In the midst of the Oglala National Grasslands lies a section of sparse desert where the effects of water and wind over millions of years have carved the dusty earth into dramatic, fantastical shapes. Though the terrain is littered with a wide variety of formations, from upended chunks of broken sandstone to deeply eroded hills of ancient volcanic ash, it is the mushroom-shaped hoodoos that gave the place its name.
Toadstool Geologic Park sits in the bed of a now-vanished river that flowed roughly 45 million years ago. The thick layer of sandstone built up over time in the river’s channel incorporated layers of volcanic that blew in from geologically active regions to the west, creating the streaks of red and green that can be seen today. The wet, subtropical environment attracted many varieties of ancient wildlife whose remains turned to fossils in the ever-compacting sediment. The area eventually dried up around 26 million years ago, starting the erosion process that created the alien contours of the present-day landscape.
Toadstool Geologic Park features a one-mile-long interpretive loop trail (an informative site map is available at the entrance). Visitors can scramble (gently) over spectacular cliffs and weather-blasted geological formations, and see the fossilized remnants of ancient horses, saber-toothed cats, brontotheres, and more.