Seen from Highway 2, the grassy landscape of the Sandhills is gently sloping, very extensive, and quite pleasant. Seen from a satellite (using Google Earth or Google Maps), the intricate repeating patterns made by giant stabilized sand dunes, shallow lakes, and green meadows are mind-boggling. The three of us have done geologic work in the Sandhills for a total of 117 years. Jim was the first of us to become obsessed with sand-sized particles (~1978), I fell for the crunchy stuff in 1981, and Joe joined us in 1990 (but, full disclosure, he has continued to flirt with silt). We’ve learned that the Sandhills are comparable in size, shape, and composition to the biggest deserts in the world today (which are also mind-boggling on Google Earth). The dunes rest on sand and gravel deposited by rivers, such as the Platte, that moved eastward from the Rockies across the Great Plains to the Gulf of Mexico. The trillions of sand grains in the dunes came from Colorado and Wyoming, but their journey toward New Orleans was paused when winds (blowing alternately from north and south) swept them out of dried-up riverbeds. The geologic history of the Sandhills spans just a couple million years—not exactly deep time, but our studies (and those of colleagues and grad students) have revealed a remarkable story involving repeated climatic and biologic shifts.
Modern-day sand dunes are restricted to deserts (as in North Africa and Arabia) and to coasts with broad beaches (think North Carolina’s eastern edge or Michigan’s west coast). Dunes form in deserts and along beaches not because they are so windy but because all other continental environments (not covered by ice) have plants that protect land surfaces from wind erosion. Most of today’s deserts lie far from the ocean, in continental interiors. Today no portion of Nebraska is a desert, but the stabilized dunes in the Sandhills are clear evidence that it was a desert when the dunes were built.
Without land plants, strong winds at the bare desert floor can erode dry sandy sediment and drive the sand along the land surface. These grains don’t billow up into clouds; most travel less than a foot above the surface. As they bounce, they form half-inch-high wind ripples. Silt grains are often too small to be raised directly by the wind, but each crash of a bouncing sand grain ejects several silt grains up and into the turbulent wind. Gravity quickly brings sand grains back to the desert floor, but silt grains can’t fall through the air nearly as fast. Gusting winds carry them high in the sky, forming dust clouds hundreds of feet high. Like on today’s Great Plains, the direction and strength of the ancient winds that built the Sandhills varied with the seasons. Reversals of wind direction allowed dunes to reach great heights—they built upward instead of spreading laterally. Silt that fell back to the desert floor (after the wind died down) was raised by the next windstorm that got sand bouncing again. Eventually, when silt grains fell on a grassy surface (beyond the edge of the sandy desert), they stayed there: the grass acted as a baffle that slowed the wind, so there were no crashing sand grains to eject the silt. This explains why the Sandhills are just sand, and why broad areas south and east of the dunes are underlain by sand-free silt (aka loess). It accumulated via fallout from many thousands of dust storms. Besides wind reversals, another factor limiting the size of dune fields is that, although bouncing sand grains can cross dried-up riverbeds, they cannot cross rivers that frequently flood. For example, at Sutherland, Nebraska, the landscape north of Interstate 80 and the Platte River is composed of sand; south of I-80 it’s all silt, clear evidence of a river barrier at the south edge of the dune field.
How do we really know that these hills are wind-blown sand dunes and were not deposited by glaciers or rivers? First, they are very similar in size and shape to modern sand dunes. Second, they are entirely made of fine to medium sand—rivers and glaciers can carry (and deposit) clay, silt, sand, gravel, and boulders, but the wind carries only sand (that builds dunes) and silt (that is deposited as loess). Third, our coring and observations of the walls of excavations and blowouts revealed that the interior of the hills are made up of broad thin layers that are not flat, but instead slope 15 to 30 degrees. These “crossbeds” slope because they were deposited on the downwind-sloping surface of migrating dunes. On some days, these soft, smooth surfaces were trampled by bison. Those sloping layers—distinctively deformed by cloven hooves—were buried during the next windstorm.
The biggest dunes are in the central and western parts of the Sandhills, near Hyannis and Bingham. Up to 400 feet high and several miles long, their southern slopes are steeper than their northern slopes. These are called transverse dunes; their unequal slopes show they were deposited by winds from the north. Crossbeds in the smaller dunes on the Sandhills’ eastern edge (Burwell, Calamus Reservoir) were deposited by seasonal winds of nearly equal strength—the deposits of one season were not destroyed by winds of the next season. These are linear dunes.
Because, on this planet, water falls from the sky pretty often, streams have shaped most ice-free land surfaces, forming rills, gullies, canyons, and valleys as they charge downhill. Regardless of the climate—hot, cold, wet, dry—there is geologic evidence that streams (by the thousands) have been draining eastward across the Great Plains for more than 30 million years. River-laid sands and gravels of the famous Ogallala Formation (which lies directly under most of the central and southern plains) represent about a third of that time span. The Sandhills rest directly on river-deposited sand and gravel. The scale of the dunes is impressive, but compared to the tonnage of sand that has been carried across the plains by the Platte, the amount of sand that comprises the Sandhills is—we hate to say it—trivial.
In the national parks of southern Utah and northern Arizona, there are big cliffs that display wind-deposited sandstones that range in age from about 300 million to about 150 million years. In Zion National Park, the Jurassic Navajo Sandstone forms cliffs up to 2,000 feet high. That formation stretches from southeastern California to central Wyoming. Will the deposits of the Nebraska Sandhills ever be cemented into rock and reach thicknesses like that? No. Plate tectonics explains not only the lateral movement of Earth’s crust and upper mantle but also their vertical movement. During the Jurassic, the western United States—near the edge of the North American Plate—was slowly sinking while wind-blown sand was being slowly deposited. Cementation of the sand took place while it was water-saturated and deeply buried; uplift and canyon cutting started about 10 million years ago. Near the center of a continent and far from a plate margin, the Great Plains region is not subsiding, so great thicknesses of dune sand can’t accumulate, and, sad to say, removal of the loose dune sand by rivers is more likely than their cementation into rock.