Holocene alluvial-fan development in the...

1834

GSA Bulletin; December 2000; v. 112; no. 12; p. 1834–1849; 11 figures; 2 tables; Data Repository item 2000112.

Holocene alluvial-fan development in the Macgillycuddy’sReeks, southwest Ireland

E. AndersonSchool of Geography and Environmental Management, Middlesex University, Queensway, Enfield EN3 4SF, UK

S. Harrison*Centre for Quaternary Science, Geography, NES, Coventry University, Priory Street, Coventry CV1 5FB, UK

D.G. PassmoreDepartment of Geography, University of Newcastle-upon-Tyne, Newcastle-upon-Tyne NE1 8ST, UK

T.M. MighallCentre for Quaternary Science, Geography, NES, Coventry University, Priory Street, Coventry CV1 5FB, UK

ABSTRACT

Holocene alluvial landforms in the Mac-gillycuddy’s Reeks, southwest Ireland, wereinvestigated to determine the controls andtiming of postglacial geomorphic activity.Detailed geomorphologic analysis of threealluvial-fan and debris cones within high-level cirque basins demonstrates evidenceof episodic phases of late Holocene surfaceaggradation and incision. Radiocarbondates from peat horizons above and belowinorganic units show that phases of aggra-dation cluster into two distinct periods, thefirst after 230–790 calibrated (cal.) yr A.D.and the second from 1510 cal. yr A.D. tothe present. An additional phase of fan ag-gradation at one site is dated after 1040–1280 cal. yr A.D. All three phases coincidewith episodes of enhanced late Holocenevalley-floor alluviation and debris-flow ac-tivity from upland Britain. Alluvial fansand debris cones have developed primarilyas a result of the resedimentation of lateMidlandian (Wisconsin) drift and talusslopes, and mobilization of materials in-volved flooding, transitional-flow, and de-bris-flow processes. Pollen analysis of peathorizons interbedded with alluvial-fan anddebris-cone sediments indicates that land-use changes were an important factor inlowering the threshold for local slope ero-sion. Phases of aggradation also coincidewith well-documented episodes of climatechange, and, hence, fan development is

*E-mail: [email protected].

probably a function of both anthropogenicand climatic forcing. A sequence of eventsmay have involved initial slope destabili-zation due to overgrazing and removal ofvegetation that was followed by debris mo-bilization and fan aggradation during in-tense rainstorms associated with climatechange.

Keywords: alluvial fans, Holocene, south-west Ireland.

INTRODUCTION

Alluvial fans and debris cones are fan-shaped bodies of sediment which form at thefoot of mountain escarpments, slope gullies,and tributary valleys by rapid deposition fromchannelized or unconfined waterfloods, tran-sitional flows, and debris flows (Blair and Mc-Pherson, 1994). They can be mutually distin-guished on the basis of surface gradient andbulk composition. Alluvial fans commonlyconsist of a variety of sediment facies typesand display surface gradients generally be-tween 28 and 128 (Blair and McPherson,1994). In contrast, debris cones are generallysteeper, with average gradients typically in therange of 128–258 (Brazier et al., 1988; Blairand McPherson, 1994), and include predomi-nantly superposed debris-flow lobes (Data Re-pository Table DR11). In upland areas of the

1GSA Data Repository item 2000112, descriptionand schematic profiles of alluvial-fan depositionalfacies, is available on the Web at http://www.geosociety.org/pubs/ft2000.htm. Requestsmay also be sent to Documents Secretary, GSA,P.O. Box 9140, Boulder, CO 80301; e-mail:[email protected].

British Isles, the majority of alluvial fans anddebris cones are deeply incised, partially orwholly vegetated, and show little sign of re-cent activity (Brazier and Ballantyne, 1989).Research on the chronology and paleoenviron-mental significance of these landforms hasdemonstrated widespread evidence of Holo-cene surface aggradation and incision. Thisactivity was particularly intense during thetenth to twelfth centuries (Harvey et al., 1981;Harvey and Renwick, 1987; Tipping and Hal-liday, 1994) and sixteenth to nineteenth cen-turies (Innes, 1983; Brazier et al., 1988;Brazier and Ballantyne, 1989).

Despite the body of evidence cited previ-ously, uncertainties remain regarding severalkey research issues. First, discerning the re-spective roles of climate and land-use changes(external forces) in controlling geomorphic ac-tivity remains a contentious issue (Ballantyne,1991a, 1991b). Alluvial records in upland ba-sins are commonly incomplete, and materialssuitable for dating are commonly lacking.Consequently, these proxy records are seldomof sufficient resolution to isolate a definitiveclimatic or cultural signal. Second, studieshave focused on single sites where geomor-phic responses to fluxes in external boundaryconditions may be difficult to differentiatefrom internal stochastic or complex response(Schumm, 1973).

