What bedrock can tell us about ancient cooking and culture

The mortar, pestle, and cutting board you use in your kitchen today are modern tools rooted in ancient tradition. Long before stainless steel and ceramic surfaces, people used manos and metates – stone tools designed to grind plants and seeds into edible forms.

A mano is a handheld stone, used in tandem with a metate, which is a flat slab or natural rock depression. Some of these bedrock grinding surfaces date back as far as 15,500 years and appear in clusters across archaeological sites around the world.

Recently, scientists from the University of Utah, working through the Natural History Museum of Utah (NHMU), have begun using innovative techniques to extract microscopic plant residues from these ancient bedrock metates.

By doing so, they’re uncovering details about the diets and lives of the people who used them.

Looking deeper into ancient tools

“People have lived here for time immemorial and have been processing native plants on ground stone tools for a long time too,” said archaeobotanist Stefania Wilks, a research assistant at NHMU and graduate student at the University of Utah.

Her work focuses on identifying food and medicinal plants processed in the past to understand both traditional lifeways and how landscapes have shifted over time.

Wilks is part of a team working with NHMU’s Curator of Archaeology, Lisbeth Louderback, also a professor of anthropology at the University of Utah. Together, they’re focusing on something extraordinarily small: starch granules.

These are microscopic structures within plant cells used to store energy. Some are so tiny that even the largest are still smaller than a tenth of a millimeter.

Because they’re so small, starch granules can’t be seen with the naked eye. The researchers have to extract them from surfaces where plants were once processed – like pottery, basketry, and now, bedrock metates.

Plant matter in ancient bedrock

Louderback suspected that bedrock metates, despite being exposed to the elements, might still contain preserved plant matter.

Specifically, she believed that deep crevices in the stone might be hiding starch granules that people unknowingly pushed into the rock while grinding food.

“Through their actions of grinding and mashing, people would have forced these starches down deeper into the stone,” Wilks explained.

The bedrock metates themselves come in many forms. In Utah, they’re often long grooves in sandstone. In other areas, they may be bowl-shaped or resemble a modern-day mortar.

While they may not catch the eye like arrowheads or pottery, they are commonly found in groups and can be rich with information.

“They aren’t sexy like an arrowhead, but they still contain valuable information about what plants people processed in the past,” said Wilks.

Digging into the dirt – microscopically

The research team focused on southern Oregon, where metates are found along basalt outcrops and surrounded by thousands of petroglyphs.

These sites are also home to culturally significant plants – especially geophytes, which have underground storage organs like roots and tubers.

Archaeologists once believed that ancient people went to these uplands mainly for hunting. Wilks wanted to test whether the grinding surfaces were actually used for plant processing. The team collected samples from both the surface and the crevices of the metates.

First, they used an electric toothbrush and water to scrub the top layer of the stone. Then they added a deflocculant, a cleaning agent similar to detergent, to release particles trapped deeper in the rock.

Another round with the electric toothbrush followed. For comparison, they performed the same process on nearby rocks that weren’t used for grinding.

When examined under a microscope, the results were clear. The surface samples showed almost no starch granules – nor did the control rocks.

But the material from inside the metate crevices? Hundreds of granules.

“It increased our confidence that what we were seeing was direct evidence that different plant species with starchy organs were processed on the metate,” Wilks recalled.

Ancient diets recorded in bedrock

With evidence in hand, the researchers began the meticulous process of identifying the starches.

Wilks compared each granule’s shape and structure to modern examples of plants still found in the region. It was a slow and careful job, but worth the effort. The researchers identified starch granules from several plant families.

One common group was the carrot family, including plants known as biscuit root. They also identified wild grasses, likely wild rye, and members of the lily family.

All of these plants have been – and in many cases still are – important food sources for local Indigenous communities.

Ancient bedrock holds valuable archives

“Starch analysis is helpful in studying human diets of the past because some plant parts don’t preserve in the archaeological record,” Wilks said.

Root vegetables, for example, decay far faster than seeds or grains. By recovering starch granules from stone, archaeologists have a new way to piece together the food stories of ancient peoples.

These bedrock metates, often overlooked in archaeological surveys, are proving to be valuable archives of cultural history.

This new technique shows that even the most weathered stones can hold on to traces of the past. Each granule recovered tells a story – not just about what people ate, but how they lived, traveled, and used the land around them.

These simple tools, still hiding in plain sight, connect us to generations long gone but not forgotten.

The full study was published in the journal American Antiquity.

Image Credit: Stefania Wilks, University of Utah

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