Holocene relative sea-level changes in Harris, Outer Hebrides, Scotland, UK

JOURNAL OF QUATERNARY SCIENCE (2010) 25(2) 115–134Copyright � 2009 John Wiley & Sons, Ltd.Published online 26 May 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/jqs.1281
Holocene relative sea-level changes in Harris,Outer Hebrides, Scotland, UKJASON T. JORDAN,1* DAVID E. SMITH,2 SUE DAWSON3 and ALASTAIR G. DAWSON4

1 Department of Geography, Environment and Disaster Management, Coventry University, Coventry, UK2 Oxford University Centre for the Environment, Oxford, UK3 School of Social Sciences – Geography, University of Dundee, Dundee, UK4 Geography and Environment, School of Geosciences, University of Aberdeen, Aberdeen, UK

Jordan, J. T., Smith, D. E., Dawson, S. and Dawson, A. G. 2010. Holocene relative sea-level changes in Harris, Outer Hebrides, Scotland, UK. J. Quaternary Sci., Vol. 25pp. 115–134. ISSN 0267-8179.

Received 22 August 2007; Revised 23 February 2009; Accepted 3 March 2009

ABSTRACT: Evidence for relative sea-level changes during the middle and late Holocene isexamined from two locations on the Atlantic coast of Harris, Outer Hebrides, Scotland, usingmorphological mapping and survey, stratigraphical, grain size and diatom analysis, and radiocarbondating. The earliest event identified is a marine flood, which occurred after 7982–8348 cal. a(7370� 80 14C a) BP, when the sea crossed a threshold lying at �0.08 m Ordnance Datum Newlyn(OD) (�2.17 m mean high water springs (MHWS)) before withdrawing. This could have been due to astorm or to the Holocene Storegga Slide tsunami. By 6407–6122 cal. a (5500� 60 14C a) BP, relative
sea levels had begun to fall from a sandflat surface with an indicated MHWS level of between 0.08 and�1.96 m (�2.01 to �4.05 m). This fall reached between �0.30 and �2.35 m (�2.39 to �4.44 m) after5841–5050 cal. a (4760� 130 14C a) BP, but was succeeded by a relative sea-level rise which reachedbetween 0.54 and �1.57 m (�1.55 to �3.66 m) by 5450–4861 cal. a (4500� 100 14C a) BP. Thisrise continued, possibly with an interruption, until a second sandflat surface was reached between2.34 and �0.26 m (0.25 to �2.35 m) between 2952–3375 cal. a (3000� 80 14C a) and 1948–2325 cal.a (2130� 70 14C a) BP, before present levels were reached. The regressive episode from the earliestsandflat is correlated with the abandonment of the Main Postglacial Shoreline. It is maintained that thefluctuations in relative sea level recorded can be correlated with similar events elsewhere on theperiphery of the glacio-isostatic centre and may therefore reflect secular changes in nearshore seasurface levels. Despite published evidence from trim lines of differential ice sheet loading across thearea, no evidence of variations in uplift between the locations concerned could be found. Copyright #2009 John Wiley & Sons, Ltd.
KEYWORDS: morphology; stratigraphy; grain size; diatoms; radiocarbon dating; Holocene; relative sea-level changes; glacio-isostasy.

Introduction

Studies of sea-level change have long featured on the agenda ofQuaternary science research, and in recent years majoradvances have been made. In areas affected by glacio-isostasy,detailed empirical studies, together with the modelling ofcrustal displacement, have provided valuable insights into thetemporal and spatial patterns of relative sea-level changeduring the Holocene. However, most studies identify onlybroad trends, and detailed fluctuations, observed at manylocations, are rarely correlated extensively over coastlinessurrounding glacio-isostatic centres, thus questioning thelikelihood of nearshore secular sea surface changes. Yetrelative sea-level changes are likely to vary across areas ofglacio-isostatic uplift, with such fluctuations in the sea surface

* Correspondence to: J. T. Jordan, Department of Geography, Environment andDisaster Management, Coventry University, Coventry CV1 5FB, UK.E-mail: [email protected]

as may have occurred being registered more clearly towardsperipheral areas. This paper contributes to an understanding ofthis problem by examining two locations towards the peripheryof an area of glacio-isostatic uplift where sediments accumu-lated in sheltered environments, and where the detailedevidence for relative sea-level change can be examined inthe context of possible differential uplift of the land. Thelocations are on the coast of Harris, in the Outer Hebrides, offScotland’s western coast: Horgabost, in the north-west andNorthton, 10 km away to the south-west (Fig. 1).

Previous work

The Outer Hebrides have for some time been a focus of interestin coastal change, often in the context of archaeologicalstudies, but have not so far provided detailed evidence forHolocene relative sea-level change. Many authors haveremarked on the evidence of ‘submerged peat’ in the nearshore

Figure 1 Location of Harris and the sites examined, showing isobases for land uplift after Smith et al. (2007)

116 JOURNAL OF QUATERNARY SCIENCE

environment (see summaries in Ritchie, 1966, 1985) and somehave concluded that this constituted evidence that the sea levelhad risen in postglacial times (e.g. Geikie, 1878). Ritchie (1966)concluded that much of the postglacial (Holocene) sea-levelrise on North and South Uist, to the south of Harris, hadoccurred before ca. 5700 14C a BP, later (1985) maintainingthat sea level rose 3–5 m along the coasts of the Uists betweenca. 8800 and 5164 14C a BP. However, despite such estimatesof the Holocene rise in relative sea level, no empirically basedrelative sea-level graph has yet been produced for any locationin the Outer Hebrides. Most research on coastal change in thearea has concentrated on the extensive sand dunes and sandplains, or machair (Ritchie, 1966, 1972, 1979), with the aim ofdetermining episodes of sand movement, although the impactof possible relative sea-level changes has also been considered(e.g. MacClenahan, 1997; Whittington and Edwards, 1997;Gilbertson et al., 1999; Dawson et al., 2004).

Given the apparent lack of empirical evidence for relativesea-level change in the Outer Hebrides, the most recentcontributions have been in terms of models of glacio-isostaticuplift. Here, based upon glacio-hydro-isostatic modelling,Lambeck (1993a,b, 1995a), Peltier et al. (2002) and Shennanet al. (2006) have produced generalised relative sea-levelgraphs for the Outer Hebrides in the context of broader studies,and Lambeck (1991, 1993b, 1995b) has produced isobasemodels which include the Outer Hebrides. Smith et al. (2006)included the Outer Hebrides in their empirically based isobasemodels, later (Smith et al., 2007) comparing the distribution oftwo of their shorelines in an area which included Harris.

Little of the work quoted above has addressed Harrisspecifically, yet the area is of particular interest in the context ofrecent studies on the pattern of glaciation across the area. Sincethe proposal that the Outer Hebrides may have supported amajor ice centre (e.g. Flinn, 1978), studies of trim-lines onSouth Uist, Harris and Lewis (Ballantyne and McCarroll, 1995;Ballantyne et al., 1997, 1998; Ballantyne and Hallam, 2001;Stone and Ballantyne, 2006) have been employed to determineice surface altitudes at the Last Glacial Maximum (LGM). In

Copyright � 2009 John Wiley & Sons, Ltd.

particular, Ballantyne et al. (1998) propose that the surfacealtitude of the LGM ice sheet above its limit west of the OuterHebrides declined away from a maximum of�600 m across thehills of N Harris to �400 m in SE Harris. These observationsimply that ice may have been thicker over the more northerlysite of Horgabost than over Northton, even taking into accountunderlying topography. This raises the question of the effects ofdifferential ice loading on glacio-isostasy and therefore onrelative sea-level changes in the area.