In the Macgillycuddy’s Reeks, alluvial fansand debris cones are a common feature at thebase of incised-valley and cirque slopes andreflect widespread slope destabilization duringthe late Quaternary drift and talus covers.Three of these alluvial-fan and debris-cone

Geological Society of America Bulletin, December 2000 1835

HOLOCENE ALLUVIAL FAN DEVELOPMENT, SOUTHWEST IRELAND

complexes—located in the CoomloughraGlen, Hag’s Glen, and Curraghmore cirque ba-sins (Fig. 1 and Table 1)—contain buried peathorizons and hence facilitate an opportunity toaddress some of the research issues previouslyoutlined. Accordingly, the principal aims ofthis paper are to (1) establish the geomorphicdevelopment and chronology of alluvial fansand debris cones and (2) determine the envi-ronmental controls responsible for periods oflandform stability and accelerated geomorphicactivity. The methodology employed in thisstudy is described in the Appendix.

STUDY AREA

The Macgillycuddy’s Reeks is a high-reliefmassif, consisting of Devonian sandstones andshales (Fig. 1). The legacy of Pleistocene gla-ciations has left the massif deeply dissectedby several glacial troughs and numerous, well-developed cirque basins. The last phase of val-ley glaciation occurred during the late Mid-landian Glenavy Stadial (26–13 ka). TheYounger Dryas cooling (11–10 ka) also pro-moted the development of six small cirqueglaciers (Anderson et al., 1998). The presentclimate of this area is temperate maritime, andthe region is frequently affected by the pas-sage of midlatitude low-pressure systems.Consequently, the weather is often stormy,and heavy rain is common throughout theyear, particularly in the mountains, where an-nual precipitation probably exceeds 2500 mm.

Coomloughra Glen Alluvial-Fan andDebris-Cone Complex

Geomorphology and StratigraphyThe alluvial-fan complex within Coom-

loughra Glen consists of two distinct fans, re-ferred to here as fan A and fan B, and a seriesof inset bars and channel lags along the lengthof the main catchment stream (Fig. 2). Fan Ais located at the base of an incised YoungerDryas till sheet (Anderson et al., 1998) andconsists of an assemblage of superposed, tran-sitional-flow lobes and splays. Investigation ofthe distal reach and marginal areas of the fan(fan Aii) reveals that component clasts displaya greater degree of iron oxide staining andvegetation cover than the clasts that form thelobes of the central fan axis (fan Ai). Suchcontrasts may be explained by temporalweathering differences and duration of vege-tation development, indicating that fan A mayhave been formed by two distinct depositionalphases.

Fan B is a debris-cone and alluvial-fancomplex that developed downslope of three

distinct feeder channels incised into YoungerDryas till and is overlain by talus (Fig. 2). Theproximal reach consists of superposed vis-cous-debris and transitional-flow lobes andsplays, which grade proximally into poorlysorted channel fills. Distally, dilute debrisflows and fluvial boulder lobes replace thesefacies, and the distal portion is made up of anexpansive assemblage of superposed fluvialcobble bars and sheetflood deposits. Fivestream-cut exposures (sections 1–5; Fig. 3) re-veal that this surface assemblage, referred tohere as lithofacies association (LFA) CG5,consists of clast-imbricated, gravel-cobble andcobble-boulder deposits, which fine distallyinto a sequence of laminated silts and sandsand are overlain by sand-and-cobble gravel.Interstratified peats and clastic units underlieLFA CG5 in sections 1 and 2 (Fig. 3). Radio-carbon dating of peat samples from LFAsCG2 and CG4 produced dates of 1510–1930cal. yr A.D. and 650–790 cal. yr A.D., re-spectively (Table 2). Uprooted vegetation infeeder-channel deposits demonstrates that thefan is still partially active.

Geomorphic relationships between fan Aand fan B are inconclusive (Fig. 2), but strati-graphic evidence appears to indicate that atleast part of fan A predates fan B. Paleocur-rent analysis of the buried alluvial units (LFAsCG3 and CGi) exposed in sections 1–5 dem-onstrates that they were deposited by water-floods derived from the catchment area of fanA and probably, therefore, represent the down-stream washout debris of transitional flowswhich formed fan A (Figs. 2 and 3). Presently,it is not clear at what stage during the devel-opment of fan A that these alluvial units weredeposited because this fan probably consistsof two temporally distinct assemblages of su-perposed transitional-flow lobes. The strati-graphic evidence outlined here, however, doesat least indicate that one of the transitional-flow events probably occurred after 650–790cal. yr A.D. (Table 2).

