The Inland Dunes on the Missouri

Last week I explored a couple of parks near Omaha, Nebraska, and thought I would take a couple of posts to talk about them.

West of the Missouri River and in the Great Plains region, you may expect unvaried topography.  Yet in Neale Woods Nature Reserve I found myself 200 feet above the nearest cornfield, on what was likely a thousand-year-old dune made of loess.

In geological terms, loess (pronounced “luss”) is material transported and deposited by aeolian (wind) action, consisting predominantly of silt-sized particles. The most familiar source of loess would be from the combination of drought and poor farming practices that became known as the Dust Bowl in the 1930s. However, the hills I stood on and the larger ridges across the river have an older and more distant origin.

As glaciers and ice sheets expanded across the landscape during the last hundred thousand years, the accumulated weight of ice scoured earth down to bedrock, grinding away stone and carrying the powdered rock along with it.  During warmer periods, the ice retreated, with melt water transporting the “rock flour” downstream. Eventually, the silt settled into glaciofluvial mudflats. Much later, this floury dust dried, to be picked up by the wind (becoming loess) and carried aloft for hundreds or thousands of miles before coming to rest in blankets across much of the Midwest and down to the Gulf of Mexico (map).  There are other sources of loess, including dust blown in from deserts, volcanic fields, or other sites with fine particles and the wind to carry them.  The upper layers of loess in Nebraska likely come from deserts or perhaps previous loess deposits, sent windborne again as local climates fluctuated.  This deposition formed high dunes of silt and fine sand, and dates to between 1000 and 11,000 years ago.

Loess tends to have a high mineral content which, with some weathering and centuries of extraction by plant roots, creates exceptionally fertile soil.  This is one reason for the productivity of the Midwest and Great Plains. Unfortunately, loess is highly erodible by wind or water, and without a thick prairie sod to protect it, tons of fertile soil are lost on each farmed acre every year.

While easily eroded, loess tends to maintain vertical integrity provided there is some protection at the top.  That may explain why I came across these cliff faces.  Where Man’s desire for level roads conflicts with dune topography, the hill gets shorn.  In more sandy terrain, that 20-foot wall would have slumped all over the road in short order.

Ranging up to 200 feet, these forest-covered dunes stand long and narrow, dissected by gullies. They might have eroded down to low berms if not pinned in place by oaks and prairie grasses. Whether for the dune geology or the prairie ecology, they are worth a look if you ever get out that way.