Methodology and techniques employed atHorgabost and Northton

The two locations examined in this work are localised areas ofsedimentation in an otherwise open and high-energy coastline.To ensure that the results achieved might be indicative of thewider coastal area and not only specific to the locationsinvestigated, the strategy of the investigation was to applysimilar techniques at each location to obtain as detailed arecord as possible and to enable comparisons to be madebetween locations. This involved the following studies.

Field survey

Each location was mapped morphologically at a scale of1:10 000 up to a surrounding altitude of 25 m, focusing on thegeomorphology and sediments. Generalised maps of thevegetation present were also produced.

Stratigraphical work

At each location, particular interest was directed at shelteredcoastal marshlands. In these areas, initial work involved the use

J. Quaternary Sci., Vol. 25(2) 115–134 (2010)DOI: 10.1002/jqs

HOLOCENE RELATIVE SEA-LEVEL CHANGES IN HARRIS, OUTER HEBRIDES 117

of an Eijkelkamp gouge auger (model P) to determine thestratigraphy. Boreholes were generally made down to depths ofup to 8 m at intervals of 50 m or less; 77 boreholes were putdown in total. This approach allowed sites with representativestratigraphies to be selected. These sites were then sampledusing either a ‘Russian’ type peat sampler (Jowsey, 1966) or apowered Stitz percussion corer (Merkt and Streif, 1970). Thepercussion corer sometimes caused compression of thestratigraphic record, particularly of the organic sediments,and this was adjusted by comparing that record with the gougerecord from the borehole concerned. The approach is based onthe identification and correlation of units in the stratigraphy andis geomorphologically based. In consequence, microfossildiagrams were not statistically zoned; instead the record isdescribed according to each unit. In the text all units aredescribed according to depth in the core unless otherwisestated.

Altitude measurement

At each location, altitudes of all boreholes and certainmorphological features were determined by instrumentallevelling on closed traverses from Ordnance Survey benchmarks. The datum on Harris is a local datum and to relate this toOrdnance Datum Newlyn (OD) the mean value of the differencebetween mean sea level (MSL) Stornoway and MSLs on themainland at Kinlochbervie and Ullapool was used. MSLKinlochbervie is 0.2022 m below MSL Stornoway, and MSLUllapool is 0.1632 m below MSL Stornoway (P. Woodworth,pers. comm.). The mean of these values, 0.1827 m, wassubtracted from the altitudes of the OD (local) bench marksused. In the text all altitudes are given in terms of OD with theequivalent MHWS Stornoway value in parentheses, thus �1.44to �0.53 m (�3.53 to �2.52 m).

The tidal context

The coasts of Harris are mesotidal. The only station in acomparable location to the sites studied and which reports therelationship of chart datum to local ordnance datum isLeverburgh,on thenorth-west coast,whichparadoxically reportsa very similar tidal regime to the more extensive record atStornoway, on the Minch coast (Admiralty Tide Tables, 2000).Tidal values for Leverburgh and Stornoway are shown in Table 1.

Given the similarity between the Stornoway and Leverburghrecords and the greater detail of the Stornoway record, all tidalaltitudes in this paper are referred to the Stornoway record. Theequivalent mean high water springs (MHWS) Stornoway valuesin OD Newlyn terms are given in parentheses after themeasured OD Newlyn value.

Table 1 Tidal values for Leverburgh and Stornoway in metres ODNewlyn

Station LAT MLWS MLWN MTL MHWN MHWS HAT

Leverburgh N/A �2.51 �1.21 N/A þ0.39 þ1.49 N/AStornoway �2.35 �2.19 �0.89 �0.09 þ0.81 þ1.91 þ2.61

LAT, lowest astronomical tide; MLWS, mean low water springs;MLWN, mean low water neaps; MTL, mean tidal level; MHWN, meanhigh water neaps; MHWS, mean high water springs; HAT, Highestastronomical tide.


Grain size analysis

Grain size analysis was undertaken at each location in order tohelp determine the depositional environment of the minero-genic sediments studied, with the principal objective ofshowing whether sediments had accumulated in a dune ormarine/estuarine environment. This was achieved by compar-ing samples from the palaeoenvironment with those accumu-lating under present-day conditions. Accordingly, modernsurface samples were collected along a beach transect atHorgabost at 5 m intervals from the swash zone and foreshoreacross the main beach area and landward into the non-vegetated seaward margin of the dunes. The location of thebeach transect is shown in Fig. 2. All the samples frompalaeoenvironments were collected from boreholes. Grain sizewas determined by dry sieving. All samples were pre-treatedwith an acid digest to remove all organic matter. Descriptionsand interpretations of the grain size profiles follow Friedman(1961, 1967), Folk and Ward (1971) and McManus (1988).

Diatom analysis

Diatom analyses at both locations followed well-established,routine protocols. A minimum of 300 microfossils was countedat each level. Samples were prepared for diatom analysisfollowing the standard techniques outlined in Barber andHaworth (1981). Identification of individual species was madewith reference to Hartley et al. (1996), Hendey (1964) and Vander Werff and Huls (1957–1974). Classification into life formand salinity, as advocated by Vos and De Wolf (1993), wascarried out using Denys (1991). Sampling in the laboratory wasundertaken at an interval of 2 cm across changes in stratigraphyand 4 cm elsewhere, although these intervals were slightlylarger after corrections for compaction (see above) had beenmade. The percentage of broken or eroded diatoms was notedin order to aid understanding of the conditions under which thesediments were deposited.

Radiocarbon dating

Ten radiocarbon dates were obtained in this work. They wereobtained from horizons where changes in the local environmentof deposition are believed to have taken place, based onbiostratigraphy and/or lithostratigraphy. Dating was undertakenby Beta Analytic, Inc. Samples submitted were of bulk peat andwere between 1 and 3 cm thick, and stems and rhizomes ofPhragmites, as well as wood and twigs, were avoided. A list ofradiocarbon dates is given in Table 2. In the text all dates arequoted in calibrated years BP with an age range of 2s, and theequivalent radiocarbon age to 1s in parentheses, thus: 3375–2952 (3000� 80). Approximate ages are quoted thus: ca. 3200(3000).Calibrationwasmadeusing theCalib4programofStuiverand Reimer (1993) and Stuiver et al. (1998).

Results from Horgabost and Northton

Horgabost

Morphology

At Horgabost (Fig. 2) a sandy beach is backed by a single duneridge over 10 m high (Traigh Horgabost). Landward of the dune


Figure 2 The Horgabost area, showing the dune area, marshland and borehole transects


ridge lies a machair-type grassland and a marshland, LianaHorgabost, comprising an area of tussocks and Phragmitesreedswamp. The surrounding area to landward is occupied bysteep slopes with rocky outcrops. The area is drained by the GillMeody, which cuts through the dune ridge in a narrow defile toreach the sea. The Phragmites reedswamp area lies at 2.4–2.9 m(0.3–0.8 m) and is within the reach of highest astronomical tide(HAT).

