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Driving over caterpillars

In October 2022, a collecting trip that crossed the Basin and Range province required driving over a lot of caterpillars. It was caterpillar, then a valley, then another caterpillar and another valley. In the Basin and Range Province, there are over 500 of them. Caterpillars, that is.

In an 1886 report from the Secretary of the Interior to Congress, geologist Clarence E. Dutton wrote, "The great belt of Cordilleras coming up through Mexico and crossing into United States territory is depicted as being composed of many short, abrupt ranges or ridges, looking upon the map like an army of caterpillars crawling northward. At length, about 150 miles north of the Mexican boundary, this army divides into two columns, one marching northwest, the other north-northeast."

The northwest marching caterpillars are the ranges of the Basin and Range. If you are driving in an east-west direction, it's up, down, up, down, over these ranges. This geomorphic province has interior drainage, with no through-flowing rivers to take sediments and dissolved minerals to the sea. As a result, many of the basins, the valleys between the ranges, have salt flats in their bottoms. These are seasonally wet or dry and are called playas, the loosely applied Spanish word for "beach." The Bonneville Salt Flats in Utah and the salt flats at Badwater in Death Valley are classic examples. 

On the first part of the route we took US Highway 6. At Ely, Nevada, we detoured onto US 93 and then US 93 Alternate, heading for the Great Salt Lake. It was caterpillars and valleys all the way. Just west of Wendover, Utah we coasted down the last caterpillar into the last valley, where I-80 crosses the Bonneville Salt Flats and skirts the south edge of the Great Salt Lake.


The easternmost edge of the Basin and Range is the Wasatch Front, behind Salt Lake City, Utah, where eastbound, you climb onto the Colorado Plateau. We zigzagged around on this trip, with the ultimate aim the K-T boundary exposures in Montana and North Dakota. We had found the boundary clay in 2021, but it was snowing. That clay formed when the meteorite that finished off the dinosaurs blasted tons of pulverized crust into the atmosphere. This fell back to earth and in some places, where it wasn't washed away by rain, it has formed a roughly two-inch layer of clay. K stands for Cretaceous, from the German spelling, the last time period with dinosaur fossils. T stands for Tertiary, where sediments lack dinosaur fossils entirely. The two inches of clay mark the extinction.


It was an interesting collecting trip, and most of the boxes are still waiting to be unpacked. Wondering why there is a long gap between these posts? Well, we're busy. We still have 20 boxes of lava types from Hawaii to unpack, but that's another story. And keep an eye out for the K-T clay.

April 04, 2023 by RC de Mordaigle
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