Keywords

1 Introduction

The Shetland archipelago (~60 to 61°N) is the northernmost part of the British Isles and occupies the northern extremity of the Orkney-Shetland Platform (OSP), the topographic divide that separates the NE Atlantic and the North Sea (Ziegler 1981; Fig. 7.1). Shetland has a small total land area (<1500 km2) but is extraordinarily rich in its geological and geomorphological diversity. The archipelago spans 120 km along a roughly north–south orientation, ~170 km north of the Scottish mainland. The largest island (Mainland) is deeply indented by inlets that subdivide it into north Mainland, the Walls peninsula (west Mainland), central Mainland (north of the main town, Lerwick) and south Mainland; northeast of Mainland lie the large islands of Yell, Unst and Fetlar (Fig. 7.2). Numerous smaller islands and skerries lie offshore from Mainland, the largest being Fair Isle, Foula, Papa Stour, Whalsay and Bressay.

Fig. 7.1
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Geology of the Orkney-Shetland Platform. (Simplified from © NERC 2016. All rights reserved. Contains Ordnance Survey Data © Crown Copyright and database right 2016. Produced using Copernicus data and information funded by the European Union-EU-DEM layers. NEXTMap Britain™ elevation data from Intermap Technologies)

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Fig. 7.2
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The Shetland Isles: relief and main locations mentioned in the text. 1: Keen of Hamar; 2: Fugla Ness; 3: Grind of the Navir; 4: Villians of Hamnavoe; 5: Ayres of Swinister; 6: Mavis Grind. 7: Vementry; 8: Petta Dale; 9: Sel Ayre; 10: Westerwick; 11: Tresta; 12: Weisdale Voe; 13: Lax Firth; 14: Whiteness; 15: Quarff gap; 16: St Ninian’s Isle; 17: Quendale; 18: Ness of Burgi. Fair Isle lies ~38 km southwest of Sumburgh Head. (Base map contains Ordnance Survey data © Crown copyright and database right. Creative Commons Attribution-Share Alike 3.0 Unported license)

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The juxtaposition of many metamorphic, igneous and sedimentary rock types on Shetland provides a remarkable array of landscapes in which the imprints of lithological and structural controls are manifest. During the cold stages of the Pleistocene, Shetland supported an independent ice cap that extended across neighbouring shelves. The impact of glacial erosion varied across the archipelago, with deep erosion along dales and voes (the local name for coastal inlets), but minor erosion along the former ice-divide and at higher elevations. Postglacial sea-level rise drowned glacial valley floors and the inner parts of the shelves. Rising relative sea levels over the Holocene have progressively exposed the outer coast to erosion under the full force of Atlantic storm waves. The cliff coasts of Shetland represent some of the finest coastal scenery in the world.

2 Geology

2.1 Geology and Structure

Shetland stands on a part of the Caledonian Orogenic belt and comprises several different crustal fragments, or terranes, assembled by strike-slip movements along three major north–south transform fault zones during the closure of the Iapetus Ocean in the Late Silurian (Chap. 2): the Melby, Walls and Nesting fault zones. The Walls Boundary Fault Zone represents the northern continuation of the Great Glen Fault Zone in Scotland. It was initiated during the Caledonian Orogeny, reactivated and dislocated during late Carboniferous inversion of the Orcadian Basin and became active again during the Mesozoic or Cenozoic, with up to 15 km of left-lateral displacement (Watts et al. 2007).

The bedrock geology of Shetland is complex (Mykura 1976; Mykura and Phemister 1976). The oldest rocks form a basement platform of late Archaean (2500–3000 Ma) ‘Lewisian’ gneiss that crops out along the north coasts of west Mainland and North Roe. Resting on the basement is a succession of Mesoproterozoic and Neoproterozoic metasedimentary rocks which display several phases of deformation and alteration. Banded gneisses, mica schists and psammites of the Moine Supergroup underlie most of Yell and parts of north Mainland, while Dalradian quartzite, limestone, mica schist and metamorphosed lavas form much of south, central and eastern Mainland, western Unst and western Fetlar. Fragments of the floor and underlying crust of the Iapetus Ocean are found in the ophiolite complex of eastern Unst and Fetlar; this complex was obducted westwards on to the edge of Laurentia during the Caledonian Orogeny. The ultrabasic and basic peridotites and gabbros provide a rare insight deep into oceanic crust and upper mantle rocks (Flinn 2014).

Rocks of the Devonian Old Red Sandstone Supergroup formed mainly in the later stages of the Caledonian Orogeny. Sandstones predominate, with volcanic agglomerate and lava confined to Papa Stour and at Eshaness. Caledonian plutonic complexes, predominantly of granite, were intruded into both the metamorphic and sedimentary rocks; granite now underlies the highest hill on Shetland, Ronas Hill (450 m).

