What lies beneath Loch Lomond?

Loch Lomond is a freshwater lake at the heart of the Loch Lomond and Trossachs National Park in the south-west highlands of Scotland. It is surrounded by beautiful landscapes and vistas influenced by past ice ages.  

Using seismic data, marine geoscientists at BGS have discovered a new sedimentary unit buried in deposits beneath the loch, giving new insights into its past glacial history.

Scotland in the last ice age

Much of the highlands of Scotland were covered by an extensive mountain ice cap 12 900 to 11 700 years ago, during the last period of cold climate (known as the Younger Dryas or the Loch Lomond Stadial). Decades of onshore research have shown how past ice ages have shaped the landscape of Loch Lomond, including carving of the present-day loch itself and its surroundings through processes such as erosion and deposition. However, this new dataset provides an interpretation of the stratigraphy now buried beneath the loch.

Mapping the loch bed and subsurface features

BGS used multibeam bathymetry surveys to gather detailed information about the features on the loch bed. The data revealed a series of flat-topped and prograding features (or the growth of a river delta further out into the sea over time) and ancient glacial geomorphological features. These features include drumlins, which are oval-shaped hills largely composed of glacial drift that form parallel to the direction of ice flow, and streamlined bedrock, created by glacial restructuring of hard beds that produces a collection of extended rock landforms, interpreted as showing the direction of the palaeo-ice advance.

It’s been incredibly exciting to have had the opportunity to interpret these datasets and present the loch surface and subsurface in a way we’ve never seen before. The seismic mapping and interpretation of the Inchmurrin Formation helps us understand past landscapes and geological events that are now buried under the loch bed. We are keen to undertake further research in and around the area, building on the seismostratigraphical framework that we observe in Loch Lomond.

Nicola Dakin, BGS marine geoscientist.

BGS geoscientists used seismic data to map the subsurface of the loch. Seismic data uses sound waves, which travel through buried layers of sediment, forming an acoustic image based on density variations between different sediment types. We interpreted the acoustic signature, linking sedimentary processes and depositional environments to past climatic cycles. This provided a framework to create an updated chronostratigraphy within the loch.

What did the survey reveal?

• during glacier advance associated with the cold Younger Dryas climate, glacial landforms were shaped underneath the ice; these can now be identified at the base of the sedimentary succession, up to 60 m below the loch bed surface
• as the ice retreated, vast volumes of water and sediment were released into the loch, leaving a sequence of layered sediments up to 44 m thick
• immediately after deglaciation of the area, exposure of steep loch margins likely resulted in landslides into the loch, producing a unit that is shown as a transparent layer in the seismic data and can represent up to 50 per cent of the sediment fill in places — we have named this new unit the ‘Inchmurrin Formation’
• as the climate transitioned from the early Holocene to the present day, a final phase of lacustrine sedimentation followed, depositing up to 127 m of the youngest, layered, grey-brown lake sediments

Global value of this work

Work is continuing to build understanding of other lochs in the area. The Loch Lomond dataset is a valuable resource that could enable BGS to offer insights into the extent and rates of landscape adjustment that accompanied the transition from glacial to non-glacial conditions. Such findings are of global importance when considering landscape stability and potential future geohazards in regions that are undergoing rapid deglaciation, such as around the European Alps, Himalayas and New Zealand’s Southern Alps.

About the author

Nicola Dakin

Marine geoscientist

BGS Edinburgh

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