The Inner Hebrides have a natural history which, in geological time, dates from the Archaean and, in human history, from the Mesolithic 7000–6500 BC. The archipelago was a great attraction to scientists and historians in the 19th and 20th centuries which has resulted in an extensive literature. It started with Martin in 1703 and following a steady interest for about two centuries has blossomed in many spheres of scientific and other interests over the past 50 years. The islands are geologically outstanding and much pioneering work has been done on their Lewisian and Torridonian basement, Mesozoic fossiliferous sediments and Tertiary ring structures which has been of value in the understanding of the geology of the European and the North Atlantic provinces. The geomorphological features include glaciated landscapes, raised beaches and those caused by marine erosion and accretion.

The character of the islands is in great part determined by a mild, wet and windy climate. The soils are mainly ‘mor’ in character with localities of ‘mull’ soils overlying limestone or calcareous sand. Similarly, the vegetation and freshwaters are mainly oligotrophic except when overlying limestone or shell-sand which support meso- and eutrophic communities of plants and animals. There are important assemblies of breeding seabirds in Rhum and Treshnish Isles and wintering flocks of geese and other wildfowl in Islay. The large islands have populations of red deer, grey seals, common seals and otters breed throughout the islands. The Inner Hebrides hold stocks of plants and animals which are genetically distinct from those of the mainland and some have Lusitanian and American affinities.

The islands are mainly used for the grazing of sheep and cattle with local small-scale arable cropping in crofting and farming. Forestry is possible only in the sheltered parts of the larger islands or in small sheltered islands; the main crops are sitka spruce and lodgepole pine but there are broadleafed woodlands around large houses in many islands and gardens with exotic, sub-tropical plants, shrubs and trees. There are pelagic and demersal marine fisheries, shell fisheries and fish farms mainly growing salmon, rainbow trout and mussels. The mineral resources on a commercial scale, are minor. The nature conservation interest is high with two National Nature Reserves (including Rhum) and 62 Sites of Special Scientific Interest.

The geology of the Inner Hebrides covers the time span from ca 2900 m.y. (million years) ago to the present and is dominated by three major crustal regimes. The foreland of the Caledonides is represented by the metamorphic rocks of the Lewisian complex on Rona, Raasay, Coll and Tiree. It is separated by the Moine thrust from the ca 525–100 m.y. old Caledonian mountain chain which is represented by the sediments of the Dalradian Supergroup of Jura and much of Islay. The younger, ca 60 m.y. old, Hebridean Tertiary Igneous Province has major centres in Skye, Rhum and Mull and basaltic lava flows cover considerable areas. Other geological features include the Mesozoic sediments and the products of both the Quaternary glacial activity and present day sedimentation.

The Lewisian complex, which forms the continental basement to north-west Scotland, crops out on the Inner Hebridean islands of Rona, Raasay, Skye, Coll, Tiree, Iona and Islay. Upon this basement, four major rock successions were deposited before the Caledonian orogeny. The upper Precambrian Moine assemblage forms only a small area of metamorphosed rocks on Mull but the c. 790 m.y. (million year) old Torridonian sediments are found on Raasay, Scalpay. the Sleat of Skye, Soay and Rhum. The upper Precambrian to Cambrian Dal radian Supergroup dominates Islay, Jura, Gigha and the islands of the Firth of Lome whilst also forming a partial rim around the Tertiary Northern granite in Arran. Other Precambrian rocks of uncertain affinity are found on Islay, Oronsay, Colonsay and Iona. Cambro-Ordovician sediments are found on Skye where they have been partially metamorphosed by Tertiary intrusions.

South-east of the Moine thrust zone, the Precambrian and lower Palaeozoic rocks were deformed and metamorphosed during the late Cambrian to early Devonian Caledonian orogeny which resulted in the development of the Caledonian mountain chain. Subsequently, Scotland north-west of the Highland Boundary fault has tended to persist as a land area undergoing erosion with sedimentation restricted to peripheral areas now preserved around the present coast. Upper Palaeozoic rocks are therefore only well represented on Arran although Old Red Sandstone (Devonian) sediments are found in the Firth of Lome and a very small area of possible Permian rocks occurs on Islay.

In the Inner Hebrides Mesozoic sediments rest unconformably on much older, harder and more deformed rocks: they are overlain by Tertiary lavas and cut by sub-volcanic intrusions. The Triassic rocks are unfossiliferous conglomerates and sandstones, deposited on alluvial fans and river flood plains in a semiarid climate. Active faults controlled sedimentation. In the early Jurassic the sea transgressed across the region; a basin of net sediment accumulation lay between uplifted regions now represented by the Highlands and the Outer Hebrides. The lower and lower-middle Jurassic is composed of marine shalesandstone alternations comparable to those providing the sources and reservoirs for North Sea oil. Ammonites provide good correlation with the European succession. In the Great Estuarine Group, environments ranged from lagoonal to deltaic to dolomitic mud-flat. Molluscan assemblages can be compared to modern brackish-water faunas. In the upper Jurassic there is a return to marine shales and minor sandstones; ammonites help correlation between Europe and the Boreal province. The lower Cretaceous is absent; thin upper Cretaceous rests disconformably on older rocks. The rocks are not metamorphosed except slightly near the Tertiary plutonic centres. Their softness makes them susceptible to erosion, and, especially where shales are overlain by basalts, extensive landslips occur.