The OSP was uplifted in the earliest Palaeocene and large parts remained above sea level thereafter, with varying rates of erosion and sediment supply to flanking sedimentary basins throughout the Cenozoic (Anell et al. 2012). Sediment volumes in the northern North Sea indicate that the depth of erosion was profound, with estimated losses of ~1 km of rock from the OSP in the Palaeocene and Eocene. As the present level of erosion approximates to that of the sub-Devonian and sub-Permian unconformities (Flinn 1977), the missing rock was former Palaeozoic and Mesozoic sedimentary cover (Wilkinson 2017). The clearest evidence of a former land surface on Shetland is provided by the hilly sub-Devonian terrain, a surface preserved where Devonian sandstones and conglomerates rest on the basement in both south and north Mainland. Aside from these patches of exhumed Palaeozoic unconformity surfaces, all present-day landforms on the OSP are of Neogene or younger age. The uplifted OSP was an important source area for sediments in the North Sea from the late Miocene onwards, and the Shetland area continued to provide fluvio-deltaic sediment during the early Pleistocene (Ottesen et al. 2018).

2.2 Geology and Landscape

Close links exist between rocks and relief on Shetland. At a regional scale, the landscape and topography are strongly conditioned by the roughly north–south orientation of major faults and the main rock units, which has determined the ‘box-shaped’ outlines of Unst, Yell, North Roe and south Mainland. Fault movements produced zones of crushed and fractured rock, later exploited by rivers and glaciers to create major valleys and the arms of firths and voes (Mykura 1976). The parallel ridges and valleys of eastern Mainland (Fig. 7.3) reflect near-vertical foliation and differential erosion of psammites, pelites, quartzites and limestones, resulting in a subdued Appalachian-style relief (Coque-Delhuille and Veyret 1988), one of the best examples of its kind in Scotland, comparable to the terrain flanking the Ladder Hills in the northeast Grampians. In contrast, west of the Walls Boundary Fault Zone, more varied relief reflects the diverse geology, which includes granite intrusions and folded Devonian sandstone and lavas.

Fig. 7.3
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Geological controls on Appalachian-style relief, eastern Mainland. a Geology. (Based on British Geological Survey mapping (© UKRI)). b Relief. (Based on Ordnance Survey data © Crown copyright and licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license)

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Three case studies illustrate how different rock types give distinctive terrains at local scales on Shetland. First, weathered bedrock is exposed in many bays and valleys, where it is often associated with zones of hydrothermal alteration. In the Burn of Tactigill, east of Tresta, the alteration zone in mica schist is up to 30 m wide and follows the line of a NNE–SSW fault (Coque-Delhuille and Veyret 1988). The most widespread type of weathering is a sandy regolith found in pockets beneath till. The presence of weathered granite up to 10 m thick also partly explains the erosion of the deep north–south valley of Petta Dale in east Mainland.

Second, the rugged peninsula of Muckle Roe displays many elements typical of granite scenery. The 5 km wide stock-like intrusion of granophyre is of homogeneous mineralogy, but is cut by major crush zones and shows wide variations in fracture spacing (Mykura and Phemister 1976). Ice flow was parallel to NNW–SSE fracture sets and has excavated rock basins and trenches and heightened inland cliffs along this trend. Surficial blocky and granular disintegration of the granite is common along hematite-stained shear zones 1–10 m wide. These vertical zones of weakness have acted as major controls on the locations of geos (narrow, cliffed inlets) and bays at the coast (Fig. 7.4a) and trenches and basins inland. Similar fracture sets form parallel geos in the sandstones of south Mainland.

Fig. 7.4
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a North Ham, Muckle Roe. Blocky disintegration of hematite-stained granophyre in the foreground. In the background prominent NNW–SSE fracture sets form geos on Harri Stack. b The skeletal serpentine soils of the Keen of Hamar support nationally rare plant communities. (Images: a Adrian Hall; b ©Lorne Gill/NatureScot)

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Third, on Unst, the mineralogy of ultrabasic rocks has strongly influenced slopes, soils and vegetation. Soils are skeletal and plant growth is locally inhibited, exposing the ground surface to disturbance by frost action and slopewash. The Keen of Hamar National Nature Reserve has many plant rarities on its extensive areas of serpentinite rubble (Carter et al. 1987; Fig. 7.4b). High alkalinity has also prevented peat development but made possible the local cementation of screes with travertine (Flinn and Pentecost 1995) and the accumulation of Late Holocene spring-fed diatomite (Flinn 1996).

