The grass on the Sandhills looks permanent. Ranch roads wind between dunes that seem to have stood forever. They have not. In 1983 Jim Swinehart, Tom Ahlbrandt and Dave Maroney published the first strong evidence that the dunes had moved in recent geologic time, and the geologists who followed found that some had moved within the last thousand years.
The sand itself is old. Its quartz and feldspar grains weathered out of Front Range granite one to two billion years old and were carried east off the Rocky Mountains. The dune field may be as much as two million years old, and the oldest sand dated by luminescence, from a core in the southern hills, is about 24,000 years old. Most of the dunes in their present shape formed within the last 15,000 years, and dating shows at least four episodes of dune migration in the last 10,000.
The medieval dunes
The most recent was medieval. In peat cores from Jumbo and Cutcomb valleys, 21 miles apart, David Loope, Swinehart and Joseph Mason found a layer of blown sand about five feet below the meadow surface, laid down 700 to 1,000 years ago. The youngest dunes, on the southeastern edge, are about a thousand years old, and they run northwest to southeast, the wrong direction for today's moist southerly winds. In a 2006 paper in Science, a University of Nebraska team argued that dry southwesterly winds took over for decades or longer, the wetlands dried, the grass died back, and the sand blew. The droughts of the last century, the 1930s included, were not severe enough to do it.
The sponge
The same sand that moves so easily also drinks. Rain sinks into it instead of running off, and below the hills lies the deepest part of the High Plains Aquifer, whose largest geologic unit, the Ogallala Group, is sediment carried east by streams from what is now Colorado and Wyoming. The base of the aquifer here is generally more than 500 feet down, the water-bearing sediments are more than 1,000 feet thick in places, and about two-thirds of the water in an aquifer that reaches from southern North Dakota to Texas lies under the Sandhills.
Where the water table reaches the surface, it makes lakes, more than 1,500 of them. Many fill basins the wind scooped out during the droughts. Others formed when moving dunes dammed a valley. Crescent Lake, on the western edge, was blocked by sand only about 4,500 years ago, and dunes across Ancestral Blue Creek raised the water table behind them. The groundwater moves quickly for groundwater, perhaps 30 feet a day, and comes out cool and steady in the Loup, the Dismal, the Elkhorn and stretches of the Niobrara. Smith Falls is a small piece of it, a spring-fed creek dropping off the Niobrara's south wall.
Charles Bessey believed the Sandhills had once carried forest and planted the national forest at Halsey to prove they could again. The record the geologists pulled from the meadows tells a different story, of hills that swing between grass and blowing sand as the rain comes and goes.