One long-standing question about the Sandhills is “How old are they?” Digging deeper, more specific questions emerge. If the question is “How old is the sand, and where did it come from?” the answer is easy, because many of the sand grains are radioactive. In the dunes, 99 percent of the sand grains are abraded crystals of two minerals—quartz and feldspar. Feldspar crystals (like bananas, potatoes, and our bodies) contain radioactive potassium. Age dating based on radioactive elements in feldspars from Rocky Mountain Front Range granites indicates that the crystals are between one and two billion years old, and so are the rounded feldspar crystals in the Sandhills.
Another question is “How old are individual dunes?” The answer is made possible by a new dating technique called optically stimulated luminescence (OSL) that also makes use of radioactivity. Instead of dating the sand itself, OSL answers the question “How long has this sand grain been buried in the dark?” While buried in the dark, silt- and sand-size quartz grains steadily accumulate energy from their radioactive (potassium-rich) components. When these grains are exposed to the sun, or to light of a certain wavelength in the lab, they release that energy. The longer they have been in the dark, the more energy is released (and measured). Moving like tanks, dunes recycle their sand as the sand grains continue to migrate downwind. Sand eroded from the upwind side is moved over the crest and deposited on the downwind side, only to be eroded, perhaps a hundred years later, from the upwind side. Today, in a big, stabilized dune where would we find the “oldest” (longest-buried-in-the-dark) sand? The answer is, at the base of the trailing (upwind) edge. So far, the oldest sand we have found (in a core retrieved from the southern Sandhills) was dated at about twenty-four thousand years, which is about ten thousand years before the last of the Ice Age glaciers left the vicinity of Des Moines, Iowa.
A third question—not answerable by direct evidence—is “How long has there been a large dune field on the central plains?” This question is difficult to answer because the way dunes move limits the use of OSL. It is likely that there were big dunes in Nebraska long before twenty-four thousand years ago, but their sand grains saw daylight and lost their stored energy because they were completely recycled as the dunes migrated.
We think the Sandhills dune field may be as old as two million years. During a large portion of Nebraska’s (and North America’s) geologic history, the surface was the floor of shallow inland seaways, less than a thousand feet deep. The last of these seaways retreated to the Gulf of Mexico and to the Arctic Ocean about sixty million years ago, and the global climate started to cool. Twenty-five-million-year-old wind-deposited rocks comprise some western Nebraska cliffs and can be seen up close at Agate Fossil Beds and Scotts Bluff National Monuments. These sandstones (as well as the loose sand of the Nebraska Sandhills) record time periods when portions of North America’s core became deserts. During the Ice Ages, much of the central United States was buried by thick glaciers. These glaciers did not reach the Sandhills, but they got close. The oldest glacial deposits in northeastern Nebraska and western Iowa are more than two million years old, the youngest are about fourteen thousand years old.
The most reasonable hypothesis for the age of the Sandhills dune field brings us back to plants. The cold climate that allowed continental glaciers to move so far south would have greatly reduced the length of the growing season. This severe cooling (starting about two million years ago) led to dramatic thinning of plant cover on the silt- and sand-rich land surface and ushered in a wind-swept desert landscape in north-central Nebraska. Sand grains in the unprotected river deposits started jumping and crashing, and silt grains took their long flights. Thus, dune fields and loess deposits soon started to take shape.
A final question involves the future of the Sandhills: “When and why were the Sandhills most recently destabilized?” This is, by far, the most important question for future humans and for future pocket gophers and other prairie creatures. In 1983, Jim Swinehart, together with Tom Ahlbrandt and Dave Maroney, published the first strong evidence that Sandhills dunes were actively migrating much more recently than the departure of nearby Ice Age glaciers. This claim is sobering, because if dune formation simply coincided with glaciation, there would be little need to worry about the return of desert conditions unless glaciers returned. Early attempts to answer this question (pre-OSL) had to rely on carbon-14 (C-14) dating, but there is not much carbon preserved in a sand dune. Bob Seger’s song “Night Moves” is relevant here: “Working on mysteries without any clues.”
After a tip from soil scientist Chuck Markley, we found a place (not within a dune, but in front of one) with a lot of carbon. Some of the wet meadows between the large dunes of the central and western Sandhills are actively accumulating peat—a deposit almost entirely composed of carbon-based molecules. Coring at Jumbo and Cutcomb Valleys in the late 1990s revealed (5 feet below the squishy muck on the surface) a persistent thin layer of sand sandwiched between older and younger peat. Radiocarbon dates from the peat above and below the sand at the two locations (21 miles apart) indicated that the sand accumulated seven hundred to a thousand years ago. The interpretation is that frequent severe regional droughts led to a lowering of the water table and drying of the meadows. The nearby dunes lost their grass cover, and winds drove the now-unprotected sand out onto the surface of the (no-longer-wet) meadows. When the period of frequent droughts ended, the water table rose and rewetted the meadows. Peat again accumulated, ultimately leaving the sand buried under five feet of peat. More recent dating of the sand with OSL has confirmed the earlier results with C-14.
OSL dating of sand from Sandhills dunes and of silt deposits from loess exposures south of the Sandhills has also shown that, during the last ten thousand years, there have been at least four episodes of active dune migration, interspersed with times when—like the last seven hundred years—dunes were stable. Apparently, even after the glaciers were long gone, droughts caused the plant cover to lose its “grip,” allowing the dunes to start migrating again. Another thing that OSL has revealed is that episodes of dune activity coincide with episodes of loess accumulation. This makes sense if dust storms really are triggered by bouncing sand grains.
Happily, the Sandhills were not mobilized during the dust bowl of the 1930s. The droughts of seven hundred to a thousand years ago, indicated by C-14 and OSL dating, probably persisted for at least several decades, maybe several centuries; their cause is unknown. There are hints that changes in atmospheric circulation shifted over the Atlantic and Pacific Oceans, with long-distance effects on the flow of moist southerly air from the Gulf of Mexico that brings rain and snow to the central Great Plains. Historical records indicate that the Sandhills may be greener now than they were a hundred years ago, so those sand grains won’t start bouncing . . . not in the near future, anyway. But they are certain to do so at some point in the future.