Sea Lochs of Scotland

Loch Fyne is over 656ft (200m) deep

The west coast of Scotland has over eighty sea lochs; sheltered, semi-enclosed fingers of sea that, together with rugged mountains, characterise the west coast scenery. A glance at a map of the west coast of Norway shows a similar coastline, and not by chance, for the same forces shaped both countries. Sea lochs are essentially fjords, carved by the tremendous erosive power of ice.

Although each sea loch is unique in size and shape, there are common features that give clues to their Ice Age origins. Sea lochs are typically narrower and shallower at their entrances, widening and deepening inside into one or more large basins. During the last ice advance 400,000 years ago, great glaciers moved slowly from the mountains to the sea. As they did so, they gouged out vast quantities of rock from the basins, and since the weight of ice was greater inland than near its melting (seaward) end, the basins were made even deeper. As the ice melted in the sea water, much of the rock carried within was dropped in a pile, which now forms part of the shallower 'sill' at the entrance to many sea lochs. When the sea level rose after the Ice Age, the inner basins filled with sea water, and are now over 262ft (80m) deep in many mainland lochs. Loch Fyne is over 656ft (200m) deep.

The shape of sea lochs has a great effect on the water movement within them, and hence on the type of marine life found in different parts. When the tides rise and fall, a large amount of water enters or leaves the loch over the shallow sill, creating fast currents. Depending on the loch size, sill depth and the narrowness of the entrance, sill currents vary from a gentle half knot or so to fast rapids of over eight knots, almost a waterfall in places. Inside the loch the current is quickly lost in the large basins, where tidal movement is only noticeable around headlands projecting into the loch, or around islands.
Water currents have a profound effect on the sea bed. Fast currents remove fine particles, leaving boulders, pebbles and coarse gravel near the sills. By contrast, the quiet water in the basins allows very fine particles to settle, and deep, soft, squelching muds collect. Gentle wave action around the loch edge takes away fine sediments, leaving coarser material in shallow water. Because sea lochs are very sheltered, wave action seldom has much effect below the immediate surface layer of the water column, leaving fine silt to settle below. Scotland's mountainous west coast has a notoriously high rainfall, much of it draining into burns and rivers, and ultimately back to the sea. The peaty freshwater mixes reluctantly, preferring to float on top of the denser sea water in a brackish layer. Freshwater at the surface may ice over in winter, while the sea water below, with its own salt antifreeze, stays liquid.