3 Pleistocene Glacial History and Landforms

The maritime setting of Shetland, its generally low topography and position at ~60° N strongly influenced the regional impacts of Pleistocene fluctuations in temperature and sea level. During cold stages, sea level fell as low as −130 m and an ice cap developed over Shetland which, at its maximum extent, advanced to the edge of the Atlantic continental shelf (Chap. 4). During warm stages, sea level rose close to its present level and a maritime climate, similar to today’s, was established.

3.1 Landforms and Landscapes of Glacial Erosion

Shetland is a narrow archipelago with a linear ice-shed zone that exhibits little or no glacial modification, flanked to the west and east by belts of more pronounced glacial erosion (Fig. 7.5). There is a discordance, however, between the radial pattern of ice flow established during the last cold stage and the NNE alignment of valleys on Mainland. A possible explanation is that the valleys were eroded mainly in periods when Shetland was occupied by an icefield of limited extent, with topographically constrained ice flow.

Fig. 7.5
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(Adapted from Hall 2013; Creative Commons Attribution 4.0 International License)

Glacial features of Shetland.

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The axial ice-shed zone is evident from the distribution of striae and ice-roughened bedrock. Ice roughening is not widely developed along the former ice shed, and remnants of weathered and altered bedrock occur frequently (Fig. 7.5), both features indicating limited glacial erosion. The intensity of glacial erosion also decreases with altitude. On Ronas Hill, there is a sharp upper limit to ice roughening at ~200 m, with glacially disturbed granite regolith at lower altitudes. Close to the summit, granite surfaces carry weathering pits up to 15 cm deep that likely relate to weathering before the last glaciation. The hill summits on Foula also show few signs of vigorous glacial erosion (Bradwell et al. 2019).

In western Mainland, ice-roughened scenery is widespread, with striae and roches moutonnées indicating the movement of ice to the west and northwest. Knock-and-lochan terrain is developed on rock types with variable, but locally wide, joint spacing, such as the gneisses, granites and psammites on Vementry. On the east coast, rock basins and ice-moulded crags indicate ice flow towards the east and northeast (Fig. 7.5). Where sharp boundaries exist between smooth slopes and roughened terrain, for example, north of Ronas Hill (Bradwell et al. 2019), the small elevation differences indicate that the depth of glacial erosion necessary to produce this ice-roughened terrain may be <10 m. Rough, boulder-strewn granite terrain known as the Giant’s Garden found northeast of Ronas Hill may have formed under a late phase of glacial ripping (Hall et al. 2020; Fig. 7.6a).

Fig. 7.6
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a Ice-roughened granite terrain at Beorgs of Housetter, near Ronas Hill. Dilated rock fractures and boulder spreads suggest that glacial ripping operated during deglaciation. b Weisdale Voe, a glacially eroded valley drowned by postglacial sea-level rise. c Fugla Ness. Interglacial peat is overlain by periglacial slope deposits and till deposited by the last Shetland Ice Cap. d Till-filled geo head, South Harbour, Fair Isle. (Images: a, d Adrian Hall; b, c John Gordon)

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Valleys on Shetland are relatively shallow, reflecting the limited relief of the landmass. The highest cliffs on valley sides occur where the valleys have been cut through a ridge of high ground, as on Ronas Voe beneath the cliffs of The Brough, or where the ice has flowed parallel to the structural grain, as on the flanks of Weisdale (Fig. 7.6b). The degree of valley connectivity is high, with glacial valleys separating hill masses, as on Whiteness. This compartmented relief is perhaps best expressed in the isolation of islands, such as Whalsay and Vementry, from the neighbouring landmass. Important through-valleys exist, notably The Daal on Foula and crossing the isthmus of Mavis Grind. The Quarff gap, southwest of Lerwick, may be a relic of the Devonian landscape (Flinn 1977).

Shetland’s shallow voes and firths are valleys that have been drowned by Lateglacial and Holocene sea-level rise (Fig. 7.6b). Many show undoubted evidence of glacial erosion, with ice-moulded bedrock on their flanks, steep sides, a straight or gently-curving course and the presence of enclosed rock basins, but the depth of glacial erosion has often been modest and nowhere exceeds 100 m.

Many voes, such as Sullom Voe, have dendritic valley systems at their heads. Valley incision has been facilitated by the steep gradients for ice flow provided by the drop between the current landmass and the seabed offshore, which falls steeply towards −100 m. Rock basins occur offshore and may lie hidden beneath sediment on valley floors onshore. Yell Sound has deeps down to −90 m. Rockhead in the deepest part of St Magnus Bay is in Permo-Triassic sandstone and lies at −200 m, buried under 80 m of Late Quaternary fill.

