As the Spearfish shale that lined the sides of the pit became waterlogged, it began to ooze like mud and crumble. Smaller animals that entered the pit escaped successfully, but the heavy, awkward mammoths could not get a firm enough footing to climb out of the sinkhole once they had entered it. They either died of starvation, or got so weak that they could no longer hold their heads up out of the water. Sliding under the surface, they soon drowned. Their remains sank to the bottom of the pit where silt and sand covered and protected the bones.
Scientists estimate that the sinkhole acted as a trap for as long as seven hundred years.
In that time it is estimated that over 100 mammoths died in the sinkhole. While three of the animals were woolly mammoths, the majority found have been Colombian mammoths. Colombia mammoths were slightly larger than their well-known cousins, the woolies with no hair. They resembled a modern Asian elephant with the same small ears, but were larger and taller with longer tusks. Mammoths were not the only creature found in the trap.