The selenium which is found in the soils of the Great Plains has been derived from geological formations, especially those formations deposited during Cretaceous time. The sedimentary rocks which make up the Cretaceous formations are the most important selenium bearers. The selenium which was laid down in these sedimentary formations has been carrie3d through the soil-forming processes and is found in the soils of certain areas. Plants growing ion these soils absorb the selenium which further finds its way into the tissues of animals that happen to feed upon the plants.
South Dakota State University
M, Alvin L.; Olson, Oscar E.; and Searight, Walter V., "Selenium in Rocks, Soils, and Plants" (1939). Agricultural Experiment Station Technical Bulletins (1939-2011). 14.