3.2 Deposits Pre-dating the Last Glaciation

The present landscape of Shetland represents the cumulative imprint of multiple episodes of Pleistocene glaciation. More recent glaciations have largely removed evidence for earlier events, but two critical sites preserve a detailed record of changing environmental conditions on the islands before the last (Late Devensian) ice sheet. At Fugla Ness, a layer of peat resting on till is overlain by periglacial slope deposits and then till deposited by the last ice sheet (Fig. 7.6c). The peat preserves a pollen record and subfossil wood remains, mainly the roots of Scots Pine (Pinus sylvestris), that demonstrate a former tree cover in Shetland. The age of the peat is uncertain. Pollen evidence suggests correlation with penultimate or Gortian/Hoxnian interglacial deposits in Ireland (Birks and Ransom 1969). However, a last interglacial age is supported by a Uranium-series date of 110+40/–30 ka on the peat itself (Hall et al. 2002).

At Sel Ayre, organic deposits overlie periglacial slope deposits and contain pollen assemblages that represent vegetation covers ranging from grasses to a rich herb flora (Birks and Peglar 1979; Hall et al. 2002), distinct from those at Fugla Ness. Luminescence ages of 98–105 ka obtained for overlying sands place these deposits in Marine Isotope Substage (MIS) 5c (Brørup Interstade). Overlying deposits indicate the return of periglacial conditions during the cold interval MIS 5b, followed later by full glacial conditions and deposition of till by the last ice sheet.

3.3 The Last (Late Devensian) Glaciation

The maximum limits of the British-Irish Ice Sheet (BIIS) during the Middle and Late Pleistocene lie close to the NW Atlantic shelf edge (Chaps. 4 and 6). Offshore data indicate that the Shetland Ice Cap (SIC) extended well beyond the current landmass to the east and north, and close to the edge of the continental shelf west of Shetland (Johnson et al. 1993; Stoker et al. 1993; Ross 1996; Bradwell et al. 2008, 2019; Clark et al. 2012). In the North Sea, the BIIS was at times confluent with the Fennoscandian Ice Sheet (FIS; Graham et al. 2011).

There has been debate since the nineteenth century as to whether Shetland supported a local ice cap during the last glaciation (Flinn 1978), or whether the islands were overwhelmed by the FIS (Peach and Horne 1879). However, firm evidence for the former presence of the FIS (or ice from mainland Scotland) is absent on Shetland. In western Shetland, ice flowsets (Golledge et al. 2008) generally conform to patterns of westward ice flow recognized from the dispersal of erratics, striae and roches moutonnées (Mykura and Phemister 1976; Flinn 1978; Hall 2013). Evidence from eastern Shetland, however, in the form of mega-scale glacial lineations (Golledge et al. 2008) and glacitectonic structures (Carr and Hiemstra 2013), has been interpreted to suggest westward ice flow. This evidence conflicts with reports of only one supposed Scandinavian glacial erratic on Mainland, the absence of evidence for glacial transport of Permo-Triassic and younger Mesozoic debris across the islands from offshore basins located to the east, and the apparent absence of any glacial deposits from the FIS (Hall 2013). Erratics on Fair Isle, however, include rock types sourced from Shetland, Scotland and, possibly, Norway (Hall and Fraser 2014).

The evidence of radial ice-flow patterns from erratics, glacial bedforms and striae provides little support for flow of external ice across Shetland (Fig. 7.5). Instead, these patterns support the existence of an independent Shetland ice cap throughout the Late Devensian (Hall 2013). Recent results show widespread and significant spatial fluctuations in the extent and flow configuration of the SIC (Bradwell et al. 2019; Chap. 4). At its maximum extent at ~26 to 25 ka, the SIC was confluent with the FIS in the northern North Sea. Subsequent retreat led to parting of these ice masses, with establishment of a seaway to the east of Shetland by ~19 ka. The SIC shrank quickly at 19–18 ka, and eventually disappeared between 16 and 15 ka. The dynamism of the last SIC was a product of high snowfall on the Atlantic margin, limited topographic control and a high susceptibility to changes in sea level and the rapid loss of ice mass through iceberg calving as sea level rose.

3.4 Landscapes and Landforms of Glacial Deposition

The depositional zone of the last SIC lies largely offshore. Sets of moraines on the West Shetland Shelf have been interpreted to represent multiple phases of expansion of the SIC during the Middle and Late Devensian (Davison 2004). The pattern of deglaciation around Shetland has become much clearer in recent years with the advent of detailed imagery of bedforms on the adjacent seabed (Bradwell et al. 2008, 2019; Chap. 4). The SIC retreated towards the spine of the islands from both west and east. Moraines on Papa Stour and on the adjacent seafloor mark late stages of ice retreat into St Magnus Bay (Bradwell et al. 2019; Chap. 6), and De Geer moraines on the floor of the Pobie Basin, to the east of Unst, indicate southward retreat of the ice margin (Bradwell et al. 2008). Moraines on Unst (Golledge et al. 2008) mark positions of the western flank of this retreating ice lobe. Clusters of cosmogenic nuclide exposure ages obtained for sites on Papa Stour, Muckle Roe and North Roe shows that the SIC had retreated onto the present land area by 17 ka, with large tidewater glaciers discharging into the main voes and sounds between the islands (Bradwell et al. 2019).