Geomorphic EvolutionThe presence of alluvial silts in the distal

reach of the fan complex indicates geomor-phic activity, and resedimented peat fragmentsin the complex demonstrate that they are Ho-locene (section 1). This geomorphic activitywas followed by a period of surface stabilityand peat development. After 650–790 cal. yrA.D., surface aggradation was triggered by de-stabilization of the upstream catchment slopesof fan A. Initial activity commenced with thedeposition of distal-reach silts and sands andwas followed by the sedimentation of gravel-cobble bars. These deposits are possibly

downstream washout debris from transitionalflows that deposited a series of stacked lobesand lobes on fan A. Fan aggradation was fol-lowed by renewed peat development, althoughsurface stability was interrupted briefly by thedeposition of channelized sandy gravel in thedistal reach (LFA CG4, section 2). The initialtiming of this episode of peat formation is un-certain, but the thickness of the peat layer insection 1 (0.28 m; Fig. 3) indicates a pro-longed period of surface stability prior to thereturn of alluvial deposition after 1510–1930cal. yr A.D. Renewed slope instability after1510–1930 cal. yr A.D. caused the depositionof fan B and possibly transitional-flow depo-sition on fan A. At this time, the principalbasin stream flowed around the outer marginof fan B (Fig. 2). Following the cessation offan aggradation, however, erosion by fan Bsurface streams and localized cobble-bar de-position appear to have caused the avulsion ofthe principal basin stream, leading to the for-mation of the present-day channel.

The Hag’s Glen Alluvial Fan

Geomorphology and StratigraphyThe Hag’s Glen alluvial fan is located just

south of Lough Callee (Fig. 4) at the foot ofa gully cut into Glenavy Stadial till. The prox-imal reach of the fan consists of several tran-sitional-flow boulder lobes. Although the mid-fan area is heavily vegetated, close inspectionreveals an expansive assemblage of super-posed streamflow boulder bars. Wells andHarvey (1987) pointed out that fluvial boulderbars could be interpreted as sieve deposits (cf.Hooke, 1967). The boulder bars in the Hag’sGlen alluvial fan, however, overlie peat de-posits that would prevent the rapid surface-water loss necessary to trigger sieve-like de-position. The surface geomorphology of thedistal reach is also largely obscured by surfacevegetation and organic topsoil. The faciescharacteristics of depositional units and thedistal-reach stratigraphy, however, can be ob-served along stream-cut exposures (sections6–13; Figs. 5 and 6). Cross-correlation of theupper distal-reach sheetflood splays exposedbetween sections 7 and 10 (LFA HGe) and thecobble-bar deposits seen in section 6 (LFAHGv; Fig. 5) is relatively straightforward be-cause sections 6 and 7 are only 8 m apart.They are also at the same elevation (1.5 m)above the present stream channel and displaysimilar lithostratigraphy and paleocurrent di-rections (Figs. 4, 5, and 6). These deposits areeither equivalent in age or predate the super-posed fluvial boulder lobes and transitionalflow because the latter display onlapping re-

1836 Geological Society of America Bulletin, December 2000

ANDERSON et al.

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TABLE 1. PARAMETERS FOR ALLUVIAL-FAN AND DEBRIS-CONE CATCHMENT AND MORPHOLOGY

Laboratory Catchmentarea(m2)

Basinrelief(m)

Reliefratio

Feederchannelgradient

Altitude(m)

Surfacearea(m2)

Apex-distalreach length

(m)

Maxthickness

(m)

Coomloughra A 0.337 529 0.47 0.49 500 4600 101 4Coomloughra B 0.095 481 0.73 0.72 500 13350 175 2.5Hag’s Glen 0.132 470 0.57 0.49 340 7640 120 5.5Curraghmore 0.676 670 0.51 0.33 300 12000 215 3.5

lationships (Fig. 4). Therefore, the prominentdepositional landform assemblages that formthe central mass of the alluvial fan postdatethe upper peat unit (LFA HGiv) in section 6and were deposited after 1510–1930 cal. yrA.D. (Table 2). Radiocarbon dates from sec-tion 11 demonstrate the greater antiquity ofthe sediment sequence exposed between sec-tions 11 and 13. Comparison of the radiocar-bon dates from the base of LFA HGii (1040–1280 cal. yr A.D.) and from the top of LFAHG4 (1040–1270 cal. yr A.D.) indicates thatLFA HGi and LFA HG5 are probably thesame depositional unit. In turn, LFA HGiprobably correlates with the lower sheetfloodfacies (LFA Hga) exposed in sections 9 and10 (Fig. 6). Consequently, the sedimentary de-posits exposed in sections 6–13 represent asemicontinuous vertical profile (Fig. 7).

Geomorphic EvolutionFormation of the Hag’s Glen alluvial-fan

complex commenced with the deposition ofsheetflood gravel and vertical accretion oflaminated silts in a low-energy environment.Resedimented peat fragments within the siltunit demonstrate that they are Holocene inage. After 1760–1520 cal. yr B.C., a period ofsurface stability and peat deposition occurred(LFA HG2), although it is not clear whetherinactivity extended across the entire fan sur-face at this time because the peat layer maybe a localized channel fill. Alluvial activityrecommenced after 550–680 cal. yr A.D. withthe deposition of channelized cobble bars andoverbank sand and gravel. Following a briefperiod of possibly localized surface stabilitybetween 890 and 1270 cal. yr A.D., sheetfloodgravel was deposited across a wide area of thedistal reach. The planar-bedded, sheetfloodcouplets imply that the floodwater was asso-ciated with autocyclic development and wash-out of standing-wave antidune bedforms.These couplets, in turn, suggest that the wholeunit may have been deposited during oneflash-flood event. After 1040–1280 cal. yrA.D., a period of surface stability and peatdeposition occurred across the entire fan. Peatformation was interrupted briefly between1400 and 1630 cal. yr A.D. by the depositionof a localized drape of sand. This layer may

