Surveying the Loch Lomond 'canyon' | A new navigation chart

The BGS and Loch Lomond and Trossachs National Park Authority (LLTNPA) have collaborated to produce a new navigation chart of Loch Lomond. The chart dataset can also be analysed for a variety of scientific purposes including geological interpretation.

The image below shows how one of Britain's largest 'lakes' would look if the water was taken out.

How one of Britain's largest lakes would look if the water was taken out

Underwater glacial features

The loch lies at the southernmost edge of the ice limit during the last glaciation to affect Scotland. The survey shows glacial features, which will add to our understanding of how quickly the ice retreated. The Highland Boundary Fault, which separates the Scottish Highlands from the Central Valley, runs through the loch and the survey also provided an opportunity to acquire underwater data across this important structural feature.



Project progress

Data were collected during a 7-week period from December 2007– January 2008. The BGS has extensive experience of using multibeam data in the marine and coastal environment, where the data have been used for a wide range of scientific research topics including mapping the habitats of marine flora and fauna.

'Sonar data has been used for several years to map the geology of the sea floor around Britain, but this was the first time we had used the technique in a loch environment.

We are very pleased that the information has also helped the National Park Authority to improve their navigational chart for loch users. The data we plan to collect from beneath the loch floor will add to the sonar information and help us to understand even more about the present-day loch environment and its geological history.

The data are also widely used by marine management organisations with responsibilities for fisheries, oil and gas, cables/pipelines, conservation etc. This project is the first occasion that BGS have used echo-sounding equipment in freshwater.

Solar powered survey boat

The echo-sounder was fixed to the LLTNPA’s solar boat, Bàta Grèine, which operated transects on Loch Lomond in order to capture the required data. The next stage of the project is to survey the loch using the BGS surface-tow boomer, which will be used to collect profiles of the rocks and sediment beneath the loch floor.

Three men in a boat

The 1861 survey used rowing boats from which lead-line soundings were taken a few hundred metres apart, possibly amounting to no more than a few thousand soundings. This basic approach was relatively accurate as the charts that are used today are based on these surveys. However, using the BGS multibeam echo-sounder system, the equivalent of millions of soundings have been taken, allowing a detailed picture of water depths and the geology of the loch floor to be acquired.

Geological history

Loch Lomond was formed as the result of glaciers moving southward during the last Ice Age. As the ice was forced into a narrow channel at the northern end of the loch, it gouged out large quantities of rock to leave a deep, steep-sided valley. The floor of this valley reaches 196 metres beneath the loch surface, deeper than the North Sea. The material removed by the ice was pushed by the ice southwards.

Following the end of the Ice Age (approximately 12 000 years ago), sea level began to rise and the southern end of the loch was connected to the sea. The platform along the southern shore is the result of this inundation of the sea. As uplift of the land continued following ice removal, the sea level began to fall rapidly and by 11 000 years ago lay at around the present level or below.

During the time between 11 000 to 10 000 years ago, a cold period known as the ‘Loch Lomond Stadial’, the glaciers readvanced across western Scotland. Ahead of the readvancing ice, a thick sequence of marine muds was deposited that became ‘bulldozed’ by the ice to form a prominent ridge that can be seen on land. Glacial meltwater flowed down the valley of the River Leven depositing sand and gravel, which raised the outlet of the loch, causing it to be a freshwater loch today.

Highland Boundary Fault

The geology of the loch is strongly influenced by the Highland Boundary Fault, a fracture formed several hundreds of million years ago that forms the boundary between the Scottish Highlands and Lowlands. The fault crosses Loch Lomond and can be seen on the elongate islands of Inchmurrin, Creinch and Inchcailloch. To the north of the fault the hard metamorphic rocks were more resistant to erosion and weathering than the softer sedimentary rocks to the south.

Evidence of the Highland Boundary Fault and the glacial features of the last Ice Age can be seen onshore around the loch, but for the first time, a sonar survey of the loch floor has revealed the detailed landscape that remained after the ice melted. The survey, conducted by the British Geological Survey and the Loch Lomond and The Trossachs National Park Authority will help geologists to understand the changes that took place in our climate over 10 000 years ago and can be used to produce detailed charts of the loch floor.

Contact

Contact Alan Stevenson for further information.