Moving stones: faults, slopes and sediments

Deposits on upland hillslopes are formed by a range of processes like debris flows, rock falls, slope wash and soil creep. The movement of sediment by these processes occurs over days, years or decades and can have far-reaching implications. Over geological timescales it can influence the relief of mountain ranges, but on human timescales it is also a potential geohazard affecting roads, bridges, and reservoirs, and a key factor in managing river habitats and water quality.  

Faults are important geological features, even when they are no longer active tectonic structures. They are often associated with highly fractured ‘damage zones’ that are relatively weak, providing abundant source material for slope processes and acting as conduits for groundwater flow. We investigated how faults control the types of deposits that are produced on upland slopes by weathering and erosion, and how the direction of a fault’s intersection with a hillside influences the way sediment is mobilised and transported to rivers and reservoirs.  

Study area: Tweedsmuir Hills, Scotland

In the Tweedsmuir Hills, in Scotland’s Southern Uplands, the rolling upland landscape is bisected by a series of brittle faults comprising highly fractured damage zones in the otherwise hard, metasedimentary rocks. The study area, at the head of the Talla Reservoir, provides a prime opportunity to compare the geomorphological imprint of slope-oblique faults that traverse across a slope at a low angle (roughly perpendicular to the slope direction) with that of slope-parallel faults (roughly parallel to the slope direction).  

Faults that traverse slopes at low angles are associated with enhanced regolith (weathered bedrock) production, which forms a more-or less continuous spread of colluvial deposits (loose sediments that move downslope under gravity) across the slope. Sediment transfer to the valley floor is limited because topographical breaks associated with the slope-crossing structures disrupt gully systems and inhibit sediment ‘flow’ downslope.

By contrast, slope-parallel faults are associated with more focused erosion along fault zones, giving rise to a deep and well-connected gully system. The alignment of slope and fault directions creates positive feedback, which enhances downslope erosion and transport to the valley floor. This feedback has resulted in approximately 20 times more rock being eroded per metre of fault length than in the slope-oblique fault system. 

Influence on infrastructure design

The movement of sediment is associated with geohazards such as debris flows and rock falls as well as slope instability that can damage upland transport, energy and water infrastructure. However, sediment movement on slopes is a natural part of how our landscape behaves and interrupting or altering the flow of sediment from hillslopes into streams can affect river environments and habitats, and influence water quality in reservoirs.

Understanding the mechanisms of active slope processes and their distributions within the landscape is necessary to ensure we can design effective approaches for managing both the impact of moving sediment on our built infrastructure, and the effect this infrastructure has on our rivers and reservoirs.

Another way of looking at it is that every slope has its own story. Our work in Talla demonstrates how geomorphological mapping and quantitative field analysis can be used to help understand the dynamics of slope systems, adding to our knowledge of the ‘language’ of slopes. By understanding how their past geological history influences their present processes, we can learn to better ‘read’ slopes and ensure we develop more positive relationships with them.

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About the authors

Dr Katie Whitbread

Survey geologist

BGS Edinburgh

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Reference

Whitbread, K, Thomas, C, and Finlayson, A. 2023. The influence of bedrock faulting and fracturing on sediment availability and Quaternary slope systems, Talla, Southern Uplands, Scotland, UK. Proceedings of Geologists’ Association, in press. DOI: https://doi.org/10.1016/j.pgeola.2023.11.003