The magmatism early in the Tertiary period resulted in (1) eruption of extensive plateau lavas (Skye, Small Isles, Mull) and of Central-Group lavas (Mull, central Arran), and (2) emplacement of central intrusion complexes (Skye, Rhum, Mull, Blackstones Bank, Arran), of sill complexes (northern Skye, Shiant Isles, western Mull, southern Arran) and of dyke swarms (almost all of the Inner Hebrides) focused on the central intrusion complexes. Each of these magmatic stages and the rock types formed are described, with particular emphasis on the central intrusion complexes and the fact that the centres about which intrusion occurred shifted with time. Some of the causes of compositional variation among rock types are briefly discussed. Most of the magmatism occurred between 61 and 58 million years ago when rifting and separation of eastern Greenland and north-western Europe was initiated.

The Quaternary features and deposits provide a record of the geological history of the past 18,000 years and only a few elements of the landscape, such as the glaciated rock platforms and cliffs of marine origin, can be ascribed to earlier times. Mainland ice almost certainly covered the islands during the last glaciation, its retreat being temporarily reversed or halted perhaps more than once. There was a final well-marked, but short-lived episode of valley glaciation some 10,000 to 11,000 years ago. Glacial deposits, raised beaches, periglacial features, landslips and the post-glacial accumulations of peat, shell sand and diatomite are discussed briefly.

Little land-derived material currently reaches the sea. Inland deposits are dominantly organic, peat succeeding lacustrine diatomite. Marine sediments result from the wave and tidal current reworking of Pleistocene glacial debris, with an admixture of Holocene skeletal carbonate. Muddy sands are accumulating in glacially overdeepened and tidally scoured hollows, reworked from moraine and outwash. Lag gravels occur on the shallows.

Beach sands have a local origin, with mixtures of shelly fauna and rock fragments eroded from adjacent cliffs. Beaches are less carbonate-rich than in the Outer Hebrides, except on Coll and Tiree. They are also finer, which encourages well-developed dune systems. Calcareous dunes are locally cemented on Coll, the cementation picking out ‘fossilised’ hoof-prints.

There is no transport of coastal bedload to the shelf, so that offshore sediments are more calcareous. Bivalves, barnacles, gastropods, worm tubes and sea urchins make significant contributions to the shellsand. Underwater television surveys show a diverse suite of sedimentary facies: algal gravels in shallow, sheltered tidal channels; carbonate-rich sand ribbons and sandwaves along the main tidal streams; highly burrowed muds in the deeper, lower energy areas, with many examples of penecontemporaneous nodule formation.

The field mapping of the soils of the Inner Hebrides has revealed a range of major soil subgroups including calcareous regosols, brown forest soils, humus-iron podzols, peaty podzols, peaty gleys and peat. The parent materials, plant communities and Land Capability for Agriculture of the soils are described, and the distribution of soils in relation to various factors controlling their formation discussed.

The climate of the Inner Hebrides is considered as a variant (a) of the climate of the British Isles, and (b) of the European Atlantic seaboard. Its chief characteristics are wetness, and mildness—expressed as minimal variation of temperature both diurnally and seasonally. Other features are the marked effect of topography, particularly on rainfall, cloudiness, and windiness. The ecological significance of these features is stressed.

The chief physical processes encountered in Inner Hebridean waters are described, including the tidal and current regimes, temperature and salinity variations, and stratification in the sea and sea-lochs. Although information on some of these topics is sparse, available data are used to provide order of magnitude estimates for the volume transport of the coastal current and run-off water and to indicate expected ranges of the major parameters of the marine environment. The islands are shown to be important as a region of tidal mixing of coastal water.

Rapid advances in scientific SCUBA diving have provided a means of investigating the previously difficult ground of the shallow sublittoral. Dhing surveys of the Inner Hebridean islands have been promoted and coordinated by the Nature Conservancy Council (NCC) and the Underwater Conservation Society (UCS) to provide appropriate data for reviewing the marine nature conservation importance of the area. Although the surveys have been extensive rather than intensive, and many areas have yet to be investigated, a short preliminary descriptive account of the communities of the shallow seas of the Inner Hebrides is presented in this paper.

The classification and description of sublittoral habitats is essentially related to the substratum type and the influence of water movement and light penetration. Although the diversity of habitat types is probably less for the Inner Hebrides than either the outer islands or the mainland, the larger islands show a range of exposure from the sheltered mud of sea lochs to wave- and tide-exposed bedrock. The juxtaposition of three marine biogeographical regions around the British Isles is reflected in the biota of the Inner Hebrides. In addition to the typical ‘Scottish’ sublittoral biota there is an associated assemblage of south-western species, and a number of northern species at their south-western limit. In many areas the diversity of the biota is reduced by the intense grazing of the sea urchin Echinus esculentus, a notably Scottish phenomenon.

