University of Glasgow-Led Isle of Jura Study Reveals How Sediment Hardness Helps Rivers Carve Through Landscapes and Could Improve Future Erosion Models

University of Glasgow-Led Isle of Jura Study Reveals How Sediment Hardness Helps Rivers Carve Through Landscapes and Could Improve Future Erosion Models

(IN BRIEF) A University of Glasgow-led study of rivers on the Isle of Jura has provided new evidence that the relationship between the hardness of sediment carried by rivers and the strength of underlying bedrock can significantly influence how quickly landscapes are eroded. Researchers examined rivers flowing across exceptionally hard Jura Quartzite and much softer Easdale Subgroup slate, taking 832 rock-hardness measurements across 35 outcrops before using the data to build three computer models of knickpoint retreat since the end of the last ice age around 13,600 years ago. The modelling showed that simply treating hard and soft bedrock as having different erosion rates did not improve predictions, whereas accounting for the contrast between transported sediment and the riverbed produced a much better match with the real landscape. The findings suggest that hard sediment can act abrasively against softer bedrock and could help improve large-scale models of river erosion, landscape evolution and future climate impacts, while also contributing to the eventual development of more detailed digital representations of geological processes.

(PRESS RELEASE) GLASGOW, 7-Sep-2026 — /EuropaWire/ — Researchers led by the University of Glasgow have used the distinctive geology of the Isle of Jura to uncover new evidence about how rivers erode bedrock, showing that the hardness of sediment carried by flowing water can be as important as the strength of the riverbed itself.

The study suggests that hard rock fragments transported downstream can act like abrasive material, scouring softer bedrock and accelerating erosion beyond what would be predicted by bedrock strength alone.

The findings, published in the journal Earth Surface Processes and Landforms, could improve understanding of fluvial erosion and help geologists reconstruct how landscapes evolved in the past while improving models of how climate change may reshape river systems in the future.

Researchers from University College London and the University of Southampton also contributed to the study.

Jura provides natural laboratory for studying river erosion

The Isle of Jura, home to around 250 people, provided researchers with an unusual geological setting in which to examine how contrasting rock types influence river erosion.

Around 13,600 years ago, as the last ice age ended, the removal of the weight of glacial ice caused the land on Jura to rise rapidly.

The resulting strain produced changes in the landscape that created small waterfalls, or knickpoints, in rivers across the island.

Knickpoints gradually retreat upstream as flowing water erodes and eventually undermines the rock at their edges.

Because the position of these features changes over time, geologists can use them to estimate how rapidly river channels have eroded since their formation.

Hard quartzite and softer slate create natural comparison

Dr Adam Smith, from the University of Glasgow’s School of Geographical & Earth Sciences and corresponding author of the paper, said Jura offered an opportunity to investigate why harder rocks do not always erode more slowly than softer ones.

The island’s bedrock rivers flow across Jura Quartzite, described by the researchers as one of the hardest rock types found on Earth, and a substantially softer slate unit known as the Easdale Subgroup.

Some rivers flow only across the hard quartzite, while others move from quartzite onto softer slate.

This geological contrast allowed the team to test whether differences in bedrock strength alone could explain variations in erosion.

Researchers take 832 rock-hardness measurements

The research team travelled across Jura collecting measurements from exposed rock outcrops using a Schmidt hammer.

The instrument uses a spring-loaded mechanism to strike a rock surface and measure the rebound, with harder rock producing a greater rebound.

The researchers collected 832 individual measurements from 35 outcrops located along Jura’s single road.

The measurements showed that the island’s quartzite is approximately three times harder than the slate.

Using these observations, the researchers produced a map of rock strength across the island that could then be incorporated into computer simulations of landscape evolution.

Three computer models test competing explanations

Back in the laboratory, the researchers created three different computer models representing alternative assumptions about how rock and sediment strength influence knickpoint retreat.

The first model assumed that rock type had no effect on erosion.

A second model assumed that harder bedrock was always more difficult for rivers to erode than softer rock.

The third model considered both the strength of the underlying riverbed and the hardness of sediment being transported by the river.

Researchers then compared the predicted locations of knickpoints generated by each model with their actual positions across Jura.

Sediment-to-bedrock hardness contrast proves most important

The results showed that simply assigning different erosion rates to hard and soft bedrock did not improve the models.

According to Dr Smith, this approach actually performed slightly worse than the assumption that erosion rates were uniform across the landscape.

The model that best matched the real-world observations was the one that incorporated the difference in hardness between transported sediment and underlying bedrock.

The researchers found that hard sediment travelling across softer bedrock can substantially increase erosion, acting in a way comparable to abrasive material grinding against the river channel.

Dr Smith said this effect can result in erosion rates considerably greater than those predicted from bedrock strength alone.

Water flow and channel steepness remain dominant controls

The research team stressed that sediment and rock hardness are only part of a much larger erosion system.

The amount of water moving through a river remains one of the principal controls on erosion and is itself influenced by rainfall and the size of the drainage basin feeding the channel.

The steepness of the river channel is another major factor.

Rock hardness therefore plays a smaller role than flow and slope, but the researchers found that its influence remains significant enough to improve the accuracy of landscape-evolution modelling.

Findings could improve large-scale landscape models

Current landscape-evolution models often divide landscapes into separate blocks of hard and soft rock.

The researchers argue that this approach can overlook the fact that sediment originating from one geological unit may be transported downstream and interact with a completely different rock type.

At the scales used in regional geological modelling, often extending across tens or hundreds of kilometres, incorporating this interaction could provide a more realistic representation of how landscapes evolve.

As computing capabilities improve, the researchers believe the findings could eventually contribute to more advanced geological models and even digital twins of Earth’s geological processes.

Such models could help researchers investigate how rivers and landscapes responded to past environmental changes and how future shifts in rainfall and climate could affect erosion.

Scotland’s exposed geology continues to support new research

Dr Smith said Scotland’s long history of geological research and its extensive exposed landscapes continue to provide valuable opportunities for scientific study.

Jura’s rugged terrain allows geologists to examine rock formations and river systems directly in ways that would be more difficult in areas where the underlying geology is concealed by farmland or urban development.

The study adds to a long tradition of geological research based on Scotland’s distinctive landscapes and rock formations.

The research paper, “How does lithology influence fluvial erodibility? A study of knickpoint retreat in small bedrock rivers on the Isle of Jura,” is published in Earth Surface Processes and Landforms.

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SOURCE: University of Glasgow