4 Periglacial Landforms and Deposits

At several sites on Shetland, till rests on brecciated rocks that appear to have been subjected to significant frost weathering and mass movement under periglacial conditions at elevations down to present sea level. At Fugla Ness and Sel Ayre the organic deposits are overlain by periglacial slope deposits which pre-date the last SIC (Fig. 7.6c).

Upland periglacial landforms and deposits are well represented on the granite regolith at Ronas Hill (Ball and Goodier 1974; Coque-Delhuille and Veyret 1988). They include blockfields and stone-banked terraces at 365–427 m, comparable to those on the Cairngorms at altitudes above 1000 m (Fig. 7.7a) Whilst the mountain-top regolith likely has an earlier origin, these features were probably last active during the Lateglacial.

Fig. 7.7
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Periglacial landforms on Shetland. a Granite blockfield, Ronas Hill. The remnants of an actively eroding aeolian sand sheet occur in the middle distance. b Wind stripes, Ronas Hill. (Images: John Gordon)

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Other periglacial features on Ronas Hill have been active during the Holocene at elevations that are unusually low for the British Isles. These active forms include solifluction terraces and deflation surfaces, reflecting a prevailing subarctic-oceanic climatic environment of extreme wind speeds, heavy but now short-lived snowfalls, high soil-moisture levels, except in high summer, and frequent freeze–thaw cycles. Also present are horizontal and oblique turf-banked terraces, wind-patterned vegetation (wind stripes) and composite stripe-terrace features (Veyret and Coque-Delhuille 1989; Fig. 7.7b). Active frost-sorted patterned ground even occurs near sea level on Fetlar and Unst, on serpentine soils that are inimical to vegetation colonization (Spence 1957; Ball and Goodier 1974; Carter et al. 1987).

5 Peat Erosion and Bog Bursts

Approximately 50% of Shetland is covered by blanket bog several metres deep developed on acid substrates (Veyret and Coque-Delhuille 1993). The bog covers the base of the slopes and ridge and hill tops up to an altitude of about 200 m, including slopes that reach 14°–15°. Areas of actively eroding bare peat, gullies and peat hags are widespread (Birnie 1993). Under extreme rainfall events, the peat may become saturated and unstable, leading to bog bursts and peat slides, as occurred in 1990 and 2003, with deep erosion of the peat, often exposing the bleached bedrock surface underneath (Veyret and Coque-Delhuille 1993; Dykes and Warburton 2008; Fig. 7.8).

Fig. 7.8
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Peat slide north of Voe, north Mainland. (Image: John Gordon)

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6 Coastal Landforms

The coast of Shetland is predominantly hard and rocky, characterized by strong structural controls and an impressive array of dramatic cliffs, caves, stacks and arches. The archipelago sits between two storm-swept seaboards, the Atlantic and the North Sea. With relatively deep water extending close to the coast, there is often limited wave attenuation, and much of the outer coast is subject to a severe wave-energy environment. The coastline includes sea cliffs up to 275 m high, some of the highest in Europe. The cliffs are the present-day terminations of ridges and hills that have been truncated by the retreating coastline. The sea caves on Papa Stour and Eshaness are amongst the longest (up to 300 m) in the world. The occurrence of till deposits within the heads of geos (Fig. 7.6d) indicates that parts of the Shetland coast predate at least the last glaciation and formed when interglacial sea levels were similar to that of the present. Nevertheless, almost all substantial glacial deposits lie offshore (Bradwell et al. 2008) and the lack of onshore glacial and periglacial sediment has restricted sediment availability for beach-building. Beaches and dunes are therefore limited to a few locations, mainly within bays or other sheltered sites where the impact of wave activity is reduced, allowing retention of locally available sediment. In general, the coastal landforms of Shetland reflect the influence of four factors: strong structural controls, the effects of past glaciations, rapid sea-level change over millennia and a relatively severe morphogenetic environment.

