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Geo-Joint: Iowa’s Loess Hills

Posted on August 31 2016

Iowa pops onto the front page every four years as those now-famous presidential caucuses start the mad cycle to become or be re-elected to the most powerful position on the planet. Outside of that spectacle and the state’s fame for corn and pig production, how much do you know about the physical look of Iowa? Today’s subject focuses on western Iowa, where the state’s topography is not the billiard table plane that characterizes its majority. In fact, there is some notable rumpling of the land in the west, along the Missouri River, which forms Iowa’s border with Nebraska. Nebraska itself isn’t known any better than Iowa for the verticality of its landscape, so how does western Iowa come to have hills?

No, it wasn’t earthquake uplift; Iowa is pretty stable. The process began back in the Ice Ages, most recently in the neighborhood of 15,000 years ago. In Iowa, the Wisconsinan Glaciation Period saw massive slabs of ice push south as far as Des Moines in tongues as thick as 1,300 feet. Of course the ice sheets to the north were on the order of ten times that. The weight of these enormous sheets and glaciers essentially ground the underlying and bordering rock to a powder. As the planet turned to a warmer phase, the ice melted and rivers flowed heavily down channels like the valley of the Missouri River. Thick with the fine sediment called “glacial flour,” the Missouri deposited its load downstream, along its banks and floodplains. During winter periods the flow off the ice sheets would slow, which exposed huge mudflats. Dry winter conditions were accompanied by strong westerly winds which carried this light material into western Iowa. Such windblown, or eolian, glacial deposits are called “loess” from the German word for “loose,” a reference to the crumbly nature of the silty, powdery soil. The word, in case you were wondering, is pronounced more or less as “luss.”

During three glacial periods over nearly 160,000 years, the process repeated itself. An area up to 15 miles wide and 200 miles long received this gift of the wind in successive winters, many thousands of times. The heavier particles travelled the shortest distance, piling up in high steep bluffs near the river. Finer material carried farther, laying down deep deposits of easily drained soil with high mineral content, that eventually became the fabulously productive farmlands of Iowa. The deposits of the somewhat weightier material built up smooth dunes along the Missouri River to a height of 200 feet or more. Plant growth helped stabilize these deposits and they served as home to Pleistocene megafauna such as the wooly mammoth and giant sloth. Over time, however, erosion from rain sculpted the evenly undulating deposits into the highly varied forms now known as the Loess Hills. Because the sediment is so easily carried away, gulllies often form in the hills, presenting access problems when roads or bridges collapse into them. Farming in the hilly area requires methods such as terracing and contouring in order to minimize erosion.

The accumulation of loess soils creates a relatively rare landscape, and the composition of the Loess Hills is only rivalled in its size by some older and deeper deposits laid down in central China. The presence of humans and their centuries of farming in that Asian location has reshaped their loess landscape so that it no longer reflects its origin. Indeed the common characteristic of loess deposits is their transience, whether by the hand of man or nature. While firm and stable when dry, loess soil lacks the clay content that strengthens other soils. Once saturated, it can quickly fail. But as it is worn away from the Loess Hills, slowly erasing the evidence of a huge glacial process, this good agricultural material heads for flatter, farm-covered regions happy to accept the delivery. Meanwhile, the curiously cut hills still standing are enough of a visual treat and a lesson in geography that a route joining the highways running their length has been named as the Loess Hills National Scenic Byway.

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