What do Council Bluffs, Iowa, the Battle of Vicksburg and the Yellow River all have in common? Loess.
Loess comes from the German löß, and has a common root with the English word, loose. It’s a geological term for a light silty dust blown by the wind that accumulated into thick layers and hills. These deposits, often taking on a pale yellowish-brown or buff color, are capable of covering huge territories with a blanket of dust hundreds of feet thick, formed over numerous repeated cycles of water and wind.
I first became aware of loess at the appropriately named Loess Hills of western Iowa, a remarkable example of this type of formation. My in-laws lived in a suburb of Omaha, Nebraska at the time, on the Iowa side of the river. Their home was located atop one of the hills. This was near the city of Council Bluffs, Iowa, where the "bluffs" of the city’s name were comprised of loess (they even hold an annual Loessfest each year).
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Notice the terrain east of the Missouri River floodplain. These are part of Iowa’s classic Loess Hills area. Anyone driving along Interstate 29 through here can look to the east and glimpse these unusual sedimentary hills at pretty much any random place. They may not seem terribly remarkable until one ponders their interesting geological history and compares them to the flatness of much of the rest of this part of North America.
SOURCE: Loess Hills Ridge by FordRanger, on Flickr
via Creative Commons Attribution-NoDerivs 2.0 Generic (CC BY-ND 2.0) license
As the Iowa Department of Natural Resources explained,
The Loess Hills of western Iowa were formed from 20,000 to 10,000 years ago of finely ground windblown silt from the glacial deposits. As the Pleistocene glaciers melted, the Missouri Valley became a major channel for tremendous amounts of water. Each winter season, as the quantity of melt water was reduced, large areas of flood-deposited sediments were left exposed to the wind. Silt, clay and fine sand were lifted by the wind, carried to the east and deposited.
Particles of loess display some interesting and unusual properties. Their angular structure combined with a propensity for extreme crumbliness and quick erosion results in characteristic bluffs with steep ridges and and rapid drop offs, as displayed in the photograph. Yet, loess is extremely stable when dry and held in place by prairie grasses. The Loess Hills of western Iowa tower up to 250 feet (75 metres) above the Missouri River floodplain quite contently until people cut into the hills for their own purposes. Any soil exposed directly to the elements quickly crumbles away.
I would hear residents remark about the uniqueness of their Loess Hills whenever I visited the area, although "unique" isn’t completely correct. They’d always append a qualifier to their description, "except for some place in China." The place in China was never named, however to their credit they understood that loess formations were unusual.
They were on the right track. The two most significant deposits of loess happen in western Iowa and in China, although the phenomenon can be found to lesser degrees in many other parts of the world. The whole reason it’s called loess rather than loose (or some variation in a Chinese dialect) is because the term derived from deposits found in Germany’s Rhine River Valley, as just one example.
There are even other places with loess in the United States, for instance the eastern bank of the Mississippi River at Vicksburg, Mississippi. An vital element of the 1863 Battle of Vicksburg during the U.S. Civil War — the siege of Vicksburg — took place when Union troops could not dislodge Confederate forces dug-in securely atop the steep loess bluffs along the river. The siege lasted more than a month until Confederate troops exhausted their supplies and had to surrender.
What about China, though?
China’s equivalent of the Loess Hills is called the Loess Plateau. As noted by Wikipedia, it covers 640,000 square kilometres (250,000 square miles)… "almost all of Shanxi and Shaanxi provinces, as well as parts of Gansu province, the Ningxia Hui Autonomous Region, and the Inner Mongolia Autonomous Region." That’s about the same size as the U.S. state of Texas, or the Canadian Provinces of Alberta or Saskatchewan!
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It’s particularly prevalent along the upper and middle Yellow River watershed. It is loess that contributes the characteristic yellow color to the Yellow River.