A side-scan sonar image of a glacially-overdeepened sea loch, upper Loch Etive, Argyll

Loch Etive in Argyll on the west coast of Scotland, is a 30 km long, up to 145 m deep sea loch of glacial origin. A unique side-scan sonar image with new accompanying bathymetric data of the upper basin of the loch has been obtained, covering an area of 19.5 km(2). These data illustrate the morphology of the loch floor and the depositional processes that have been active in the loch since the end of the last (Younger Dryas stadial) glaciation, approximately 10 ka sp. The new bathymetry reveals the complex physiography of the loch with the deepest (> 100 m) regions confined to the SW with slope angles of 5-15 degrees. Towards the head of the loch water depths are shallower, with <50 m common and the loch floor is also smoother with slope angles of 2-5 degrees. Using the side-scan image, areas of high and low acoustic backscatter have been identified. The high backscatter areas correspond to regions of exposed outcrops of in situ granite, with localized glacial erratics on the loch floor, and the low backscatter areas to deeper-water basins containing fine-grained sediments. Other features observed are submarine cliffs, downslope creep of water-laden sediments on the steeper loch slopes, and outwash submarine fans from rivers.