6.1 Structural Controls on Coastal Landforms

Shetland is dominated by the largely north–south or NNE–SSW orientation of major faults and lithological boundaries. Selective weathering and erosion along faults, most recently by glaciers, has resulted in the formation of several large firths and numerous linear sea lochs (voes) that extend far inland. The overriding control of structure is best seen in southwest and northeast Mainland where rising sea level has flooded low ground to form a series of long parallel inlets controlled by the strike of the underlying structure (Fig. 7.3). At the local scale, changes in lithology, minor faults, fracture zones and dykes have influenced the development of almost all the smaller inlets, geos and other landforms that fret the entire coast (Fig. 7.9a, b). Many voe and bay heads expose rocks that are altered, weathered or otherwise fractured, with low resistance to wave attack. Lithology has also strongly influenced the propensity of cliffed coastlines to landsliding: 72% of 128 recorded coastal rock-slope failures on Shetland are seated on metasedimentary rocks with relatively few on other lithologies (Ballantyne et al. 2018).

At Eshaness, flat-lying and gently-inclined andesitic tuffs, ignimbrites and lavas form almost vertical cliffs that rise from deep water to heights of ~45 m; locally these have stepped profiles, with steep cliff faces in the resistant tuffs and wide terraces cut across blocky, andesite lavas (Hansom 2003a). The coast here is fully exposed to Atlantic storm waves and, in common with Papa Stour to the south (Hansom 2003b), marine exploitation of numerous vertical and horizontal fractures in these extrusive rocks has resulted in the development of numerous stacks, arches, geos and subterranean passages, some of which have extended into spectacular blowholes (Fig. 7.9c).

Fig. 7.9
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a The cliffed coast of Westerwick, central Mainland, is typical of the rocky and fretted Shetland coast with steep cliffs, stacks and skerries. b View north over Ness of Burgi, Sumburgh, south Mainland. Steeply dipping sandstone with SSW–NNE orientated fractures has been exploited by waves to form parallel geos. The lower parts of the 10–20 m high cliffs have been cleared of loose rock by storm waves that have also stripped vegetation, soil and regolith from the cliff tops. Cliff-top storm deposits occur on the most exposed parts of the headland. c Christie’s Hole in Papa Stour is a till-capped cliff, geo and subterranean extension where part of the roof collapsed in 1981. (Images: a, b James Hansom; c Adrian Hall)

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6.2 Glaciation Control on Coastal Landforms

Although the sides of some voes and firths in southwest and northeast Mainland exhibit evidence of ice-moulding and glacial steepening, most Shetland inlets are relatively unmodified, possibly pre-glacial, valleys that have been drowned by sea-level rise. The north–south axis of the Shetland ice cap during successive glacial cycles meant that ice flowed at right angles to much of the coast, across the structural grain, producing glacially moulded and streamlined cliff-top surfaces but leaving cliff faces relatively unmodified. The preservation of sediments pre-dating the expansion of the last SIC below the west-facing cliff top at Sel Ayre (Sect. 7.3.2) illustrates the lack of glacial modification of lee-side cliffs. In general, only limited amounts of glacial deposition have occurred on land in Shetland; this has resulted in a lack of sediment available for substantial beach development.

6.3 Sea-Level Control on Coastal Landforms

The present form of the Scottish coastline reflects the interaction of eustatic sea level rise since the Last Glacial Maximum (~26 to 19 ka) and glacio-isostatic uplift caused by the shrinkage and eventual disappearance of the last ice sheet. The amount of glacio-isostatic uplift declines with distance from the axis of uplift in the western Grampian Highlands. On Shetland, >450 km from the uplift axis, eustatic sea-level rise has outpaced uplift throughout the Lateglacial and Holocene (Shennan et al. 2018), leading to progressive drowning of the coastline; postglacial emerged beaches are therefore absent. A relative sea-level (RSL) curve for Shetland constructed by Bondevik et al. (2005) indicates that at ~8 ka RSL stood at −30 to −20 m and by ~4 ka it lay at −4 m, thereafter gradually rising to present levels. At several locations, terrestrial peat deposits of mid-Holocene age now occur in the intertidal and subtidal zones due to subsequent inundation (Mykura 1976; Birnie 1981; Bondevik et al. 2005).

Although many of the cliffs on the outer coasts of Shetland plunge to depths of up to 80 m, there is in some locations a distinct break of slope at or just below the present intertidal zone, with nearshore rockhead sloping gently away from the base of cliffs, though intertidal or sub-tidal rock platforms produced by postglacial cliff retreat are narrow or localized (Flinn 2014). Small, structurally-controlled bedrock ramps also exist at present sea level in several locations, such as on Foula, Fetlar and north of Eshaness. Intriguingly, small, glacially eroded, low-relief bedrock benches at ~20 m in NE Unst, at Ness on North Roe and at Eshaness have been interpreted as emerged shore platforms (Coque-Delhuille and Veyret 1988), possibly implying interglacial sea levels markedly higher than those of the present. The sea floor adjacent to Shetland displays steps and nearly horizontal surfaces, commonly at depths of ~24 m, ~46 m and ~82 m below present sea level (Flinn 1964). The ~46 m and ~82 m surfaces have parallels with similar submerged platforms around St Kilda (Chap. 9), and probably indicate marine plantation during multiple periods of low sea level.

