RAIN AND WINDS buffeted the northwestern Scottish coast as Zachary Killingback inspected a rock stuck in the mud. It wasn’t just any old stone: Weighing in at nearly a quarter of a million tons and measuring longer than a jumbo jet, the boulder had careened to its position some 1.2 billion years ago, which means it may be the oldest rockfall yet found on land.
Killingback, a master’s student at England’s Durham University at the time, wanted to know just what happened in the few catastrophic seconds when the humongous boulder gave way. Rocks have fallen off cliffs ever since Earth was cool enough for rock to form, but few ancient rockfalls have been found in the geologic record. This one in Scotland offers a window into what was happening on the planet before animals took their first breaths, before plants stretched roots into the ground, before modern continents had even taken shape.
As a team describes in a new study published in the journal Geology, the boulder plunged less than 50 feet into watery sediments, the force of impact cracking the rock and injecting mud into the fractures. While the cliff it fell from has eroded away, the rockfall remains. Every rock has a story, and scientists are tasked with taking what’s known about our planet’s many physical processes to tease out tidbits of its past.
“It does show you how much amazing detail you can pull out of one block of rock if you approach it really carefully,” says Cara Burberry, a structural geologist at the University of Nebraska-Lincoln who was not involved in the study. “They’ve documented it really beautifully.”
Northwest Scotland is a wonder to take in, with turquoise waters washing into small beach alcoves nestled along the coast. The rolling landscape records billions of years of our planet's history as supercontinents formed and broke up, and rivers and lakes ebbed and flowed.
“It’s geology Disneyland for the Brits,” says Alex Webb, a geologist at the University of Hong Kong who was not involved in the study.
Generations of scientists have visited this ancient landscape, now a popular site for undergraduate field excursions. “If it wasn’t for COVID, I’d be on these very outcrops today,” says study author Bob Holdsworth, a structural geologist at Durham University.
During one of these student trips, Holdsworth and his colleagues noticed that something was off with a block of rock near the village of Clachtoll. The boulder is part of the Lewisian gneiss, rock as old as three billion years that was squeezed under intense pressure as it formed, causing minerals to align in stacked layers known as foliation. Across most of the region, these layers trend northwest-southeast. But the boulder’s layers are rotated 90 degrees.
Holdsworth and his colleagues had an inkling that the rotated layers and other curious features of the rock’s fractures might be the result of a precipitous plunge, but they needed more data to make the case. So Killingback took on the challenge for his master's thesis research.
Gathering clues in the field
The site of the rockfall was Killingback's favorite field trip as an undergraduate student. Organized excursions with his classmates were often challenging for Killingback, who is autistic. Navigating through crowds of students, processing rapid-fire instructions, and the barrage of sensory stimuli in the field presented constant obstacles.
The Clachtoll trip, however, was different. Rather than a professor guiding them through the geologic sights, he says “you were sort of let loose—I loved it so much.”
He returned for his master's work in September 2016 to carefully map the boulder’s structure. The wind tore through the hills as rain pounded down, but Killingback worked through each task as efficiently as he could before dashing back to the safety of his car to review notes and plot next steps. Even in his vehicle, the winds made themselves known. “I thought I was going to blow away every night,” he says.
On his last day in the field, undergraduates on a trip flooded the site. Killingback finished up his work as the group scampered over the rocks, then he returned to the lab to reconstruct a glimpse into the planet’s ancient past.
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