich with petroglyphs etched in windblown desert sand deposits of the ≈190-180 million-year-old Early Jurassic Aztec Sandstone (one and the same as the Navajo Sandstone of Zion National Park).
The geology of the southern Nevada/Lake Mead area is complicated by exposed Mesozoic thrust faults, exhumed tilted fault blocks of Precambrian crystalline basement rocks, extensive Tertiary volcanism, strike-slip faults of the right-lateral Las Vegas Valley shear zone, and the left-lateral Lake Mead fault system.
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| Image courtesy of Geologic Tours in the Las Vegas Area |
Sedimentary rocks of the Horse Spring Formation were a result of crustal extension in southern Nevada between ≈17-10 million years ago. The rocks most likely were deposited in large basins that contained periodic lakes during a time of less arid desert conditions. There is much tectonic history with these rocks, notably regarding movement of Frenchman’s Mountain, presently near Las Vegas, 50 miles along a fault-guided path.
However, what we were looking for on this day was evidence to support a theory that the Colorado River had flowed north along a course into Canada ≈20 million years ago. The basal Horse Spring Formation would be the conglomerates of the ancient Colorado River channel (click here for James Sears’ abstract from symposium).