represent the overbank fines from a channel-ized flood or washout debris transported dis-tally following the sedimentation of a depo-sitional lobe. Sometime after 1520 cal. yrA.D. but before 1930 cal. yr A.D., a thick andlaterally extensive assemblage of transitional-flow lobes and splays, streamflow boulderlobes and sheet-flood gravel was deposited.Morphostratigraphic relationships betweenthese depositional landforms indicate that theyprobably represent the geomorphic responseto a single flash flood.

Pollen DiagramsPollen diagrams and zone descriptions of

peat sampled from depositional units LFAHgii and LFA HG1v from section 6 and fromLFA HG2 and LFA HG4 from section 11 arepresented in the Data Repository (see textfootnote 1). These data are shown as FiguresDR1 and DR2, respectively.

Curraghmore Alluvial Fan

Geomorphology and StratigraphyThe Curraghmore alluvial fan is located im-

mediately west of Lough Curraghmore (Fig.1) at the foot of a gully cut into a GlenavyStadial till (Fig. 8; Anderson et al., 1998). Thefan consists of three distinct landform assem-blages: (1) proximal- and medial-zone, tran-sitional-flow lobes and splays; (2) a distal-zone sequence of interbedded alluvial unitsand peats buried beneath a surface cover ofpeaty soil; and (3) a series of fluvial, boulderlobes and cobble bars set between the proxi-mal transitional-flow deposits and a flankingtill sheet.

Inset around and between the frontal mar-gins of the transitional-flow lobes are chan-nelized alluvial fills (Fig. 8) consisting ofclast-imbricated gravel cobbles with sparseboulders and sandy matrix (sections 15 and17; Fig. 9). These deposits are interpreted ascut-and-fill deposits. These fill deposits un-conformably overlie a buried sequence of flu-vial boulder lobes or bars in section 15. Thepaleocurrent direction of the upper alluvialunit in section 15 is parallel with the generalorientation of the transitional lobes and aban-doned interlobe channels. Their geomorphic

setting suggests that energetic waterfloodsformed them immediately after the depositionof the transitional-flow sediment assemblage.The interbedded sequence of peats and allu-vial units probably covers much of the distalportion of the alluvial fan, although its exactsurface extent and thickness is not known be-cause the sediment sequence is only partiallyexposed along one stream-cut section (section14; Fig. 9). This section exhibits six distinctlithofacies units. Peat units bracketing thelower alluvial layer have been dated to 15056 40 yr B.P. and 705 6 40 yr B.P. (Table 2).However, the younger date may be erroneous-ly old because the overlying contact is ero-sional. The upper alluvial units are dated toafter 190 6 90 yr B.P.

A crosscutting relationship 7 m upstreamfrom section 14 demonstrates that the cut-and-fill units postdate the distal-zone sequence ofinterbedded alluvial layers and peats (Fig. 8).Consequently, the bulk of the proximal- anddistal-zone sedimentary landforms appear tobe younger than the upper peat unit exposedin section 14 (Fig. 9) and were therefore de-posited after ca. 1640 cal. yr A.D. This hy-pothesis, however, rests on the assumptionthat the transitional-flow deposits and adjacentcut-and-fill units are equivalent in age. Al-though the geomorphic evidence is compel-ling, it is not conclusive.

Geomorphic EvolutionAlluvial activity commenced after 440–630

cal. yr A.D. with the deposition of distal-reachsands and gravels, which may grade upstreaminto buried boulder lobes or bars (section 15).After 1390–1510 cal. yr A.D., peat formationoccurred, although fan stability was interrupt-ed briefly by the sedimentation of a localizedsand layer. Assuming that the stratigraphicscheme discussed above is correct, the evo-lution of the fan after ca. 1640 cal. yr A.D.may be outlined as follows. After ca. 1640 cal.yr A.D., geomorphic activity commenced withthe deposition of distal-reach sands and grav-els and sedimentation of transitional-flow de-posits. The transitional-flow deposits mayhave been formed either by two or more geo-morphic events or were possibly depositedduring the same flood with each lobe repre-


ANDERSON et al.

Figure 2. (A) Map of Coomloughra Glen alluvial-fan and debris-cone complex. MN—magnetic north. (B) Key to sediment types forgeomorphic maps shown as Figures 2A, 4, 8.