Sea-level control may also have influenced the pattern and timing of coastal landslides on Shetland. Cosmogenic 10Be exposure ages obtained for two coastal landslides on Fetlar demonstrate that these occurred at ~4.5 ka and ~4.8 ka, at a time when rising sea levels allowed storm waves to access the cliff foot and initiate instability (Ballantyne et al. 2018). Moreover, 24% of 128 recorded coastal landslide sites on Shetland exhibit evidence of recent activity in the form of fresh scarps, areas of stripped bedrock and accumulations of debris at the base of cliffs. The chasm of The Sneck on Foula is an outstanding example of an impending rock-slope failure.

6.4 Morphogenetic Environment and Coastal Landforms

The Shetland coast is subject to strong winds with a high frequency of stormy conditions. Mean annual wind speed is 6.5–7.5 m s−1, with 58 days of gales per year, mainly from the southwest, northwest and north (Barne et al. 1997). During a storm in January 1992, gusts reached 34.7 m s−1 (125 km h−1) at Lerwick, with an unofficial gust of 67 m s−1 (241 km h−1) recorded at Muckle Flugga, Unst. The Shetland wave climate is equally severe, with waves at the Schiehallion oil platform, 160 km west of Eshaness, regularly exceeding 20 m and with the highest individual wave in January 1992 reaching 28 m, only 1.1 m less than the highest individual wave ever recorded, at Rockall, 250 km west of Scotland (Holliday et al. 2006). Foley (2019) has suggested that between 1900 and 2009 there may have been an increase in the intensity, but not the frequency, of cyclonic weather associated with severe gales and associated storm waves. Deepwater waves generally attenuate as they approach shore and this is the case within the Shetland voes and inlets, where small beaches and spits have developed. Along much of the outer Shetland coast, however, steep or vertical cliffs plunge into deep water; wave attenuation is therefore often very limited at such locations and the shore itself can be exposed to exceptionally high wave energies (Fig. 7.9a).

Large unbroken incident waves regularly impact steep cliff faces on Shetland with enough force to quarry large blocks from the upper cliff face and top surface, before decoupling into green-water bores capable of transporting the detached blocks landward over the cliff-top surface and depositing them in cliff-top ridges (Hall et al. 2006; Fig. 7.10a). Well-developed examples include Grind of the Navir and nearby Villians of Hamnavoe, at Eshaness (Hall et al. 2008) and the cliffs of Out Skerries and Mousa on the North Sea coast (Hall et al. 2006). At these locations, cliff-top storm deposits (CTSDs) of large boulders up to 2 m long have accumulated in multiple ridges up to 3 m high and 50 m from the cliff edge (Fig. 7.10). Modelling by Hansom et al. (2008) suggests that cliff-top bore velocities of up to 7 m s−1 and bore depths of up to 5 m are required to effect boulder transport at Grind of the Navir, a conclusion supported by video recordings during storms in 2011 and 2013.

Fig. 7.10
figure 10

a Six cliff-top storm deposit (CTSD) ridges at Grind of the Navir lie at up to 20 m above sea level and were emplaced and regularly reorganized by overtopping storm waves. b The wave-eroded basin at Grind of the Navir is accessed by bores of overtopping water that have transported large boulders (in the foreground) landwards to accumulate in CTSD ridges, tens of metres from the cliff edge. (Images: James Hansom)

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Repeat surveys (Fig. 7.10b) and modern human-derived flotsam trapped within the ridges demonstrate that CTSDs are subject to frequent reorganization during major storms (Hall et al. 2008). However, radiocarbon dating of marine shells within the ridges and of peat above and below storm-deposited boulders, together with optically stimulated luminescence ages of intercalated sand, provide approximate ages of enhanced CTSD emplacement during periods of increased storm activity during AD 400–550, AD 700–1050, AD 1300–1900 and since AD 1950 (Hansom and Hall 2009). Despite claims to the contrary, CTSDs do not appear to be tsunami deposits and no records of significant tsunami impact on Shetland coasts exist for the past ~1500 years (Hall et al. 2010). As elsewhere on North Atlantic coasts (Suanez et al. 2009), stormwave impact appears to have been responsible for both the origin and ongoing modification of CTSDs.

However, three possible older tsunami layers (mainly sand) have been recorded at various sites in Shetland (Bondevik et al. 2005). The earliest, dated to ~8.1 ka, is attributable to the well-documented Storegga submarine landslide west of Norway (Dawson et al. 1988). The two younger ‘tsunami’ events are estimated to have occurred at ~ 5.5 ka and ~1.5 ka (Dawson et al. 2006), but their origin is unknown and evidence for post-Storegga tsunami events has yet to be found outside of Shetland (Smith et al. 2019).

