Mammoth Site Geology
The sinkhole formed approximately 26,000 years ago when a cavern in the Minnelusa limestone collapsed. The collapse caused a vertical shaft called a breccia (BREH-chee-uh) pipe to form. The ground surface of Spearfish Shale, a rock strata, also caved in. This opened a sixty-five foot deep 120 X 150 foot sinkhole. This type of sinkhole is called "karst" (named after a region in Italy). The breccia pipe provided a chimney-like opening for a warm artesian spring to percolate up through the rocks to create a steeply-sided pond.
Enticed by the warm water and pond vegetation, the mammoths entered the pond to eat, drink or bathe and then could not escape. The mammoths were unable to find a foothold to scale the steep shale banks. Trapped in the pit, the mammoths ultimately died of starvation, exhaustion, or drowning.
The watering hole, active for about 350-700 years, slowly filled with layers of drying silt, sediments, and dying mammoths. The mud, which had aided in trapping the mammoths, now entombed and preserved the mammoth remains.
Eventually the sinkhole filled, and the artesian spring diverted to the lower elevation of Fall River, as the river cut deeper in the valley floor. Over thousands of years, the "hardened mud plug" inside the dried-up pond has remained stable. The surrounding dirt, the soft red Spearfish shale, ultimately eroded, leaving the sinkhole a hill.