The largest field of sand dunes in the Western Hemisphere is located southeast of the Black Hills region and occupies much of western Nebraska. The Sand Hills, developed on the northernmost occurrence of Late Tertiary High Plains deposits (light yellow), are evident on the map as a fine-scale hummocky texture. The dunes formed as recently as 10,000 years ago, during and after Pleistocene time. Under modest changes in climatic conditions, these dormant features could shed their vegetative cover and again become active.