West Highlands SMR

Figure 1: West Highlands Scottish Marine Region with the thicker white line delineates the extent of the West Highlands SMR. For a map of all SMRs and OMRs, see here

West Highlands SMR

• Coastline length (km) 3,656
• Sea area (km²): 10,420
• Deepest point (m): 322
• Shallowest point (m): coastline
• Average depth (m):  82
• Tides (m): 3.4 – 5.1
• Salinity:  34.27 – 34.59
• Sea surface temperature (°C): 8.1 – 13.8

The West Highland SMR (Figure 1) has a complex coastline with sea lochs, bays, islands, etc. Many of the sea lochs have sills that restrict the water exchange. It receives large freshwater inflow from land runoff and numerous small rivers, which influences the salinity in its coastal areas. Sea surface temperatures measured at Tiree showed large inter-annual variations, while salinities on the shelf only showed weak seasonality. The northward residual current, the Scottish Coastal Current (SCC), transports water through the Minch. The SCC is a low salinity current that carries water from the Irish and the Clyde Seas. Seasonal variation in circulation exists in both strength and positioning, due to changing wind patterns and water properties. The West Highlands SMR includes a mostly sheltered coastline because of its sea lochs, bays, etc., and it is protected from Atlantic swell by the Outer Hebrides and thus wave heights are generally small.

Holocene (current geological epoch) sea-bed sediments

The Minch and the Sea of the Hebrides serve as a sink for a limited volume of sediment, which mainly originates from the mainland to the east. Seabed sediments are largely derived from reworked glacial deposits and shell fragments. A diverse range of sediments occur, with algal gravels in shallow, sheltered tidal channels, for example in Loch Dunvegan in north-west Skye. Carbonate rich sand ribbons and sandwaves occur along the main tidal streams, and highly burrowed muds occur in the deeper, lower-energy areas, many with examples of cementation taking place shortly after deposition.

Pleistocene geology

The major ice sheets that covered the Scottish Highlands in numerous episodes during the Pleistocene caused deep erosion of the western coast and offshore parts of this SMR, excavating deep sea lochs and inter-island channels. Unlike onshore, a much fuller Pleistocene succession is preserved on the sea bed between the Outer Hebrides SMR and the West Highlands SMR In the Minch two sequences, consisting of a stiff till overlain by a dark grey glaciomarine clay, are overlain by a late Devensian pebbly clay. In the Sea of the Hebrides the late Devensian till is overlain by a glaciomarine unit up to 130m thick.

Solid (pre-Quaternary) geology

The Mesozoic sediments within the Sea of the Hebrides and the Minch occupy a fault-bound sedimentary basin which infills much of the area. The Minch Fault, lying close to the east coast of Outer Hebrides SMR, forms the boundary between the Lewisian basement and the basin, while the eastern boundary of the basin coincides approximately with the present mainland coast: Mesozoic sediments along this boundary extend to within a few kilometres of the coast, and extend onto the land in a few places. The basin is infilled with Permo-Triassic red sandstones and conglomerates, which are up to 1,000 m thick offshore. The overlying Jurassic sediments are locally up to 1,500 m thick, with the thickest sequence preserved close to, and east of, the Minch Fault. Tertiary intrusions and lavas in the Little Minch and the Sea of the Hebrides occur locally on the sea bed and commonly have a rugged form. These igneous bodies are absent from the Minch, which displays a smoother sea floor at a depth of about 100 m. A full succession of Jurassic rocks is exposed on a number of the islands forming the Inner Hebrides.