Stratigraphy

The principal focus of interest was the marshland area of LianaHorgabost, the adjacent tussock area, and the machairgrassland to seaward. In this area, a number of boreholeswere put down to determine the stratigraphy. Boreholes weremade at intervals of approximately 50 m along transect linesrunning in a NW–SE direction across the area, broadly from theedge of the dunes into the marshland. Where the sedimentsencountered were particularly complex, the distance was


halved to approximately 25 m. It was not possible to bore in thewaterlogged centre or western side of the marshland. Boreholelocations are shown in Fig. 2 and generalised stratigraphicaldiagrams from two transects are shown in Fig. 3.

The basal unit of the stratigraphy is a dense grey sand withoccasional shells. The surface of this deposit is relativelyconsistent in altitude, ranging from �1.44 to �0.43 m (�3.53 to�2.5 2 m) in 19 boreholes over an area of 1 km2. It is widelyoverlain by a compact peat, sometimes sandy or silty,sometimes humified, and up to 0.56 m thick. Overlying thepeat are horizons of sand and silty or sandy peat, the sandoccasionally containing shell layers. Inland, these horizonsmerge into an apparently continuous sand or peaty sand layer,sometimes shelly. Over most of the area, a Phragmitesreedswamp peat then occurs to the surface, but along theseaward margin of the area the shelly sand of the dunes can betraced tapering into the overlying Phragmites reedswamp peatand here the peat overlying the dunes becomes moreherbaceous as it becomes shallower. The boreholes chosenfor detailed analysis (C1 and Z3, Figs 2 and 3) are representative


Tab

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Copyright � 2009 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 25(2) 115–134 (201DOI: 10.1002/jq


0)s

Figure 3 Top: The Horgabost area viewed from the south, showing the Phragmites reedswamp area (light coloured) and borehole transects. Below:Generalised stratigraphy along two transects at Horgabost


of the stratigraphy encountered. Core C1, to seaward, wastaken from close to the dune edge, while core Z3, to landward,was taken nearer to the hillslope inland.

Stratigraphical, grain size and microfossil analysis: core C1

From core C1, 11 units have been identified which are describedbelow. Figure 4 depicts the results of diatom analyses.

LH1 (4.04–4.31 m) – grey sand. The first of the diatom unitsidentified, LH1, comprises a predominantly marine andmarine/brackish diatom assemblage with a freshwater layerat 4.21 m. The majority of the species encountered were Paraliasulcata, Caloneis westii or Diploneis interrupta, with Fragilariaexigua being notable in the freshwater episode.

Grain size analysis was conducted at 4.12–4.26 m, whichshowed the sediments were a well-sorted fine to very fine sand.The sand showed a positively skewed frequency distributionaround a very platykurtic curve. Possible origins for thesediment could be a swash/lower beach.

LH2 (3.75–4.04 m) – dark, grey/brown humified peat – verysandy. Following the initial brackish–marine diatoms found at


the base of unit LH2, the unit is then almost entirely dominatedby a freshwater assemblage consisting of Fragilaria construensvar. venter and F. exigua. A 14C date of 4760� 130 (5841–5050) was obtained from 4.00–4.03 m, where the underlyingmarine influence terminates.

LH3 (3.18–3.75 m) – light-brown silty peat. LH3 sees a returnto the marine and marine/brackish diatoms found in unit LH1,with P. sulcata, C. westii and D. interrupta dominating theassemblage once more. There is a small ‘pulse’ of freshwaterspecies from 3.40 to 3.50 m including F. exigua, Fragilarianitzschoides and Fragilaria pinnata.

LH4 (2.79–3.18 m) – grey sand. In unit LH4 the diatomspectrum discloses a transition from the prevailing marine/brackish species-dominated assemblage, to a point at around3.08 m where the freshwater species Fragilaria leptostauronvar. dubia appears and dominates the count until the end of unitLH4 into unit LH5.

Grain size analysis was carried out at 2.79–2.83 m, whichshowed a moderately sorted fine sand. The mesokurticdistribution has a negatively skewed tail. The indication isthat it represents an upper beach/dune origin.


Figu

re4

Dia

tom

dia

gram

from

bore

hole

C1,

Horg

abost

Copyright � 2009 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 25(2) 115–134 (2010)DOI: 10.1002/jqs



LH5 (1.88–2.79 m) – light-brown silty peat. The diatomspectrum for LH5 is similar to that of unit LH3, with the marine/brackish water diatoms P. sulcata, C. westii and D. interruptadominating. The main difference is that P. sulcata declinestowards 2.30 m and is replaced by the brackish-water Naviculaperegrina. Coupled with this change, a rise in the number offreshwater species Fragilaria takes place. A 14C date obtainedfrom 1.89–1.92 m, 820� 50 (906–664), dates the fall in marine/estuarine influence towards the top of this unit and the initiationof a freshwater environment.

LH6 (1.86–1.88 m) – very shelly, peaty sand. The LH6 unitmarks the lowest point, or end, of the decline of the marine andbrackish water diatoms from unit LH5. Coinciding with thisdecline is a peak of freshwater diatoms, mainly F. construensvar. venter.

LH7 (1.10–1.86 m) – sandy, humified peat. The diatomspecies for LH7 are similar to LH6. There is a clear mix ofdiatoms from different environments, including brackishspecies and freshwater species. Once again, the dominantspecies are D. interrupta, N. peregrina and the freshwaterFragilaria spp., with Navicula digitoradiata occurring through-out. Towards the top of this unit there is an increase infreshwater species and a decline in brackish-water diatoms14C dated (1.11–1.13 m) from 430� 40 (537–325).

LH8 (0.58–1.10 m) – grey, shelly sand. Borehole evidenceshows that the LH8 deposit is part of the tapering inland edge ofthe dune. It has a predominantly freshwater diatom assem-blage. The most frequently occurring diatom is Cocconeisplacentula, followed by F. construens. There is a backgroundcount of brackish water N. peregrina, but this does not accountfor a large percentage of the overall assemblage.

The grain size analysis carried out at 0.58–0.62 m shows thesediment to be a medium to fine sand that is moderately wellsorted. The origin is akin to the modern upper beach/dune area.

LH9 (0.54–0.58 m) – sandy/silty peat. LH9 sees a dramaticdecline in the total amount of freshwater species and a relativerise in brackish species, namely D. interrupta and N. peregrina.

LH10 (0.30–0.54 m) – shelly, organic sand. Unit LH10contains a mixed environment of brackish-water and fresh-water diatoms. The assemblage displays a more diverse rangeof habitats than has previously been found in any of the otherunits.

Grain size analysis at 0.30–0.34 m showed a moderatelywell-sorted fine to medium sand, with a symmetrical but veryplatykurtic distribution. Again, this would be most like themodern upper beach/dune area.

LH11 (0.0–0.30 m) – Phragmites–Carex peat. The upper unit,LH11, is a freshwater peat that had a sample for 14C datingtaken from the base of this unit at 0.28–0.30 which yielded adate of 120� 40 (274–0).

Stratigraphical, grain size and microfossil analysis: core Z3

From core Z3 eight units have been identified. Descriptions ofthe units are given below and the diatom analysis is shown inFig. 5.


