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Earth Notes: Ephemeral Footprints

Four human footprints preserved in stone.
National Park Service
Footprints at White Sands National Monument

This is Earth Notes…

Footprints made tens of thousands of years ago may look like they’ve been erased by time and weather, but like invisible ink, they can sometimes reappear under the right conditions.

These kinds of ephemeral trackways have been found in White Sands National Park in New Mexico and near the Great Salt Lake in Utah. But it’s likely many more exist.

They form when a footprint fills with coarse sand and becomes covered in clay or silt. The material inside the print dries at a different rate than the ground surrounding it. So a rainstorm or a hard frost can make the shape visible for a short time.

David Bustos, resource manager at White Sands National Park, describes it like Easter egg hunting. Half-hidden tracks left by people and Ice Age animals have stories to tell. Giant ground sloths rear up on their hind legs when startled by a human, and children often stop to splash in puddles.

Bustos once visited a familiar spot in the park and was astonished to find thousands of tracks made by people and animals glittering in a hard frost. Then the sun rose, and they vanished.

Ephemeral trackways have much to teach about the past. And they’re significant to present day Indigenous peoples whose ancestors made the footprints. But they’re in danger of disappearing for good because of erosion from drought and climate change. Scientists are working to photograph these kinds of sites and study them with ground-penetrating radar before they’re lost forever.

This Earth Note was produced by KNAU and the Sustainable Communities Program at Northern Arizona University.

Melissa joined KNAU's team in 2015 to report on science, health, and the environment. Her work has appeared nationally on NPR and been featured on Science Friday. She grew up in Tucson, Arizona, where she fell in love with the ecology and geology of the Sonoran desert.
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