M

senting a distinct sediment pulse formed asnew supplies of debris were entrained up-stream. Floodwaters associated with eachpulse caused localized cut and fill of the pre-existing fan surface adjacent to lobes andsplays and incised to the present position ofthe distal-reach channel. In addition, the tran-sitional-flow landform assemblage appears tohave blocked and deflected the flow of theprincipal fan channel to the south, which inturn prevented further distal-reach surface ag-gradation. The boulder lobes and cobble barsinset between the transitional-flow depositsand late Midlandian till represent the last sig-nificant episode of geomorphic activity andwere deposited by channelized floodwaters.They may have formed immediately after thediversion of the channel or may reflect morerecent flood events.

DISCUSSION

Consideration of the external causes of Ho-locene fan-alluviation and debris-flow activityin upland areas of the British Isles has focusedon the relative roles of human activity and cli-mate change (e.g., Innes, 1983; Ballantyne,1991a). Similar debates also have concernedgeomorphologists and archaeologists workingon valley-floor alluvial sequences in uplandand lowland Britain (e.g., Macklin and Lewin,1993; Macklin, 1999). The anthropogenic hy-pothesis recognizes that vegetation distur-bance (especially by soil tillage and grazing)lowers the threshold for soil erosion, leadingto debris mobilization and gullying during pe-riodic high runoff. Conversely, the climatichypothesis is based on the apparent coinci-dence between evidence of geomorphic activ-ity and established periods of climate change,although Ballantyne (1991b) proposed that in-creased frequency of intense rainstorms as op-posed to general climatic deterioration mightbe the key factor. Indeed, in a recent reviewof European alluvial records for the period1300–1900 A.D., Rumsby and Macklin(1996) concluded that periods of enhancedriver aggradation and incision coincided withphases of climatic transition, whereas the mostsevere phases of climatic cooling were asso-ciated with reduced fluvial activity.

Recent reviews of Holocene alluvial valley-floor development at various scales (Passmoreand Macklin, 1997) and throughout the United

Kingdom (Macklin and Lewin, 1993; Mack-lin, 1999) have argued that river response toanthropogenic activity and climate changeshould be viewed in terms of a continuum be-tween these factors. In particular, they sug-gested that land-use changes might be criticalfor sensitizing river systems to changes inflood frequency and magnitude. It is common-ly the case, however, that geomorphologic in-vestigations lack either chronological controlsand/or climate and land-use data at appropri-ate spatial and temporal scales (e.g., Tipping,1994; Macklin, 1999). Intercalated peat hori-zons in the alluvial-fan complexes provide arare opportunity to address some of these is-sues by facilitating investigations of the chro-nology of fan development and local vegeta-tion history.

Alluvial-Fan and Debris-ConeDevelopment During the First MillenniumA.D.

Accelerated geomorphic activity is reflectedby the occurrence of thin clastic units in thedistal reach of all fans. Pollen spectra from theHag’s Glen alluvial fan indicate that the localvegetation cover after 550–680 A.D. consistedof acidic grasslands and woodlands character-ized by birch, willow, pine, hazel, and oak pri-or to fan aggradation (Data Repository Fig.DR1). The pollen record also reflects the im-pact of human activity on the landscape at thistime. This is not unexpected, and several otherauthors have reported evidence of Bronze Ageand Iron Age land-use changes from pollenrecords from southwest Ireland (Dodson,1990; Monk, 1993). Initial evidence includeshigh values of Poaceae pollen and a suite ofherbaceous taxa that is normally associatedwith pastoral use. These herbaceous taxa in-clude Plantago lanceolata, Rumex acetosa/acetosella, Lactucae, Asteroideae, Vicia-type,Brassicaceae, Trifolium-type, and Pteridiumwithin local pollen assemblage zones (LPAZs)HG11a and HG11b (Fig. DR2). Because thearboreal pollen curve in LPAZs HG11a andHG11b fluctuates around 25% to 30% of totalland pollen, it appears that agricultural activ-ities were not associated with significantwoodland clearance. The first noticeable lossof woodland cover took place at the beginningof LPAZ HG11c when Pinus, Betula, Quer-cus, and Salix pollen values all decreased (Fig.

DR2). Charcoal values from this assemblage,however, suggest that fire was not involved inwoodland clearance (Fig. DR2). This reduc-tion in woodland coincides with the rise inCalluna values at 65 cm (ca. 2480 yr B.P.),indicating an expansion of acidic, organicsoils. Plantago lanceolata, Melampyrum, Lac-tucae, Fabaceae, Brassicaceae, and Pteridiumare also recorded in low percentages just be-fore and/or during this expansion, suggestingthat pastoral agriculture in the local vicinitymay have created conditions conducive forpeat growth by removing woodland cover andmodifying the local hydrologic regime. Re-covering arboreal percentages in the middle ofLPAZ HG11c may reflect reduced agriculturalactivity, although Betula and Salix never at-tained their former values (Fig. DR2).

