Among those marks: footprints.

When the climate shifted at the end of the Ice Age — a process that took thousands of years and was not linear or smooth — Lake Otero contracted.

The water receded.

The lakebed sediments dried, hardened, and were eventually buried under accumulating layers of gypsum-rich material.

The footprints, pressed into the wet lakebed mud when people and animals walked the shore, were sealed into those sediments and preserved in the dark, under increasing layers of overburden, for thousands of years.

The gypsum dune field that covers much of the basin today formed from the minerals left behind as Lake Otero evaporated — gypsum dissolved from the surrounding rock, concentrated in the lake water, and deposited in massive quantities on the lakebed as the water disappeared.