Date of this Version
Swinehart, J. B., and R. F. Diffendal, Jr. 1989. Geology of the pre-dune strata. In Bleed, A., and C. Flowerday, eds., An atlas of the Sand Hills, Conservation and Survey Division, University of Nebraska, Resource Atlas No. 5: pp. 29-42; rev. ed. Atlas No. 5A, 1990.
Over the last 98 million years, four general geological processes have acted to shape the ancient landscapes buried beneath the Sand Hills. Three of these affected the area directly, either depositing sediments on the land surface or eroding it, while a fourth took place west of Nebraska, but affected the region nonetheless.
Shells of clams, oysters, and numerous other kinds of creatures similar to forms that live today in the seas are preserved as fossils in the chalks, limestones, and shales that form the oldest rocks beneath the Sand Hills that will be described. These deposits indicate to geologists that seas once covered the area and that marine (oceanic) processes of sediment transport and deposition were active during this time.
After the seas retreated, streams played a major role in depositing the strata overlying the marine sediments. Nonmarine fossils of plants and animals occur in alluvial (stream-deposited) sands, gravels, and silts laid down within the last 37 million years. The third process that actively shaped the area was eolian (wind-blown) deposition. Winds have played a major role in the geologic evolution of the entire Great Plains. They have formed the Sand Hills and considerable volumes of older, nonmarine strata. In addition, erosion by both wind- and water-related processes has helped carve the ancient landscapes buried beneath the Sand Hills.
Volcanic activity far to the west and southwest of Nebraska is the fourth process that affected the Sand Hills. Fine-grained debris from volcanic eruptions was carried by high-altitude winds and deposited on the plains. The work of rivers, winds, and, to a lesser extent, volcanoes continues today. Ash from the recent eruption of Mount St. Helens in Washington was carried eastward, and fallout from the eruption cloud was deposited far downwind, including a light dusting of the Sand Hills.