Road Trip Part 2: Wyoming’s Great Divide Basin

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.

Day 3 itinerary

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Welcome to the Great Divide Basin! A whole lot of flat sage brush 7,000 feet in the air…

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.

Heller et al figure 1 captionHeller et al figure 1

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…

Heller et al figure 6Heller et al figure 6 caption

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.

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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.

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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!

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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!

Source Cited:

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.

Road Trip Part 1: Why are the high plains so flat?!

Day 1: Memphis to McPherson, Kansas

Day 2: McPherson to Boulder, Colorado

My father and I pulled out of Memphis early one Monday morning and I, having procrastinated packing into the wee hours of the morning, slept through Arkansas as he drove. I only woke up when the tail end of Hurricane Harvey dropped a solid curtain of rain on the car somewhere around Forrest City. I’ve already explored Arkansas’ landscape a bit in my Petit Jean State Park blog post, so I don’t feel too guilty about skipping it in this account.

I’m familiar with the Ozarks in western Arkansas and eastern Oklahoma, but the endlessly rolling hills of Kansas were a new phenomenon to me. The early pioneers weren’t exaggerating when they described a “sea of grass”!

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With most of my geological education focused in Tennessee, Montana, and the highways between the two, I have to admit my knowledge of the Midwest was mostly limited to knowing it is FLAT. In Iowa this endless pancake of a landscape was bulldozed by glaciers, but what about the non-glaciated, pancake-flat parts of the Great Plains? What’s with them?

USF glacial drift mod

Glacial drift map from University of South Florida https://etc.usf.edu/maps/pages/4500/4546/4546.htm

It turns out that unlike areas where glaciers shoved sediment in from the north, the sediments under the High Plains of Kansas, Oklahoma, and Colorado came from the west.  In order to understand these plains, we have to turn to their opposite – the Rocky Mountains. Luckily my dad and I were driving straight to them! We met my friend Alyssa for dinner on Day 2 in ground zero of the eroded source of the High Plains – Boulder, Colorado.

Boulder is perched right on the boundary between tilted layers of 315-70 million year old rocks rocks, and the masses of Precambrian granite continental basement that were uplifted during the Laramide Orogeny that reshaped the American West between 70 and 60 million year ago. In that period, the Farallon oceanic plate dove under the North American plate at an unusually shallow angle, resulting in volcanism unusually far inland.  Additionally, the friction between this subducting plate and the overlying continent formed the Colorado Plateau as it rumpled the North American plate like a rug on a hardwood floor. The figures below show the shallow angle of the Laramide Orogeny, a cross-section view through Boulder, and a map view of those tilted layers exposed in Boulder along with their names and ages.

Diagram from https://en.wikipedia.org/wiki/Laramide_orogeny

cross-section view of Boulder….

 

 

 

 

 

diagram from http://bcn.boulder.co.us/basin/watershed/geology/

Map-view of exposed rocks in Boulder….

diagram from http://bcn.boulder.co.us/basin/watershed/geology/geolmap.html

Unfortunately I didn’t have time to go hiking with Alyssa in the foothills, but two weeks later my sister went adventuring with her in the Flatirons! Heather took tons of amazing photos including several in the Flatirons area where those tilted rocks are dramatically exposed

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View of the Flatirons, (c) Heather van Stolk

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My wonderful sister brought me along in sketch form! I can’t wait to go there in person! image (c) Heather van Stolk

Nowadays we see those layers of sedimentary rock cut through and exposed at the surface, but originally they would have extended westwards and upwards to cover those older granite rocks.  However, over the 60 million years since mountains were uplifted streams have been hard at work eroding those rocks from the higher areas and washing them downstream to lower elevations. This effect is called a “sediment apron” of a mountain range.  The Rocky Mountains are enormous, so that sediment apron extends all the the way through central Nebraska! The diagram below shows the general process of the sediment removal from the mountains, deposition on the High Plains, and gradual erosion from the High Plains.

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Schematic cross-section of the Colorado Front Range and adjacent High Plains (from Anderson et al., 2012, Figure 4). ‘LGM’ stands for “last glacial maximum”, when glaciers had their maximum impact on the North American landscape.

Cross section source here

This thick apron of sediment is the cause of the gradual, sloping rise up to the base of the Colorado Front Range of the Rockies. This area is in the “rain shadow” of the Rockies where precipitation is pretty scarce, and so the High Plains have not been as highly dissected by streams as the Cumberland Plateau which I am more familiar with and have written about on this blog. The relatively soft, homogeneous composition the sediment causes the High Plains area to be eroded gently and gradually by what streams there are.

I found a helpful introduction to the High Plains thanks to the writer at “In the Company of Plants and Rocks”, who did a great write-up of their trip through the high plains of Colorado.

In retrospect, the Plains landscape would be much easier to understand if we had been driving west to east instead. However after Boulder we had our sights on Salt Lake City and took the northern route across Wyoming to get there.

Up next:

Why is there a basin on top of the Continental Divide?

What’s with the colorful jumble of rock in the Wasatch Range?