Mammoth Site Geology
The Creation of the Mammoth Site's Karst Sinkhole 
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.