The boundary between LPAZs HG11c andHG11d also coincides with a reduction ofwoodland cover associated initially with de-clining Salix values between 52 and 48 cm(ca. 1615 cal. yr B.P.). Betula and Corylus av-ellana–type briefly increase at this depth, pos-sibly as a successional response to the loss ofwillow, but this is followed by a drop in per-centages of both taxa. The fall in the arborealpollen values during the first part of LPAZHG11d is associated with a rise in herbaceoustaxa, including Poaceae, Cyperaceae, Potentil-la-type, Ranunculaceae, Succisa pratensis,and Plantago lanceolata, suggesting that bothpastoral activity and the areal extent of acidicgrasslands had increased. Again charcoal val-ues remain low, suggesting that fire was notinvolved in woodland clearance (Fig. DR2).The 14C date within HG11d (550–680 cal. yrA.D.) implies that this phase corresponds withthe early Christian period, which was markedin Ireland by increasing agricultural activityand widespread forest clearance (Edwards,1985).

The evidence from the pollen record dem-onstrates that catchment vegetation prior tothe deposition of an inorganic layer (LFAHG3) had been modified by human activitybetween 1760–1520 cal. yr BC and 550–680cal. yr A.D. Therefore, it is possible that fanaggradation may have been facilitated, at leastin part, by the gradual removal of local veg-etation and possibly overgrazing, which desta-bilized soil and sediment cover on the slopesadjacent to the fan. The phase 230–790 cal.yr A.D., however, also coincides with evi-


ANDERSON et al.

Figure 3. Coomloughra Glen, sections 1 to 5. LFA—lithofacies association.

dence of a major shift to wetter climatic con-ditions (Fig. 10). In blanket peats across up-land areas of the British Isles, climatic changeis reflected by increased surface wetness, and,in west Ireland, this phase has been dated toafter 550–740 cal. yr A.D. (Blackford andChambers, 1991). Furthermore, climatic de-terioration at this time in Ireland is recordedby narrow tree-ring growth, especially around541 A.D. (Baillie and Munro, 1988). Global

climatic change during the fourth to sixth cen-turies A.D. was also proposed by Wendlandand Bryson (1974). Therefore, climate changepossibly associated with increased storminessmay also have been a contributing factor inpromoting geomorphic activity between 230and 790 cal. yr A.D. Indeed, some evidencein the pollen record supports a climatic inter-pretation for fan aggradation. Cyperaceae pol-len and Sphagnum spores increase simulta-

neously with the fall in arboreal-pollen valuesbetween 48 and 44 cm in HG11d (Fig. DR2).This increase combined with a decrease inCalluna pollen support a shift to locally wethydrologic conditions. Fan aggradation, atleast in the Hag’s Glen, therefore, is most like-ly to be related to a combination of humanactivity and climate change and is primarily aresponse to external forcing. A sequence ofevents may have involved initial slope desta-



TABLE 2. RADIOCARBON DATES OF PEAT SAMPLES FROM ALLUVIAL-FAN AND DEBRIS-CONECOMPLEXES

Laboratory number Location and depth Uncalibrated age(yr B.P.)

Calibrated age

Coomloughra GlenSRR-5957 Section 1: 0.66m 240 6 40 A.D. 1510–1930SRR-5958 Section 1: 0.93m 235 6 40 A.D. 1510–1920SRR-5959 Section 1: 1.22m 1305 6 40 A.D. 650–790

Hags GlenSRR-5960 Section 6: 0.68m 240 6 40 A.D. 1510–1930SRR-5961 Section 6: 0.80m 450 6 40 A.D. 1400–1510SRR-5962 Section 6: 0.84m 445 6 45 A.D. 1400–1630SRR-5963 Section 6: 1.14m 820 6 45 A.D. 1040–1280SRR-5964 Section 11: 0.23m 835 6 40 A.D. 1040–1270SRR-5965 Section 11: 0.28m 1020 6 45 A.D. 890–1160SRR-5966* Section 11: 0.36m 1410 6 45 A.D. 550–680SRR-5967 Section 11: 0.74m 3360 6 45 1760–1520 B.C.

CurraghmoreBETA-117295* Section 14: 0.30m 190 6 90 A.D. 1640–presentSRR-5955 Section 14: 0.60m 705 6 40 A.D. 1390–1510SRR-5956* Section 14: 0.72m 1505 6 40 A.D. 440–630

Note: Peat samples extracted from beneath erosive contacts.

Figure 4. Map of Hag’s Glen alluvial fan showing the location of sediment sections. See Figure 2B for explanation of sediment types.MN—magnetic north.

bilization due to overgrazing followed by de-bris mobilization and fan aggradation duringan intense rainstorm or series of rainstormsassociated with climate change.