LH(Z)1 (4.11–4.20 m) – grey sand, slightly peaty. The diatomspectrum of the basal unit, LH(Z)1, comprises a marine/brackishassemblage. The count is dominated by the marine diatomP. sulcata, and the marine/brackish diatoms C. westii andDiploneis didyma. The diatom assemblage changes up-coretowards the top of the unit, with brackish/freshwater speciesbeing most notable. Species include D. interrupta, Fragilariafasciculata, F. leptostauron var. dubia, F. nitzschoides andF. pinnata.

Grain size analysis was carried out at 4.19–4.17 m. It showeda poorly sorted medium to fine sand with a negatively skewed,leptokurtic distribution. This is indicative of a swash/lowerbeach environment.

LH(Z)2 (3.67–4.11 m) – sandy peat, humified with depth.A 14C date from the base of the overlying LH(Z)2 unit gave5500� 60 (6407–6122). From these lower horizons the diatomrecord is marked by a sharp rise and subsequent fall in thebrackish water species D. interrupta and N. peregrina. After theinitial brackish peak, the freshwater diatoms F. construens var.venter, F. exigua, F. fasciculata and F. leptostauron var. dubiadominate the remainder of the unit. There is a short return ataround 3.90–3.85 m to a brackish assemblage dominated byC. westii. Following the initial rise in brackish water species,N. digitoradiata continues to be ever-present throughout therest of this unit into the overlying unit LH(Z)3, as does themarine diatom P. sulcata. At 3.70–3.67 m a 14C date of4500� 100 (5450–4861) was obtained.

LH(Z)3 (2.79–3.67 m) – grey sand with shell layers and peatyhorizons, especially towards the base. At the base of LH(Z)3there is a gradual change from predominantly freshwaterdiatoms to marine/brackish assemblages. As the freshwaterdiatoms of F. exigua, F. fasciculata and F. leptostauron var.dubia decline, the marine diatom P. sulcata, marine/brackishdiatoms C. scutellum and D. smithii, and the brackish diatomsC. westii and D. interrupta increase in number. With theexception of Cocconeis scutellum, it is this mix of diatoms thatdominate the majority of this unit, although the percentageproportions differ throughout. At 3.54 m there is a spike in therecord of P. sulcata. The brackish diatoms D. didyma andN. peregrina are both present above 3.60 m, but at no time dothey number any more than 20% of the total count. Towards thetop of this unit there is also an increasing amount of the marinediatom Grammatophora serpentina, but this never exceeds 10%of the total number of diatom frustules counted. 31 cm of core atthe upper boundary of this unit were lost, but from the gougerecord the lithostratigraphy is similar.

Grain size analysis was conducted at a number of depths,firstly at 3.55–3.51 m, which showed the sample to be amoderately sorted fine to very fine sand with a mesokurticdistribution with a negatively skewed tail. Again at 3.43–3.49 mthe sediment was moderately sorted fine sand with aleptokurtic, negatively skewed distribution, and at 3.21–3.17 m again moderately well-sorted fine to very fine sandwith a very negatively skewed distribution. All horizonsanalysed indicate a swash/lower beach environment.

LH(Z)4 (2.60–2.79 m) – sandy peat. The diatom assemblageof LH(Z)4 is similar to that in the unit below. With the exceptionof the brackish species D. didyma and N. digitoradiataincreasing (albeit by less than 15–20% of the total count)and the subsequent reduction in the total amount ofG. serpentina recorded, the remainder of the assemblage isthe same as that found in the lower unit. The main difference in


Copyright � 2009 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 25(2) 115–134 (2010DOI: 10.1002/jq


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the two units is the increase in the percentage of brackishdiatoms, notably D. didyma and N. peregrina.

LH(Z)5 (2.36–2.60 m) – shelly grey sand. Diatom analyses ofLH(Z)5 disclose a well-established brackish component similarto that of the underlying unit, but in contrast there is a decline inthe number of fully marine and marine/brackish species. Thiscomponent appears to have been replaced by a freshwaterspecies, F. exigua. As well as this change, there is also anoticeable change in the proportion of C. westii toD. interrupta, with the former decreasing and the latterincreasing.

Grain size analysis at 2.60–2.54 m showed that the sedimentwas poorly sorted fine to very fine sand with a very leptokurticcurve, with a pronounced negative skewness. Analysis was alsocarried out at 2.40–2.44 m, which showed a moderately sortedfine to very fine sand with a platykurtic negatively skewed tail.Both samples indicate a swash/lower beach environment.

LH(Z)6 (1.83–2.36 m) – light-brown silty sandy peat. Thediatom spectrum of LH(Z)6 shows a similar diatom assemblageto that of units LH(Z)3 and LH(Z)4. Unlike the underlying unitLH(Z)5, the freshwater component of the assemblage is notdeveloped and instead exhibits a series of peaks and troughs.Marine and brackish marine diatoms are consistently present,with sharp peaks in both P. sulcata and D. smithii, the latterreaching a count of 30% at 2.25–2.10 m. The peaks of thesespecies follow one another in the record, the D. smithii peakspossibly reflecting the aftermath of marine inundationsassociated with earlier P. sulcata peaks.

Grain size analysis at 1.94–1.90 m gave a moderately sortedfine to very fine sand with a platykurtic distribution with anegatively skewed tail, which is indicative of a swash/lowerbeach environment.

LH(Z)7 (1.14–1.83 m) – silty, possibly sandy, peat. UnitLH(Z)7 predominantly consists of brackish water species. A14C date of 2130� 70 (2325–1948) from 1.82 to 1.79 m marksthe rise in brackish water diatoms. The main brackishcomponents are C. westii, D. interrupta and N. peregrina,with a background count of the marine diatom P. sulcata neverexceeding 15% of the total. At this level, freshwater diatomspecies make an appearance, and from 1.17 to 1.14 m a14C date of 740� 50 (759–562) marks the commencement offreshwater conditions.

LH(Z)8 (0–1.14 m) – fibrous, dark-brown peat, humifiedwith depth and shelly at surface. In the upper LH(Z)8 unit,diatom analysis from 1.14 to 0.75 m discloses the continued fallin brackish water species, namely N. digitoradiata andN. peregrina, and a rise in freshwater diatom counts. The

Table 3 Recent published records of modern sandflat altitudes (m, OD) in

Location Tidal Station MHWS MHWN MTL MLW

Dornoch Firth Meikle Ferry 2.3 1.3 0.38 �0.6Beauly Firth Inverness 2.6 1.5 0.5 �0.5Bute Rothesay Bay 2.0 1.4 0.5 �0.4Firth of Clyde Ardrossan 1.6 1.0 0.2 �0.5Ayrshire coast Troon 1.6 1.0 0.2 �0.6

MHWS, mean high water springs; MHWN, mean high water neaps; MTL, mesprings.


freshwater assemblage is dominated by F. construens,F. construens var. venter, F. exigua, F. pinnata and F. ulna.There is some interchange between these species with amarked decline in the number of F. pinnata and a subsequentrise in F. ulna at around 1.0 m, for example. Throughout thisunit, P. sulcata is present, possibly reflecting the nature of thebroader depositional environment.

