Ten thousand years ago, when the weather in eastern Idaho was much cooler and wetter than it is today, lush vegetation was grazed by soon-to-be-extinct one-humped camels, mammoths, pony-sized horses, and large bison. These animals may have gone for a drink along the sandy shores of Mud and Market Lakes, about 20 miles southwest of where the St. Anthony Sand Dunes lie today.
Although Mud and Market Lakes still exist, they shrank considerably when the climate warmed at the end of the last ice age. As the shoreline sands dried out, that sand, as well as sand from floodplain deposits of the Snake, Teton, and other rivers, began to be blown into what we now know as the St. Anthony Sand Dunes.
Why did the wind-blown sand develop into dunes rather than being spread in an even layer across the landscape? The answer has to do with the very nature of sand. Geologists define sand as a particle small enough to be moved by the wind but too heavy to be carried very far. When a sand grain is lifted by the wind, it bounces along the surface. If the surface is hard, like the black, volcanic basalt rock exposed in some areas near the dunes, then bouncing sand grains just keep bouncing as they are pushed by the wind—like a basketball bounced on a gym floor. If, instead, the surface is soft because it is already sandy, then bouncing sand grains will lose momentum—like a basketball bounced on the beach. Because sand grains bouncing on sand slow down, if there happens to be a small pile of sand—perhaps accumulated in the wind shadow of a bush or a rock—then sand grains will tend to slow down and stop on that pile, and can eventually accumulate into a dune.
Although wind moves individual sand grains into dunes, it cannot move entire dunes. When sand grains are blown over the crest of a dune, they fall into the dune’s own wind shadow and accumulate on the other side of the crest. How, then, does the dune ever move forward? As more and more sand accumulates at the crest of a dune, it forms a steeper and steeper slope. When that very steep slope, or that slip face becomes unstable, gravity pulls the sand down in a small avalanche. You are familiar with slip faces and have created your own avalanches of sand if you have ever tried to climb up the downwind side of a dune! One slipping avalanche at a time, the dunes have marched steadily to the northeast, at about 10 feet per year, to reach their present location 20 miles from where they once began.
The shape of a dune depends on the quantity of available sand and on wind conditions. Most dunes at St. Anthony are barchan dunes, which form where sand is in limited supply and the winds blow steadily from the same direction. Barchan is Arabic for ram’s horn, and barchan dunes are so named because they form sharp crescents that end in a pair of horns that face in the direction of sand movement. At St. Anthony, the horns of the dunes face northeast, so the winter winds from the southwest seem to move the sand.
The St. Anthony Sand Dunes are the largest tract of sand dunes in Idaho, covering approximately 175 square miles. The active dune field trends northeast for 35 miles and is five miles wide, with dunes ranging from 50 to 400 feet tall. Older dunes, now stabilized by plants, cover much of the surrounding area. The rounded hills on the north side of the dunes, the Juniper Buttes, are extinct volcanoes.