SCOTLAND’S SUBMARINE LANDSCAPE Getting to the bottom of Scotland’s fjords Perspective, colour- Martyn Stoker, British shaded view of seafloor Geological Survey, Edinburgh looking west from Ullapool towards the Over the last 500 thousand years, Summer Isles. Shallow banks are white/pink; glaciers have carved and shaped deep troughs are the stunning fjord landscape that blue/dark blue. characterises the west coast of Scotland. The North-West Highlands is one of the best places to view the effects of glacial erosion, from its spectacular mountain peaks and deep ice- sculpted corries to the U-shaped valleys and sea lochs. The special interpretation was further the fjord region was ice-free by nature of this unique landscape has enhanced by the importation of the about 13,000 years ago. been acknowledged in its acoustic data into a 3D- designation as a UNESCO visualization package that Since the retreat of the glaciers, ‘European Geopark’ produced a digital terrain model of natural processes of weathering, (www.northwest-highlands- the marine landscape. erosion and slope readjustment geopark.org.uk). Less well-known have continued to modify the is the dramatic marine landscape A striking contrast was revealed. fjords both above and below that forms an integral component Shallow marine banks, less than present-day sea level. On the of the region’s natural heritage. 15–20 m below present-day sea mountainsides, scree and rockfalls level are juxtaposed with deeply provide recent evidence. Below Between 2005 and 2009, the incised fjord troughs, up to 180 m water, similar features show up in Submarine moraine ridges British Geological Survey deep, with steep sides, flat bottoms our swath bathymetry data. offshore Achiltibuie (location undertook a study of the fjord and undulating axial profiles. The Extensive slumping has scarred the marked by cross on main image) coastline in the Summer Isles troughs represent the offshore submerged sides of the fjords. In region, west of Ullapool, including continuation of the modern sea Little Loch Broom, about 1.5 km2 Loch Broom and Little Loch lochs. The views from the of the sea floor in the outer part of Broom; an area of about 200 km2. mountain tops of Ben Mór the loch has slid and been Our main scientific objective was Coigach and An Teallach are displaced. Whereas this event most to initiate a primary marine already impressive, but to likely occurred shortly after the geological survey of Scotland’s appreciate the true size of the retreat of the Little Loch Broom fjords, which, somewhat fjords in the Summer Isles region glacier, evidence of more recent surprisingly, have received you’d have to add an additional slumping and mass failure, within relatively little attention in terms 180 m to the mountains’ height inner Little Loch Broom, suggests of their glacial history. In addition above present-day sea level. that this area may still not have to the science, we wanted to raise fully adjusted to nonglacial awareness of the unseen marine The shallow banks have a distinct conditions. landscape for socio-economic ribbed appearance that reflects the reasons. All marine activities, be preservation of a spectacular suite There is a growing cultural they local aquaculture or fishing of about 40 sea-floor moraines, 5 awareness that we need to monitor industries, or national to 15 m high and 50 to 125 m excessive pressures on our infrastructure activities, such as wide, which can be traced from the landscapes. To do this we must cable laying, require information Summer Isles to Loch Broom. The recognize that geology provides about the marine landscape and the preservation of these submarine the Geodiversity that underpins Submarine landslide in Little processes that influence sea-bed landforms contrasts markedly with landscapes, habitat and Loch Broom (location marked the much more subdued Biodiversity both above and below by asterisk on main image) habitats, in order to maintain a healthy ecosystem alongside morphology (and common the waves. Around the Summer human use of the marine absence) of moraines onshore. Isles, these landscapes are still environment. These submarine moraines chart evolving. Understanding the decay of the last British ice Geodiversity helps us establish We mapped the seafloor using sheet as it receded from the whether the use and development swath bathymetry and high- adjacent continental shelf, through of the coastal region can be resolution seismic-reflection the fjord system, and back towards sustained without adversely profiling techniques, which the adjacent mountains. A affecting the environment. transmit acoustic pulses that are combination of radiocarbon and reflected off the sea floor and sub- cosmogenic-isotope dating has To find out more, please contact sea-floor layers. By interpreting indicated that the Summer Isles [email protected] or 0131 650 0374. these reflections, including their became ice free at about 14,000 calibration with geological cores, a years ago; that the glaciers had fairly sophisticated picture of the retreated to the Ullapool area by sea floor and its underlying about 13,500 years ago; and that geology was established. This Loch Broom.