Environmental interpretation of the evidence at Horgabost

The landforms, lithostratigraphy and biostratigraphy at Horga-bost permit inferences to be drawn on coastal changes whichtook place there in the middle and late Holocene. Beneath themarshland of Liana Horgabost, a widespread surface of greysand with grain size characteristics similar to those of thepresent beach environment and containing predominantlymarine and brackish marine diatoms probably accumulated asa sandflat. Published altitude measurements taken at the innermargin or highest point on modern sandflats in Scotland aregiven in Table 3. These indicate that such surfaces accumulateto above mean tide level (MTL) and mainly between MHWSand mean high water neaps (MHWN). The altitude range of thesurface at Horgabost, �1.44 to �0.43 m (�3.53 to �2.52 m) isslightly greater than that of modern features, but not unduly so,given that this is a borehole record. By comparison with themodern record in Table 3, it is assumed that the sandflat atHorgabost accumulated to MHWS �0.5 m, and that the errorrange, at �1.44 m less �0.43 m, amounts to 1.01 m.

The presence of such a feature on the exposed western coastof Harris implies that the site was relatively sheltered at the timeit formed. The widespread peat on the grey sand surfaceprobably commenced accumulation as a saltmarsh peat, beforedeveloping later as a freshwater peat. The radiocarbon date of5500� 60 (6407–6612), from 1 cm above the surface of thegrey sand at borehole Z3, probably closely dates thecommencement of saltmarsh peat accumulation upon thesandflat surface.

The change from a saltmarsh peat to a freshwater peat up to0.2 m above the grey sand surface at both boreholes is dated atborehole C1 as having taken place by 5841–5050(4760� 130), and this point is taken as MHWS tides of thetime, since it is marked by the virtual disappearance oftychopelagic diatoms from the record. By ca. 5500 (4800),therefore, tidal influence had largely withdrawn from the sitesof both boreholes analysed as the saltmarsh peat becameincreasingly freshwater and accumulated widely across theLiana Horgabost area. However, by 5450–4861 (4500� 100)peat accumulation at the site of Z3 was replaced by sand astidal influence developed again across the site. This is equatedwith the resumption of MHWS tide influence, given theappearance of tychopelagic diatoms (e.g. Paralia sulcata; seeHendey, 1964). Grain size analysis supports this sand as beingof a beach character, and diatom analysis indicates a marine or

Scotland in relation to tidal altitudes

N MLWS Sandflat altitude range Reference

0 �1.50 1.6–1.9 Smith et al. (1992)�1.6 2.0–2.5 Firth and Haggart (1989)�1.1 0.6–1.3 Smith et al. (2007)�1.2 0.3–0.8 Smith et al. (2007)�1.3 0.9–2.0 Smith et al. (2007)

an tidal level; MLWN, mean low water neaps; MLWS, mean low water



brackish/marine environment. The return of marine influenceoccurred after a relatively brief period, however, since the datesof 5841–5050 (4760� 130) and 5450–4861 (4500� 100)overlap at 2s.

The return of a marine or brackish/marine environment atLiana Horgabost was of a different character from that whichobtained earlier, being reflected in horizons of sand and sandy/silty peat. At the seaward borehole, C1, these horizons (unitsLH3–5) include a sand layer (LH4) in which polyhalobousdiatoms decline slightly, but grain size analysis indicates abeach or dune environment. At the more landward borehole,Z3, these horizons (LH(Z)3–6) include a shelly sandy peat layerin which a slight fall in polyhalobous and mesohalobousdiatoms occurs but where grain size supports a beachenvironment. The changes in the diatom record for thesehorizons may reflect changes in stream run-off into the area, butin combination with the grain size characteristics a perimarineenvironment is indicated. Interestingly, these sandy horizonsoccur at similar altitudes, with a range of between �0.40 and0.63 m (�2.49 and �1.46 m) in the 19 boreholes in which theywere encountered.

The subsequent decline in the returning marine or brackish/marine environment is dated at 2325–1948 (2130� 70) atlandward Z3. At C1 this event is not dated; it can only bepresumed to have occurred sometime before the date of 906–664 (820� 50), a date which marks the later development offreshwater conditions there. This horizon at borehole Z3, wheresandy peat is replaced by peat, probably relates to theestablishment of intertidal conditions, analogous to the sandflatsurface dated at the base of the core.

The subsequent events at Liana Horgabost reflect the closerapproach of the dune field to the area and the reduction inmarine influence, possibly due to the development of the duneridge constricting the embayment, together with flooding fromthe land as relative sea levels rose: in the present environmentat Horgabost, streamflow is sometimes backed up and themarsh flooded during HAT and MHWS. At landward boreholeZ3, marine and brackish marine influence is greatly reduced bythe date of 759–562 (740� 50), which marks the rise in countsof freshwater diatoms to levels only previously achieved in thebasal peat at that site. At seaward borehole C1, a similar degreeof freshwater influence is recorded after the date of 906–664(820� 50) referred to earlier but, interestingly, a measure ofmarine influence increases in association with the appearanceof the tapering wedge of dune sand, dated as having reached itsinland limit between 537 and 325 (430� 40) and the probablyless reliable date of 274–0 (120� 40), when the dunes becamestabilised. The presence of marine and brackish/marinediatoms in these levels suggests continuing estuarine conditionsto the present, as the altitude of the marshland with respect topresent day MHWS indicates.

Northton

Morphology

At Northton the area (Fig. 6) comprises a Carex and Phragmitesmarsh (generalised as freshwater marsh in Fig. 6) that is fringedto seaward by a small, �50–100 m wide saltmarsh, and lying atthe head of a deep embayment in the coastline, is not fringed bydunes but is relatively sheltered from high-energy coastalprocesses. The marsh area itself is approximately 350 m inlength, N–S, and reaches a maximum width E–W of �150 m.The average altitude of the surface of the marsh is �2.4 m(�0.3 m), but there is a significant seaward slope from amaximum altitude of �3.6 m OD at the head of the


embayment, to an altitude of �2.0 m (��0.1 m) at the mouth.Most of the marshland area is therefore within the reach ofMHWS, and almost all within HAT. A small burn flows alongthe eastern side of the marsh. To the east, the hillside of theMaodal reaches a height of 251 m OD. The location is definedto the south and west by gentler-sloping hillsides that reachaltitudes of around 80–100 m OD. The marsh at Northton issituated at the head of a small tidal inlet. Water levels within theinlet become ponded after high tide or during increasedstreamflow because the outflow of water is restricted by acomplex of sandbanks situated along the seaward margin to thenorth.

Stratigraphy

The borehole stratigraphy at Northton (Fig. 7) was determinedfrom a number of cross-valley (E–W) transects that also link upto produce a down-valley (S–N) profile. From the southern endof the location boreholes were put down at �20 m intervals.The transect lines were spaced at�50 m intervals, the sequenceending where the bog becomes saltmarsh at the edge of theinlet. A feature of the location at Northton is a rock ridge lyingto seaward beneath the marsh. This ridge attains a maximumaltitude of �0.08 m (�2.17 m) and is reached in the fiveseaward boreholes in Fig. 7. The ridge effectively makes theNorthton location a buried isolation basin with a threshold toseaward. Landward of the rock ridge, the basin is occupied by asequence of clays, silts, sands and peat, overlain by a horizon ofsand, silty to seaward, in turn overlain by peat to landward,then Phragmites peat, then saltmarsh peat to seaward. The sandhorizon, reached by 21 boreholes, forms a relatively uniformsurface at 0.05–1.61 m (�2.04 to �0.48 m) and merges with themodern sandflat surface to seaward. Variations in altitude in theboreholes are probably due to differential compaction of theorganic sediments beneath, since the lower parts of the surfaceoccur above the deep basin landward of the rock ridge. Up-valley the sandflat tapers into peat with sand layers, the latterprobably of fluvial origin. The higher parts of the sandflatsurface at Northton lie �0.5 m below MHWS Stornoway and�0.6 m above MHWN there (see Table 1).

