Day Three: Boulder, Colorado to Salt Lake City, Utah
We headed out of Boulder early in the morning, and as my father drove first I clutched my thermos of tea and looked over the map for the day. I hadn’t ever looked at southern Wyoming with any interest before, but we were going to be driving through most of it. The mountain ranges and high plateaus in Wyoming were created by the same processes that created the Colorado Rockies: the Laramide Orogeny that elevated the American West between 70 and 60 million years ago. The atlas had the Continental Divide marked in bright yellow, and to my surprise it seemed to acquire a split personality just north of the Sierra Madre Mountains, skirt a vast empty area on the map, and then reunite south of the Wind River Range.
A few hours later I took the wheel in Rawlins, and signs announced that we were crossing the Great Divide for the first time today and entering the Great Divide Basin.
If I had poured out my thermos onto the ground in Rawlins, it would eventually flow towards the Atlantic.
If I dumped that same tea out in Green River, on the western side of the Great Divide basin, it would flow towards the Pacific.
But if I poured it out by one of the many oil derricks dotting the Great Divide basin… it would go pretty much nowhere.
So why does the defining drainage divide of the continent have a hole punched in it in the middle of Wyoming?
Google was less useful than usual on this question, so I had to wait until I got my journal access through Oregon State (SCORE!) to do some serious database sleuthing. And even there I couldn’t find much – I guess there aren’t many scientists considering the middle-of-nowhere Wyoming. However I did find a 2010 article by Paul Heller, Margaret McMillan, and Neil Humphrey at the University of Wyoming and University of Arkansas that presented a potential cause.
These authors propose that the Great Divide Basin originally drained through Sand Gap, on the northeast side of the basin, to the Platte River around 50 million years ago in the early Paleogene period. (shown in figure 1 below) They based this on a comparison of bedrock elevations at the 4 most likely historic outlets of the basin.
The next crucial step is climate: The high elevation but relatively low relief of the Wyoming basins meant that they have gotten little precipitation throughout the past 50 million years compared with the neighboring high peaks to the east. This leads to a difference in erosion between the basin areas and the majority of the area of the North Platte River headwaters and watershed. More sediment was removed north and east of the Great Basin, causing the Earth’s crust to bounce back in those areas by a few hundred meters over millions of years. The science-y ways to name these processes are differential erosion and isostasy.
By around 10 to 8 million years ago, this uplift east and north of the Great Divide basin tilted the basin to the south just enough that water no longer had any reason to flow out of Sand Gap. Instead, it flowed into lakes with the basin itself and evaporated, causing the saline soil that confounded settlers’ effort to cultivate the area. Figure 6 from Heller et. Al, below, shows the direction of that tilt…
Going back to the tea theme earlier in the post, I found it easier to think about this in terms of a teacup (the Great Divide Basin) with a chip in the edge (Sand Gap) on a balance (the earth’s crust). This is a farfetched analogy, but hang with me here. In the beginning the balance is evenly weighted – tea is poured into the teacup and flows out the chip in the side, and there is an equivalent weight on the opposite side of the balance that keep the bar level.
However as weight is removed by the North Platte River from the northeastern side of the balance, the opposite side tilts down to the southwest. In this tilted position the bottom of the chip is at a relatively higher elevation than before, and with the cup being refilled less often than previously tea can no longer flow out of the chip. Instead it evaporates there and leaves behind residue, much like what I find on Monday morning when I don’t wash out my mug before leaving my grad student office on the previous Friday…
After almost two hours of driving through the basin we drove past the sandstone formations of the Rock Springs uplift and passed the *other* continental divide into the Green River Basin.
Around the Wyoming/Utah border we started descending from the Rocky Mountain plateau down into the Basin and Range geologic province. The western side of this plateau gets relatively much more rain, so we saw our first tree-covered mountains since Laramie earlier in the day!
Unfortunately, my valiant little Honda Civic had some seriously weird noises going on after we swerved and braked hard to avoid an accident that day.
The downside: We had to spend an extra day in Salt Lake City while a mechanic checked it out.
The upside: We have family there, and they had the time to take us up into Little Cottonwood Canyon in the Wasatch Range to play tourist.
More details about the fantastic landscape around the Great Salt Lake to come in Road Trip Part 3!
Heller, Paul L., Margaret E. McMillan, and Neil Humphrey. “Climate-Induced Formation of a Closed Basin: Great Divide Basin, Wyoming.” Geological Society of America Bulletin 123, no. 1–2 (2011): 150–157.