In locations sheltered from the most severe storm waves, beaches, dunes and tombolos have developed where sand and gravel are locally available. Minor sand, gravel and boulder beaches are common at cliff-foot and bayhead settings, but extensive beaches are mainly limited to the southwest coast of Mainland, where the St Ninian’s Isle tombolo (Fig. 7.11a) is the largest geomorphologically active sand tombolo in Britain. Despite ongoing rising sea levels in Shetland, the 500 m long tombolo has been in existence since at least AD 1700. Strikingly symmetrical in plan, it is subject to wave activity from opposing north and south directions. With waves breaking simultaneously along the entire length of the two flanking beaches, the planforms of both beaches appear to be in dynamic equilibrium, despite overtopping during storms. With 50% shell-sand content, the tombolo is currently nourished by nearshore shell banks and more locally from thin layers of till- and rock cliff-sourced gravels (Hansom 2003c). Excess sand is blown from both ends of the tombolo into climbing dune systems, the largest of which lies at the mainland end.

Fig. 7.11
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a The strikingly symmetrical St Ninian’s Isle tombolo in south Mainland is the largest geomorphologically active sand tombolo in Britain. b Ayres of Swinister. Only the South Ayre (right) is a true tombolo; the others are spits that enclose The Houb tidal lagoon. The central spit is younger, less well developed and formed of smaller and more angular sediment than the outer ayres that are exposed to ocean waves. (Images: a © Colin Smith, Creative Commons Licence; b © Robert W. Watt, Creative Commons Licence)

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Twin sand beaches backed by dunes join Sumburgh Head to Mainland and bear all the hallmarks of a wide tombolo with a low-lying, sand-infilled centre that now forms the foundations of an airport runway. Other sandy beaches and dunes occur nearby in relatively sheltered sites. At Quendale, close to Sumburgh Head (Kelley et al. 2018), dune systems became mobile during the exceptionally stormy conditions of the Little Ice Age and buried settlements and farmland (Bampton et al. 2017). Significant changes to beaches on Unst also occurred at this time (Preston et al. 2020). Elsewhere in Shetland, many short spits and tombolos (locally called ayres) jut out from sheltered coasts, locally linking individual islands or connecting islands to Mainland. Many are found some way along, and often on both shores of, Shetland voes, at points where the transport capacity of waves has diminished sufficiently to allow sediment deposition. In such conditions, the ayres have built out from each shore and some have joined to form gravel tombolos. An outstanding example is the Ayres of Swinister in north Mainland, where three gravel spits are fed by angular and subangular gravels eroded from the adjacent rocky shore (Fig. 7.11b). They all extend from the north shore although only one, the South Ayre, forms a complete tombolo connecting an offshore island to Mainland. The other two extend out from the Mainland coast as spits that have rotated over time to become mid-bay barriers orientated to face incoming wave directions, and enclose a tidal basin called The Houb (Hansom 2003d). Subtidal peat dated to ~5.3 ka (Birnie 1981) partly floors The Houb and appears to underlie only its central barrier, suggesting that marine flooding arrested further peat development. No peat outcrops can be seen eroding out of the foreshores of the North and South Ayres, which suggests the central barrier formed later and was fed by locally derived, wave-winnowed sediments once The Houb had formed.

The past impact of sea-level rise on Shetland beaches has been partly documented by the Dynamic Coast project but since the coast is mainly rocky, only limited erosion has been identified since the 1970s (Hansom et al. 2017). However, it is likely that as sea-level rise accelerates, more widespread erosion will affect Shetland’s beaches.

7 Conclusions

The Shetland archipelago is outstanding for the scenic quality of its terrestrial and coastal landscapes. The underlying geology and rock structure have been exploited by long-term differential weathering and erosion to shape the dramatic coastline and the varied topography and landforms of the islands. Glacial erosion during the Pleistocene emphasized pre-existing valley patterns, the lower parts of which were drowned by rising postglacial sea level to form the intricate, strongly indented coastline. Offshore, in the bays and on the continental shelves, recessional moraines record the landwards retreat of the Shetland Ice Cap during the last deglaciation. Geomorphological highlights include the Appalachian-style relief in east Mainland, the assemblage of spectacular rock-coast landforms, storm-generated cliff-top deposits, coastal spits, bars and tombolos (including the largest active sandy tombolo in Britain), active wind- and frost-related periglacial landforms at relatively low altitudes, and peat slides and bog bursts on the blanket bog. The entire archipelago is included within Shetland UNESCO Global Geopark, a fitting accolade for a remarkable landscape fashioned by its diverse geology and geomorphology.