Two boreholes, F1 and H1 (Fig. 7) were sampled forlaboratory analysis. Borehole F1 lies at the deepest part of thesedimentary basin, while borehole H1 reaches the crest of theburied rock ridge.

Stratigraphical, grain size and microfossil analysis: core F1

From core F1, six units (N1–6) have been identified and aredescribed below. Figure 8 depicts the results of diatomanalyses.

N1 (5.21–7.39 m) – grey clay. The clay of unit N1 is underlainby 0.05 m of sand, its thickness unknown. In an adjacentborehole, the clay was found to lie between �3.68 and4.81 m OD (�5.59 and �6.72 m), with 0.69 m of peat beneathover bedrock. No analyses were made of this sediment.

N2 (3.87–5.21 m) – organic sandy silt with peaty layers.Diatom analysis of unit N2 discloses a freshwater assemblage.The main constituents are Cymbella laevis, F. construens,F. construens var. venter, F. exigua, F. ulna and Pinnularia viridis.

Grain size analysis carried out 4.27–4.23 m indicates amoderately sorted fine sand with a skewed, platykurticdistribution, probably originating in a fluvial environment.


Figure 6 The Northton area, showing marshland and borehole transects


N3 (3.31–3.87 m) – peat, undifferentiated. In unit N3, themain difference from the underlying unit N2 is that the trulyfreshwater component of the oligohalobous halophobousdiatoms expands greatly in number, accounting for almost45% of the total count. The halophobous species responsible forthis alterationareEunotiaexigua,P. subrostrataandT.flocculosa.However,up-core these salt-intolerant speciesdecline innumberand more salt-tolerant freshwater types appear. There is then afurther change as the brackish and marine diatoms Naviculamonolifera, P. sulcata, D. interrupta and N. peregrina begin toincrease. This whole transition from minimum to maximum salttolerance occurs in the uppermost 25 cm of this horizon. A14C date of 7370� 80 (7982–8348) was obtained from 3.57 to3.53 m, where counts of freshwater diatoms decrease.

N4 (2.62–3.31 m) – grey silty sand. The diatom assemblageof N4 is a mixture of marine, brackish and freshwater speciesthroughout the unit. Following the rise of marine and brackish


species at the termination of underlying unit N3, the brackishspecies decline in number, whereas the fully marine diatomscontinue to dominate the count. Above this, marine influencedeclines and freshwater species characterise this unit between3.15 and 2.90 m. The freshwater dominance of the total countthen begins to decline before marine and brackish diatomsincrease, with the assemblage being dominated by D. didyma,D. interrupta and N. peregrina.

Grain size analysis was carried out at 2.97–2.93 m andshowed a poorly sorted medium to fine sand with anextremely leptokurtic, symmetricaldistribution.Thedepositionalenvironment is akin to a swash/lower beach environment.

N5 (0.77–2.62 m) – sandy and silty peat. The diatomspectrum of N5 reveals a marine to brackish environment.As the minerogenic content of the core declines, so the marineinfluence appears to increase. At the base of the unit theproportion of marine to brackish diatoms is around 1:4, but as


Figure 7 Top: The Northton area viewed from the east, showing borehole transects. Below: Generalised stratigraphy for a down-valley transect atNorthton


the silt and sand are seen to decrease up-core the marinediatoms increase in number, so that there is a more equalbalance of around 1:1. This change in the proportion of marinespecies is mainly due to increases in the diatom P. sulcata.Other marine species such as G. serpentina, Navicula lyroides,N. monolifera and P. stelliger do increase also, but not asdramatically as P. sulcata. There was no piston core recoverybetween 1.85 and 0.96 m, but the gouge record indicates nochange in the stratigraphy in this section. Above the missingcore section, diatom analysis discloses a similar spectrum asbelow, although P. sulcata is reduced and D. interruptaincreases.

N6 (0–0.77 m) – Phragmites peat. Unit N6 is marked by arapid change over 0.15 m from a brackish to a freshwaterenvironment. D. interrupta declines as N. peregrina increases,


then F. exigua increases at around 65 cm. However, there iscontinuing marine influence as indicated by the presence ofN. monolifera and P. sulcata.

Stratigraphical, grain size and microfossil analysis: core H1

From core H1 four units have been identified (N(H1)1 toH(H1)4). Details of the units are given below and the results ofthe diatom analyses are shown in Fig. 9.

N(H1)1 (2.25–2.27 m) – peat. There were no diatoms presentin the basal unit (N(H1)1).

N(H1)2 (1.59–2.25 m) – grey sand. In unit N(H1)2 thediatom assemblage consists of marine to brackish species. The


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marine diatoms G. serpentina and P. sulcata, the marine/brackish diatom D. smithii and the brackish species C. westiiand D. interrupta dominate the assemblage. Few fluctuations inthe diatom count occur, except that at 1.90 m no diatoms arepresent. It could be that the horizon was depleted of its siliceouscontent due to the mobilisation of the available biogenic silicabecause of an increase in alkalinity or salinity common inmarsh and bog environments post deposition.

Grain size analysis was carried out at 1.90–1.86 m andshowed a moderately well-sorted very fine sand with asymmetrical and very platykurtic distribution, indicative of aswash/lower beach environment.

N(H1)3 (1.35–1.59 m) – sandy peat/peaty sand. The overlyingunit, N(H1)3, sees an increase then a decline in marineconditions. The same species constitute the bulk of theassemblage throughout, although the proportions differ. Themain change noticeable in this unit is the introduction of afreshwater component, F. leptostauron var. dubia, at around1.54 m, although this species declines as the overlying unitN(H1)4 is reached.

N(H1)4 (0–1.35 m) – Phragmites peat. The upper unit,N(H1)4, has a fairly consistent diatom assemblage throughout.The diatoms are largely brackish to freshwater species,although there is a background count of marine types. Thekey species found are the brackish diatoms C. westii,D. interrupta and N. peregrina, and the freshwater speciesF. construens var. venter, F. exigua and F. ulna. At the base ofthis unit, at around 1.34 m, the marine to brackish species thatdominated the underlying units fall in total number to bereplaced by freshwater assemblages. There is another change ataround 0.84 m when the remaining brackish component dropsaway and the freshwater element increases. A 14C date takenfrom 1.56–1.58 m, 3000� 80 (3375–2952), dates the dimin-ishing marine and brackish components and the rise infreshwater conditions.

Environmental interpretation

The deep coastal sedimentary basin at Northton providesintriguing evidence for Holocene relative sea-level change. Thebasal 3 m or more of sediments in the basin (units N1–N3) are ofterrestrial origin, as both the grain size and diatom recordsindicate. During the accumulation of all but the upper horizonsof these sediments, marine conditions were probably excludedby the �0.08 m (�2.17 m) threshold to seaward. However, anumber of changes are registered at the top of these sedimentsand above.

