Many of the boulders scattered across the Swiss landscape did not originate where they now stand. Instead, they were carried by ice nearly 24,000 years ago. For the first time, researchers at the University of Lausanne (UNIL) have reconstructed the journeys of these giant rocks across the entire Alpine region using a groundbreaking simulation. The model makes it possible to visualize the paths taken by millions of rocks that helped shape today's landscapes.
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Why is the city of Lausanne built on a series of terraces? Why did Lake Geneva form where it did? And how did boulders weighing hundreds of tonnes come to be scattered across the Swiss landscape?
Much of today's landscape was shaped by the major ice ages, the last of which reached its peak around 24,000 years ago. At that time, vast glaciers flowed out from the peaks of the Alps like giant bulldozers, eroding the terrain, transporting huge quantities of rock and sediment, carving valleys, and sculpting the basins that would later become the lakes we know today. Researchers at the University of Lausanne (Unil) have developed the first computer simulation capable of reconstructing and visualizing, across the entire Alpine region, the paths taken by erratic boulders and rock fragments transported by glaciers from the mountain summits down to the valleys. Their findings have been published in the journal Earth Surface Dynamics .
"Until now, extensive field studies have allowed researchers to determine where many of the erratic boulders found across the lowlands originally came from. However, the routes they followed were often counterintuitive and difficult to reconstruct," says Tancrède Leger, a researcher at Unil's Faculty of Geosciences and Environment and lead author of the study. "Our simulation shows how glaciers transported rocks across the landscape and, for example, reveals the pathways by which some of them crossed mountain passes." Alongside the simulations, the researchers have developed a catalogue detailing the origin and transport pathways of glacial deposits across different regions of the Alps. "Geologists planning to investigate moraines or sedimentary basins can submit a query for a specific area, and we can provide information on the likely origin, composition and types of sediments and boulders they may encounter there."
A unique tool worldwide This research was made possible thanks to a unique tool developed by the team of Guillaume Jouvet, a professor at the University of Lausanne. Named IGM , it enables the simulation of glacier coverage and ice flow under a range of parameters, including topography and past or present climate conditions (see box). "In our case, we fed the model with millions of points representing erratic boulders and sediments, and we computed their trajectories," explains Tancrède Leger. "The velocity of ice flow influences how it entrains rock material, and the
presence of rock walls upstream affects rockfalls, which are then transported on the ice. All of these factors were taken into account." The broader aim of this work is to help address major geological questions, including the formation of Alpine environments and lakes, and more generally to improve our understanding of how landscapes form and evolve over long timescales. "This type of high-resolution simulation makes it possible, for the first time, to visualise these processes in a tangible way."
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Graphics processing units at the heart of progress
Until recently, computing the trajectories of millions of boulders and rock fragments within a numerical model was extremely challenging, as it would have required prohibitive computational power. This has now become feasible thanks to recent advances in graphics processing units (GPUs). Originally designed to render images and power video games, GPUs have become a key tool in scientific computing. Their architecture, composed of thousands of cores capable of performing many calculations in parallel, makes them particularly well suited to training artificial intelligence models. The rise of AI has therefore strongly accelerated the development of these chips. " We developed at UNIL the first open-source model harnessing the power of graphics cards to simulate glacier evolution — in the past, the future, or under various climate scenarios." says Guillaume Jouvet, professor at the Faculty of Geosciences and Environment (FGSE). Thanks to GPUs, the model is now up to hundred times faster than before.
Named IGM and developed at Unil, the tool is already used by glaciologists worldwide. It has already led to 18 scientific publications, including Tancrède Leger's study on erratic boulders, as well as work on modelling the last major glaciation of the Alps.
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Source : T. P. M. Leger, G. Jouvet, S. Kamleitner, B. D. Finley, M. Bernard, B. Allegri, F. Herman, A. Vieli, A. Henz et S. U. Nussbaumer, First Alps-wide reconstruction of LGM glacial sediment transport enabled by GPU-accelerated particle tracking , Earth Surface Dynamics, 2026