Marked channel incision and/or aggradationduring the third through eighth centuries A.D.has also been recorded in several tributariesand trunk reaches of the south Tyne basin,northern England (Fig. 10; Macklin et al.,1992a, 1992b; Passmore and Macklin, 1997).It is concluded herein, however, that climatechange cannot have been the sole cause of val-ley-floor destabilization, because hydroclima-tologic changes of a similar if not greatermagnitude had occurred in the Tyne basin ear-lier in the Holocene without resulting in sig-nificant upland erosion. Instead, it is suggestedthat extensive deforestation during Iron Ageand Roman times had lowered the thresholdfor valley-floor erosion by indirectly aug-menting runoff rates and stream powers. Evi-


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Figure 5. Hag’s Glen alluvial fan, sections 6 and 7. LFA—lithofacies association.

dence of valley floor alluviation in Ireland,dated to between 1500 and 1200 yr B.P., hasbeen reported by Glanville et al. (1997) fromthe upper catchment area of the River Liffeyin the Wicklow Mountains (Fig. 10). Althoughthese authors acknowledged that fluvial activ-ity coincided with sub-Atlantic cooling andthat local pollen records do not show land-usechanges during this period of time, they didnot speculate on the probable causes of allu-viation. Evidence of valley-floor alluviationhas also been reported from continental Eu-rope at this time. For instance, Becker andSchirmer (1977) documented evidence of re-newed flooding after 350 A.D. in the Danubebasin, Austria, and after 560–711 A.D. in theMain Valley, southern Germany. In contrast,however, fluvial basins in North America werecharacterized by relative stability after 1800 yrB.P. (Knox, 1983).

Fan Aggradation in the SecondMillennium A.D.

Geomorphic activity in the Macgillycuddy’sReeks after 1040–1280 A.D. is recorded sole-ly on the Hag’s Glen alluvial fan (LFA HG5;Fig. 6). Pollen analysis of an earlier peat ho-rizon (LFA HG4; LPAZ HG11e) indicates astable open landscape existed from 890–1160cal. yr A.D. to 1040–1270 cal. yr A.D. Theoccurrence of high percentages of Poaceae andthe presence of Potentilla-type, Campanula-ceae, Gentianaceae, and Ranunculaceae sug-

gests that the dominant vegetational commu-nity was acidic grassland with alpine elementssuch as Jasione-type. Evidence of grazing isdemonstrated by the presence of Rumex ace-tosa/acetosella, Pteridium, Lactucae, Astero-ideae, and Brassicaceae in the upper half ofLPAZ HG11e and the gradual increase ofPlantago lanceolata throughout LPAZsHG11e to HG11c to about 7% total land pol-len (Fig. DR2) just beneath the overlying clas-tic unit LFA HG5 (Fig. 6). Consequently, fanaggradation appears to coincide with evidencefor more intensive grazing around the fan andwas, therefore, an important factor in condi-tioning slope response to rainstorm events.The rise in Cyperaceae values also suggeststhat soils become wetter during the last partof LPAZ HG11e, which may have lowered thethreshold for soil movement.

Several other authors have documented al-luvial-fan and debris-cone aggradation in up-land Britain during the tenth and eleventh cen-turies (Fig. 10). Harvey et al. (1981) describeda debris cone, which occurs at the base of adeeply incised solifluction slope in the How-gill Fells, northern England. This cone ismade up of debris-flow deposits overlying aburied organic soil horizon dated to between2580 6 55 yr B.P. and 940 6 95 yr B.P. Pol-len analysis revealed that the soil horizon co-incides with vegetational changes indicativeof the rapid transition from a mixed landscapedominated by Alnus and Rumex to a moreopen landscape dominated by moorland veg-

etation, such as Ericaceae. Harvey et al.(1981) suggested that these pollen changes re-flected a significant increase in the area of de-forestation and inferred that this occurred inresponse to the expansion of sheep grazingfollowing the arrival of Scandinavian settlersin the tenth century. Harvey and Renwick(1987) also attributed tenth century alluvial-fan aggradation in the Bowland Fells, northernEngland, to increased human activity. Tippingand Halliday (1994) demonstrated that the ag-gradation of a large tributary fan in the South-ern Uplands extending onto the valley floor ofthe River Tweed occurred between 894–1045cal. yr A.D. and 1282–1397 cal. yr A.D. How-ever, they were unable to relate fan develop-ment with proxy records of climate change oranthropogenic activity.

These episodes of geomorphic activity fromthe British uplands together with the evidencefrom the Hag’s Glen demonstrate that thetenth and eleventh centuries were associatedwith widespread slope destabilization. In turn,this suggests that the geomorphic activity mayhave been promoted by regional climatic forc-ing. However, other proxy records demon-strate that this period was associated with cli-matic amelioration and reduced North Atlanticstorminess (Lamb, 1982), and alluvial recordsfrom North America (e.g., Knox, 1983) andEurope (e.g., Becker and Schirmer, 1977) in-dicate that this period was associated with val-ley-floor stability. Therefore, the high runoffrates that initiated debris mobilization proba-



Figure 6. Hag’s Glen alluvial fan, sections 8 to 13. LFA—lithofacies association.


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Figure 7. Correlation and age of the Hag’s Glen alluvial-fan lithofacies association. LFA—lithofacies association.

Figure 8. Map of Curraghmore alluvial fan showing the location of sediment sections. See Figure 2B for explanation of sediment types.MN—magnetic north.