In borehole F1, the first presence of marine conditions at thesite is recorded in the top 10 cm of peat in unit N3, when a sharpchange takes place in the diatom record from freshwaterassemblages to fully marine assemblages with the tychopelagicdiatom Paralia sulcata notable. This record is continued into theoverlying grey sand of unit N4 for a further 10 cm, and heregrain size analysis indicates a swash or lower beachenvironment. However, the record of marine conditions endsabruptly with the return of freshwater conditions, whichcontinue to the top of that unit. A radiocarbon date of 7982–8348 (7370� 80) taken from above the last horizon offreshwater diatom assemblages in the peat below marks theend of freshwater conditions prior to the start of this marinerecord. The ‘spike’ of fully marine diatoms probably marks amajor flooding event, as may be inferred from the highproportion of broken and eroded diatoms (in the grey sand up to


75% of those counted). This could have been due to a storm, butwith a high-energy environment offshore (e.g. as shown byDawson et al., 2004), and therefore many storms, the singularityof the record (no other ‘spikes’ of fully marine diatoms havebeen found) is puzzling. An alternative origin, given the likelyage, is the Holocene Storegga Slide tsunami. Recent work hasemphasised the size of the tsunami (e.g. Bondevik et al., 2003)and demonstrated that it occurred widely, well beyond theareas outlined by Bondevik et al. (1997) and Smith et al. (2004).For example, evidence has been reported from East Greenland(Wagner et al., 2006) and possibly the Rhine delta (Cohen et al.,2006), hence although relative sea levels offshore from Harriswere probably metres below the threshold at the time, it ispossible that the tsunami could have entered the basin atNorthton. The grey sand in borehole F1 lies below the altitudeof the threshold, so may have lain in a waterlogged, possiblylacustrine environment after the departure of marine con-ditions, which could explain the increase in freshwater diatomsabove the basal layers of the unit (although marine and brackishmarine forms are still present).

The grey sand of borehole F1 continues to seaward, wherethe diatom assemblages are predominantly brackish–marine(unit N(H1)2, borehole H1), probably reflecting a greaterproximity to marine influence than at borehole F1 (unit N4).The silty or sandy peat (unit N5, borehole F1 and unit N(H1)4,borehole H1) which replaces the grey sand at both boreholesand across the area generally is marked by the return of marineconditions as rising relative sea levels overwhelmed thethreshold and tidal conditions developed, as supported bygrain size analysis. At borehole H1, the base of this unit(N(H1)4) is dated at 3375–2952 (3000� 80).

Above the silty or sandy peat, landward of the threshold, thesurface is occupied by marshland to seaward (unit NH(1)4,borehole H1) and Phragmites reedswamp (unit N6, boreholeF1) to landward. Seaward of the threshold, saltmarsh occupiesthe surface. These surfaces are taken to relate to within 0.5 m ofMHWS, as at Horgabost. The diatom record is of predominantlybrackish–freshwater species, possibly reflecting the effect ofsandbars holding back freshwater draining across the site.However, the consistent presence of Paralia sulcata throughoutthese last stages of peat accumulation indicates that marineinfluence was maintained.

Discussion

Relative sea-level change and crustal movementat Horgabost and Northton

The morphological and stratigraphical evidence at Horgabostand Northton provides a consistent record of late Holocenerelative sea-level change. Figure 10(A) depicts the changesidentified in the form of a graph derived from a combination ofthe evidence at both locations, with the ages of the datedhorizons plotted against their altitudes. The ages are plottedwith a range of 2s, and are expressed in terms of calibratedyears, with the radiocarbon equivalents shown on thehorizontal scale. The range of errors for index point altitudesfollows that adopted by Smith et al. (2003), in which a standardsurveying error is applied and site-specific errors due tosediment compaction and indicative meaning (sensu van dePlassche, 1986) errors are calculated. The surveying error is thatgiven by Sutherland (1981), and is �0.021 m. The sedimentcompaction error is calculated to range between 40% and 68%of the thickness of underlying peat for transgressive contacts or


Figure 10 (A) Relative sea level graph derived from a combination of the evidence at Horgabost and Northton, Harris. (B) Graphs of sea-level changefor the area of the Outer Hebrides from Lambeck (1993a) for Stornoway, Lambeck (1995b) for two points�80 km west of South Uist; Peltier et al. (2002)for ‘Hebrides’ and Shennan et al. (2006) for ‘Hebrides’


for regressive contacts where the underlying minerogenichorizon is 0.5 m or less, with a pro rata reduction to zero wherethe underlying minerogenic horizon is 2.5 m thick or more. Thecalculation of indicative meaning errors is based on theassumption that all the points relate to an intertidal, estuarineenvironment and the indicative range (following van dePlassche, 1986) is the range of altitudes for the buried sandflatsurface at Horgabost. This range amounted to 1.01 m over the19 boreholes which reached that surface. The indicativemeaning of the horizons measured at Horgabost and Northtonhas been discussed above: index points IP1, 4 and 5 are taken asrelating to MHWS less 0.50 m (based on sandflat geomorphol-ogy), while index points IP2 and 3, also incorporating an


indicative meaning related to sandflat geomorphology, aretaken as relating to MHWS� 0.10 m (based on the microfossilrecord). The measured values with error ranges for both indexpoints and limiting points in Fig. 10(A) can be obtained fromTable 2.

The evidence for middle and late Holocene relative sea-levelchange on Harris supports a fluctuating rise up to the presentday. The pattern is believed to have been as follows. At ca.8200 (ca.7400) (the age of limiting point LP1), relative sealevels lay below ��0.08 m (�2.17 m), the altitude of thethreshold at Northton. As relative sea levels rose at Northton, astorm or possibly a tsunami temporarily flooded the marshlandto landward. Ultimately, relative sea levels there reached the



marshland and then did not withdraw. Meanwhile, relative sealevels had reached the Horgabost embayment and a sandflatsurface accumulated up to �0.43 to �1.44 m (�2.52 to�3.53 m), 0.50 m below MHWS at the time. Upon that surfaceat Horgabost, saltmarsh peat began to accumulate at boreholeZ3, then began to fall at ca. 6200 (ca. 5500) from an altitude ofbetween �0.42 and �2.46 m (�2.51 to �4.55 m), 0.50 belowMHWS (index point IP1). Peat accumulation continued thereuntil, by ca. 5400 (ca. 4800) (index point IP2) MHWS hadbegun to fall slightly at the site of borehole C1 to reach between�0.30 and �2.35 m (�2.39 to �4.44 m) and freshwater peatdeveloped across the area. By ca. 5100 (ca. 4500) (index pointIP3) MHWS had risen again to between 0.54 and �1.57 m(�1.55 to �3.66 m) at borehole Z3, Horgabost. By ca. 3100 (ca.3000) (index point IP4), relative sea levels had reachedbetween 1.77 and �0.26 m (�0.32 to �2.35 m), 0.5 m belowMHWS of the time, in the sandflat surface at Northton, butcontinued to inundate that site up to MHWS. By ca. 2100 (ca.2100) (index point IP5), relative sea levels had reachedbetween 1.84 and �0.18 m (0.25 to �2.27 m), 0.5 m belowMHWS of the time, at borehole Z3, Horgabost, and inundationwas probably continuing there too. Thus by ca. 2100–3100 (ca.2100–3000), relative sea levels had reached close to presentlevels at both sites. The variations in the diatom record afterthose dates at Horgabost are probably due to the effect of dunemovements and the changing hydrology there (dated bylimiting points LP2–5), but the occurrence of marine diatoms inthe stratigraphic record at Horgabost to later than ca. 400 (ca.400) suggests that the rise may have continued there. There isthus a pattern of a continual rise in relative sea level from beforeca. 6200 (ca. 5500) in the area, with the exception of a short-lived fall around ca. 5100–5400 (ca. 4500–4800). Based uponthe measured altitudes in Table 2, over the last 3100 (3000) a,from index point IP4, the rate is between 0.07 and 0.73 mm a�1,and over the last 2100 (2100) a, from index point IP5, the rate isbetween 0 and 0.98 mm a�1. The mean rise from both indexpoints to present levels is 0.45 mm a�1.

