Pack Rat Middens Offer Clues of Range Creek’s Past

By Jude Coleman

A massive complex Pleistocene pack rat midden (in the alcove in the center of the image) in a cave in eastern Nevada.

Where some might see a humble mound of rat poop, Marti Sorensen sees a valuable ecological record. The piles, called middens, contain more than just feces; they’re built over dozens of years by pack rats as the rodents accumulate bits of plant debris and other refuse. Middens preserve well — some are thousands of years old — which means they often hold tiny fossils that researchers can excavate. They’re particularly useful in arid places, where vegetation is sparse and water is scarce. 

“The plants and animals that are surviving there are so evolved and adapted to living in such a harsh environment. It's beautiful to see that there's actually so much going on there,” said Sorensen, a paleoecologist at the University of Utah. She works with pack rat middens in Range Creek Canyon, aiming to piece together a history of the canyon’s vegetation and climate. The project has been in the works since 2005, but Sorensen joined in 2022 as an undergrad student during a summer research program. She continued on with the project for her master’s program. 

Capitol Dome and Wilcox Ranch. Image courtesy of Marti Sorensen.

Located in eastern Utah, Range Creek Canyon holds countless examples of a rich archaeological history, such as granaries, dwellings and rock art. The Natural History Museum of Utah manages a field station in the canyon, which is used in collaboration with interdisciplinary experts from the University of Utah. From around 400 CE to 1,200 CE, the canyon was occupied by people of the Fremont culture. But then they vanished. Archeologists aren’t exactly sure why the group left the canyon, or, shortly after, the Great Basin. Rat middens can reveal some clues about the Fremont people’s lifestyle, for example their agricultural practices. While Sorensen’s work highlights the height of their occupation in the canyon, it also focuses on the broader ecology of the canyon throughout the last 9,000 years.

Pollen under a scope. Image courtesy of Marti Sorensen.

One way that paleoecologists study ecological history is through fossilized pollen grains. Different types of plants have unique pollen structures, which can be seen under a microscope. That means researchers can determine the kinds of plants growing in an area in the past by looking at ancient pollen. They can also do this with macrofossils: bits of preserved plant matter that can be observed without a microscope. For example, pieces of leaves or shreds of stems. For pollen and plant matter to persist as fossils instead of decomposing, they need to land in the right conditions. In this case, anoxic — or oxygen poor — environments are ideal, said Sorensen. Researchers often turn to mucky sediment cores for plant matter preserved in this way, such as from lake shores or bogs. But Range Creek Canyon has few continuously wet areas. 

"That's where the pack rats come in,” said Sorensen. “Their fecal matter essentially encases the plant materials and preserves them, so it creates those anoxic conditions that the bog cores do.”

Stains in alcoves of a cliff in Range Creek Canyon show the presence of pack rat middens. Image courtesy of Marti Sorensen.

Scientists have used these poop-pile time machines for decades. Pack rats form their middens communally over many generations. They bring leaves, seeds, bones and other debris — including garbage from humans — into the pile, defecating and urinating as they go. The urine and feces act as a glue, holding the midden together as it grows. Wind-borne pollen grains also land on the middens and get incorporated with the waste. 

For Sorensen’s master’s thesis, she used pollen and macrofossil samples from 45 pack rat middens throughout Range Creek Canyon. Finding the middens was a collaborative effort started by Sorensen’s mentors and colleagues around two decades ago, she said, including TKTK. A good place to look was under rock overhangs, which protected the piles from rain and weather. Some middens also hid in crevices within the rock, only accessible by rope. 

After collecting samples from the middens, the hardened urine and feces needed to be dissolved to reveal what fossils were inside. Macrofossils could be picked out from the sludge using a screen, while the pollen required a few more steps for retrieval. Once the fossils were separated, the team dated pieces of vegetation — mainly pine and juniper needles — to determine how old the middens were. The youngest was around 100 years old, while the oldest was over 9,000 years old.

“That one was definitely a huge surprise for us,” she said. “That sample itself was definitely very rare.”

Image courtesy of Marti Sorensen

Sorensen had two main goals with the work. First, she wanted to see how the fossilized vegetation in the piles compared to the fossilized pollen. She also wanted to know how the pollen record in the middens stacked up to the pollen record from existing sediment cores, taken from a handful of bogs. She found that among plants adapted for dry conditions, there was a mirror between the pollen and macrofossils. Middens were ideal for showcasing local vegetation, while the sediment cores between reflected regional plants with airborne pollen and wetland plants.   

“If we use them together, we can tease out more of a localized story to that region,” she said.

Her findings also highlighted a bump in aridity in the mid-Holocene, around 6,000 years ago, revealed by a shift toward drought-tolerant plants. Now working on her PhD, she plans to explore those shifts more by looking at seasonal plant patterns stowed in the middens. That can help answer questions about the climate in the canyon and how it changed over time. She also plans to look for relationships between the plants and pollen found in middens and their proximity to archeological sites. 

Sorensen would like to thank her mentors on this project, including:

• Prof. Larry Coats – Associate Professor (University of Utah)
• Dr. Shannon A. Boomgarden - Director of the Range Creek Field Station (NHMU), and Adjunct Assistant Professor (University of Utah)
• Dr. Andrea R. Brunelle – Professor (University of Utah), Director of the RED Lab (University of Utah)
• Dr. Mitchell J. Power - Associate Professor (University of Utah), and Curator of botany at the Garrett Herbarium (NHMU)
• Range Creek Field Station staff & students