Benthic communities, controlled predominantly by the environmental parameters of sediment type and hydrography, have been determined by computer cluster analysis and by reference to previously established offshore communities.

A gravel association floors much of the central and northern parts of the sound while a deep mud association is flanked by a muddy sand association in the south.

Baseline surveys of Islay and Skye show that the shores of the latter island have the more diverse mollusc population. Detailed investigation of the Mollusca associated with algae corroborates the general survey. The reason for the comparative paucity of Mollusca on Islay appears to stem from a number of factors of which the lack of variety of habitats and the shorter shores with little tidal range are the most obvious. Additional work carried out by diving and dredging enhanced the data from Skye, supporting the view that a combination of approaches produces the best results in a baseline survey, and helps to minimise any false impression of a marine fauna which may arise in surveys arbitrarily restricted to above or below low water mark or to a single method of obtaining data. The relationship of the marine molluscan fauna of Islay and Skye to that of west Scotland as a whole is briefly discussed.

About 10% of the British grey seal population and 20% of the British common seal population breed around the Inner Hebrides. Grey seals occur on the more exposed coasts whereas common seals inhabit the more sheltered waters and the inner reaches of sea lochs. There is some evidence of an increase in grey seal numbers, but there are insufficient data to assess the exact status of the common seal.

The Inner Hebrides make up only a small proportion of the land area but a relatively higher percentage of the area of standing waters of Great Britain. Analyses of the 1:63,360 maps have provided the total numbers of standing and running waters on the islands (1,542 and 1,615 respectively) and are the basis for estimating the total resource there. The majority of lochs are small and relatively low lying. Most of the streams too are small and rarely larger than 4th order. Chemically, the waters are variable and range from some very acid nutrient-poor lochs on Islay to the unique marl lochs of Lismore. The algae of these islands have only occasionally been sampled and relatively little is known either about the higher aquatic plant communities—the ones studied are similar to those on the mainland. Few studies of the invertebrates of the inland waters have been carried out, and none of the surveys has been comprehensive. A review of the published and new data has enabled a broad account of the invertebrate communities to be presented. Only 15 species of fish occur here and most of the waters are dominated by the indigenous salmonids (salmon, trout and charr), sticklebacks or eels. Comparisons of the inland waters of the Inner Hebrides with those of other island groups (the Outer Hebrides and Shetlands) and the mainland of Scotland (especially Tayside) show that they have greater afiinities with the latter.

Apart from limited studies on salmon and sea trout no previous investigation of the status and distribution of Inner Hebridean freshwater fishes has been made. Only euryhaline fishes were able to recolonise Hebridean freshwaters after the last glaciation. Thus seven indigenous, freshwater fishes have been recorded in the Inner Hebrides: Atlantic salmon, trout, Arctic charr, three- and ten-spined stickleback, European eel and brook lamprey. Trout occur both as anadromous and sedentary forms but charr as the sedentary form only.

Flounders penetrate far upstream and may be found in lochs while grey mullet occur seasonally in streams a short way inland from the sea. In contrast to the Outer Hebrides there are established populations of non-indigenous species; American brook charr occur in lochs only on Eigg and Mull and pike and perch only on Islay.

Populations of rainbow trout are artificially maintained. On some small islands, freshwater bodies are temporary so that there are no permanent fish populations while on others, where freshwaters are present as ditches and small streams, only eels can exist. A survey showed that freshwater eels were the most widely distributed fish, followed by trout and three-spined stickleback in that order.

Salmon and sea trout are found in all the major river systems and are also caught in nets on the coasts of the larger islands. These coastal fisheries also exploit salmon in passage to river systems elsewhere in Scotland.

The Inner Hebrides support a diverse flora and a wide range of vegetation types today. Native woodland is rare, even on the larger islands of Skye and Mull. The present-day floristic and vegetational diversity and the scarcity of native woodland raise important ecological questions, answers to which can be provided by pollen analysis of loch or bog sediments.

Pollen analysis of sediments deposited during the last glacial stage on Skye and on Lewis suggest that some plants may have survived the last glaciation within the Hebrides. Pollen analysis of late-glacial (13500–10000 years ago) sediments on Skye indicate that even at the close of the last glaciation there was considerable floristic and vegetational diversity related to geology, altitude, and climate.

Post-glacial (10000–0 year ago) vegetational histories are available from Skye, Soay, Canna, Tiree, and Jura. Forest was rare or absent on Canna and Tiree as well as on the Outer Hebrides throughout the postglacial. In contrast Soay and southern and eastern Skye were well forested, with birch, alder, hazel, and small amounts of elm and oak. Pine was confined to eastern Skye. Since 5,000 years ago there has been extensive forest destruction by man, especially on the better soils, and the spread of bog and heath on the poorer soils. Northern Skye supported birch, hazel, and willow scrub during the post-glacial. This has largely been removed by man since about 5,000 years ago. The present-day vegetation of the Inner Hebrides is predominantly anthropogenic.