The proposition that differential glacio-isostatic upliftoccurred between the two sites examined cannot at presentbe sustained. Thus, while at Horgabost MHWS had reached0.08 to �1.96 m (�2.01 to �4.05 m) (i.e. including thecorrection of 0.50 for the sandflat surface as being belowMHWS) before ca. 6200 (ca. 5500), at Northton it can only besaid that marine flooding had occurred across the basinthreshold and peat surface at �0.08 m (�2.17 m) sometimeafter ca. 8200 (ca. 7400) BP, and while the attainment ofpresent MHWS at Horgabost at ca 2100 (ca. 2100) BP was laterthan at Northton (ca. 3200 (3300) BP), the altitudinal margins oferror of the index points overlap.

The wider context

Figure 10(B) depicts the modelled graphs of Lambeck (1993a,1995b), Peltier et al. (2002) and Shennan et al. (2006) for thearea, compared with the trend of relative sea level from thepresent study. Although these graphs cannot be closelycompared because they relate to different areas in thisglacio-isostatic setting, they do provide evidence of generalinterest. In the case of the flooding event at Northton, all themodelled graphs indicate relative sea levels substantially lowerat the time, ranging from ��5 m (Lambeck, 1993a) to ��20 m(Lambeck, 1995b) mean sea level. This reinforces the like-lihood that the flood was a major one given that it exceeded thethreshold of ��0.8 m (�2.17 m) there. In terms of the pattern ofrelative sea-level change, it will be noted that the modelledgraphs do not show any fluctuations, except for a change in the


rate of rise around 6000 14C a BP. There is disagreement as towhen relative sea levels reached present: Lambeck (1993)depicts relative sea levels for Stornoway at between ��5 m and��8 m below mean sea level at ca. 6000 14C a BP, whileLambeck (1995b) depicts relative sea levels some 80 km west ofHarris as having reached ��5 m below present at ca. 600014C a BP, whereas Peltier et al. (2002) and Shennan et al. (2006)depict relative sea levels for ‘The Hebrides’ as having attainedpresent levels at ca. 7000 cal. a BP in the case of some models.None of these graphs envisage fluctuations in the middle or lateHolocene. However, from nearby areas there is empiricalevidence to support the fluctuations recorded on Harris. Thus,on Skye, Selby and Smith (2007) record that the rise in middleHolocene relative sea level was interrupted by a fall at ca.7500 cal. a BP, but resumed and fell again after ca. 4200 cal. aBP before reaching present levels. Farther afield, evidence thatfluctuations during the middle Holocene occurred has beenreported from Arisaig by Shennan et al. (1995) and from DubhLochan, north-west of Ulapool, by Shennan et al. (2000) assummarised by Smith (2005). These fluctuations have beeninterpreted as evidence of overlapping shorelines (Smith et al.,2007), a circumstance that might be expected in these areasperipheral to the centre of glacio-isostatic uplift in Scotland.

Figure 1 depicts the possible pattern of Holocene shorelinesin Scotland as envisaged by Smith et al. (2007). From thispattern, the earliest sandflat surface at Horgabost may equatewith the Main Postglacial Shoreline of Sissons et al. (1966),widely present in Scotland, since its age continues the trend ofthe shoreline becoming younger towards the periphery of theisostatically uplifted area. Thus, while at Horgabost the sitestudied from this sandflat surface, dated at 6407–6122(5500� 100) relates to �2.01 to �4.05 m MHWS Stornoway,at Philorth, Fraserburgh, Smith et al. (1982) obtained aconformable date of 6717–6306 (5700� 90) for a buriedsandflat margin at 0 m MHWS Fraserburgh, and at Wick,Dawson and Smith (1997) obtained a conformable date of6934–6572 (5940� 60) for a relict mudflat margin at �0.75 mMHWS Wick. The sandflat surface at Horgabost is overlain byat least one, and possibly two, later shorelines. Sometimebetween the dates of index points IP3 (5450–4861 (4500� 100)BP) and IP5 (2325–1948 (2130� 70) BP) at borehole Z3, a briefepisode of a decline in marine influence is recorded. Althoughat present there is no convincing evidence for a fluctuation, thisevent could relate to the later Blairdrummond Shoreline (e.g.Smith et al., 2006). The later sandflat surface both at Horgabostand Northton, and the attainment of the present shoreline,constitute the final shoreline overlap.

Conclusion

The evidence examined in this paper discloses some detailabout relative sea-level changes along the coast of Harrisduring the middle and late Holocene but is inconclusive wherelocal differential glacio-isostatic uplift is concerned. Theearliest event recorded is a flooding event shortly after ca.8200 (7400), perhaps a major storm or the Holocene StoreggaSlide tsunami. Somewhat later a fluctuating rise in middle andlate Holocene relative sea levels is registered. At ca. 6200 (ca.5500), MHWS lay �2.0–4.0 m below present MHWS Storno-way before falling to slightly lower levels after ca. 5400 (4800)then rising after ca. 5100 (4500). There may have then been asecond fluctuation before present levels were reached after ca.3100–2100 (3100–2000). These fluctuations may havereflected secular changes in nearshore sea surface levels. Such



changes would be more likely to have been registered in areasperipheral to the glacio-isostatic centre, such as Harris.However, no variation in isostatic uplift between the two sitesexamined could be found, probably because they are too closeto each other, notwithstanding the apparent substantialdifference in ice thickness over the two sites.

The evidence for fluctuating relative sea levels on Harris issupported by evidence in nearby areas, and it seems likely thatsuch evidence is widespread around the periphery of theScottish glacio-isostatic centre. The variations recorded maynot have been of great magnitude, but may have beeninstrumental in the evolution of the present coastline ofScotland.

Acknowledgements These locations on Harris were the subject ofrecent unpublished work by Jordan (2004), which forms the basis of thepresent account. Many people provided invaluable help and adviceduring the work reported here. Professor Michael Tooley is thanked forhis advice on field evidence and kindly provided essential help in thefield. Dr Philip Woodworth is thanked for information on tidal datums.Dr Robin Cullingford, Matt Hill, Dr Alaric Rae and Lea Winter providedhelp in the field. The work was funded by the European CommissionDirectorate General for Science, Research and Development and by theCentre for Quaternary Science, Coventry University. DES acknowl-edges the award of a Leverhulme Trust Emeritus Fellowship.

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Holocene relative sea-level changes in Harris, Outer Hebrides, Scotland, UK