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”!

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?

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

View of the Flatirons, (c) Heather van Stolk

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.

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?

 

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