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 o btained, covering an area of 19.5 km(2). These data illustrate the morpholo gy of the loch floor and the depositional processes that have been active i n the loch since the end of the last (Younger Dryas stadial) glaciation, ap proximately 10 ka sp. The new bathymetry reveals the complex physiography o f 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 shal lower, with < 50 m common and the loch floor is also smoother with slope an gles of 2-5 degrees. Using the side-scan image, areas of high and low acous tic 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 b asins containing fine-grained sediments. Other features observed are submar ine cliffs, downslope creep of water-laden sediments on the steeper loch sl opes, and outwash submarine fans from rivers.