Rain and wind hit the north west coast of Scotland as Zachary Killingback inspected a rock stuck in the mud. It wasn’t just an old stone: weighing nearly a quarter of a million tons and longer than a jumbo jet, the boulder had crashed into its position some 1.2 billion years ago, meaning it may be the most ancient landslide never found on earth.
Killingback, then a masters student at Durham University in England, wanted to know what happened in the catastrophic few seconds when the huge boulder gave way. Rocks have fallen off cliffs since the Earth was cold enough to form, but few ancient falls have been found in the geological record. This in Scotland offers a window into what was happening on the planet before animals took their first breath, before plants stretched their roots into the ground, before modern continents even took shape.
As a team describes in a new study published in the journal Geology, the boulder plummeted less than 50 feet into aqueous sediments, the force of the impact broke the rock and injected mud into the fractures. While the cliff it fell from has eroded, the landslide remains. Each rock has a history, and scientists are tasked with taking what is known about our planet̵
“It shows you how many amazing details you can extract from a block of rock if you get close closely,” says Cara Burberry, a structural geologist at the University of Nebraska-Lincoln who was not involved in the study. “They documented it really beautifully.”
Northwest Scotland is a marvel to behold, with turquoise waters pouring into small beach niches nestled along the coast. The rolling landscape records billions of years of our planet’s history when supercontinents formed and shattered, and rivers and lakes began to flow.
“It’s Disneyland for the Brits geology,” 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 student field trips. “If it weren’t for COVID, I would be on these outcrops today,” says study author Bob Holdsworth, a structural geologist at Durham University.
During one such student trip, Holdsworth and his colleagues noticed that something was wrong with a block of rock near the village of Clachtoll. The boulder is part of the Lewisian gneiss, a three billion-year-old rock that was crushed under intense pressure as it formed, causing the minerals to line up in overlapping layers known as foliation. In most of the region, these strata tend to be northwest-southeast. But the layers of the boulder are rotated 90 degrees.
Holdsworth and his colleagues felt that the twisted layers and other curious features of the rock fractures might have been the result of a precipitous plunge, but they needed more data to support the case. So Killingback accepted the challenge of researching his master’s thesis.
Collecting clues in the field
The rockfall site was Killingback’s favorite trip as a college student. Excursions organized with his classmates were often challenging for Killingback, who is autistic. Navigating throngs of students, elaborating rapid-fire instructions, and the barrage of sensory stimuli in the field presented constant obstacles.
Clachtoll’s journey, however, was different. Rather than a professor guiding them through geological places, he says “you have somehow been liberated – I loved it so much”.
He returned for his master’s work in September 2016 to carefully map the structure of the boulder. The wind slashed the hills as the rain pounded hard, but Killingback did each task as efficiently as possible before rushing back to the safety of his car to review the notes and trace the next steps. Even in his vehicle, the winds made themselves known. “I thought I would fly away every night,” he says.
On his last day in the field, traveling college students invaded the site. Killingback finished his work as the group ran through the rocks, then returned to the lab to reconstruct a glimpse into the planet’s ancient past.
A destructive descent
The geological history that Killingback and his colleagues uncovered goes something like this: About 1.2 billion years ago, a basin was forming on what is now the north-west coast of Scotland. Lakes filled the region and rivers flowed through, depositing layers of muddled rock and red sediment. A powerful earthquake may have shaken the ground, possibly from the earth that extended into the basin, and the boulder fell off a cliff. It twisted slightly during the fall, causing its inner layers to orient at right angles to the rest of the region’s rocks.
When the rock landed, cracks formed from the top and bottom. Today the cracks are filled with red mud and slight differences testify to the fall. The sediment in the cracks on the top is stratified, a sign that over time it has been progressively washed out in the cracks. The cracks in the bottom, however, do not have such layering and are filled with much finer sediment, indicating a rapid injection from the force of the impact. Burberry calls this model a “smoking gun” for rockfall.
The rock slipped on impact, highlighted in part by a large crack in the front of the boulder, Killingback says. The team also brought small cores of rock to the lab to test how much force it takes to separate the rock. That figure helped determine how far the rock fell: probably less than 50 feet.
“How did we not notice this before? It makes a lot of sense, “says geologist Catherine Mottram of the University of Portsmouth, England, who brings her undergraduate students to the region every year but was not part of the study group.” I literally sat on this. [boulder] several times and ate my lunch. “
Stories hidden in the rock
For Killingback, the biggest challenge was translating his ideas into a written report, a difficulty not uncommon for people with autism. “I basically think in pictures,” he says. “I had my whole thesis in mind … like a silent documentary.”
It took another two years to turn that mental film into a written short story, he says. “I was just secretly wishing someone would come up with a technology for me to put a USB stick in my head and download everything.”
Killingback devised coping strategies, including explaining the various pieces of the study to his mother and then immediately writing his words. Eventually, it paid off, resulting in an illuminating study.
“It’s kind of a unique opportunity to see a process that we always know is happening but rarely see preserved in the rock record,” says Christopher Jackson, a geologist at Imperial College London who was not involved in the new research.
Part of the fun of geology is the detective work involved in identifying each rocky clue. In Killingback’s paper, Webb says, “the joy of understanding this kind of bleeds from the page.”