Figure 9. Curraghmore alluvial fan, sections 14–18.

bly were caused by exceptionally intense fron-tal or convection storms, and the apparent co-incidence in the timing of fan aggradation isprobably a function of site-specific factors andthe poor data set on regional alluvial-fandevelopment.

Fan Aggradation after 1510 Cal. Yr A.D.

This period represents the most significantphase of fan aggradation recorded in the Mac-gillycuddy’s Reeks. Historical records revealthat the forest cover in upland areas of CountyKerry was substantially modified in the six-teenth to eighteenth centuries following the ar-rival of Elizabethan settlers. Woodland de-struction then resumed during the seventeenthand eighteenth centuries for the production ofcharcoal for local ironworks (McCracken,1971; Watts, 1984). The pollen diagram forsection 6 (Fig. DR1) of the Hag’s Glen allu-vial fan, however, suggests that a stable openlandscape, dominated by wet acidic grasslandsand small amounts of birch and oak woodland(;15% total land pollen), existed in this lo-

cality from 1040–1280 cal. yr A.D. to 1510–1930 cal. yr A.D. Indeed, these vegetationcharacteristics are very similar to the present-day flora. Agricultural activity, particularlygrazing, is reflected throughout this time byRumex acetosa/acetosella, Urtica dioica–type,Artemisia, and Plantago lanceolata. There isno evidence of increased grazing in the upperpart of HG5 (Fig. DR1), but a sharp charcoalpeak just beneath the overlying clastic unitLFA HGv indicates that vegetation burningmay have affected slope stability.

Pollen evidence for wetter conditions (ei-ther climatic changes or reflecting hydrologicchanges) in the Macgillycuddy’s Reeks duringhistorical times is unclear. Sphagnum sporevalues remain below 5% total land pollen,whereas Cyperaceae values are much highercompared to prehistoric times (Fig. DR1) butfall gradually throughout LPAZ HG4 prior tothe deposition of unit LGA HGiii. Higher Cy-peraceae values during LPAZ HG5 (up to30%), suggest that wetter soil conditions al-lowed the spread of sedges, but these valuesdip before the deposition of LFA HG5. A

number of factors suggest that climatic forc-ing, however, triggered slope instability. First,based on radiocarbon dating, fan aggradationafter 1510 A.D. appears to have been broadlysynchronous. Second, the generation of mul-tiple transitional-flow lobes and waterflooddeposits implies a number of high-dischargeevents in response to a distinct phase of in-tense rainstorms and increased wetness. Last,this period of geomorphic activity coincideswith the well-documented phase of increasedstorminess during the Little Ice Age of the six-teenth to nineteenth centuries (Fig. 10; Lamb,1979, 1982).

Increasing evidence exists that short-termhydroclimatic changes during the Little IceAge caused widespread slope destabilizationand valley-floor alluviation throughout theNorthern Hemisphere (Grove 1988), includingwestern and central Europe (cf. Rumsby andMacklin, 1996), Scandinavia (cf. Matthews etal., 1997), and the western United States (cf.Meyer et al., 1997). In Britain, increased riverflooding in response to Little Ice Age storm-iness has been demonstrated in the Tyne basin


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Figure 11. Correlation between sections at Hag’s Glen, Glen Coomloughra, and Curraghmore of alluvial-fan and debris-cone inorganiclayers.

(Macklin et al., 1992b; Passmore et al., 1993;Rumsby and Macklin, 1994) and inferred byTipping (1994) to interpret Holocene alluvialfills in the Tweed valley, southern Scotland(Fig. 10). Brazier and Ballantyne (1989) attri-buted accelerated debris-cone aggradationsince the fifteenth century in the westernCairngorm Mountains, Scotland, to climaticforcing and were able to reject anthropogenicactivity as a forcing agent because the slopesabove the debris cones were thickly woodedand there was no evidence that this woodlandcover had ever been disturbed. Instead, theysuggested that debris mobilization might havefollowed a rare and exceptionally intensiverainstorm that stripped away surface vegeta-tion and lowered the threshold for subsequentgullying and fan aggradation. Recently, anumber of high-resolution climate reconstruc-tions based on proxy and historical sources

have demonstrated that the Little Ice Age con-sisted of a complex series of alternating andnonsynchronous, cold and warm episodes(Bradley and Jones, 1995). Significantly,Rumsby and Macklin (1996) have demonstrat-ed that phases of enhanced fluvial activityacross Europe coincided with periods of cli-matic transition. Nevertheless, the impact ofhydroclimatic forcing must have been condi-tioned to some degree by land-use changes de-spite the strong association between debrismobilization and climate change during theLittle Ice Age in catchments with a long his-tory of pastoral or arable farming.

The evidence presented here suggests thatanthropogenic activity and climate forcingcannot be viewed as mutually exclusive caus-es of late Holocene alluvial-fan and debris-cone development in upland areas of the Brit-ish Isles. Instead, it appears that such activity

was conditioned by land-use changes anddriven by increased storminess. A sequence ofevents may involve initial slope destabiliza-tion related to overgrazing and removal ofvegetation and followed by debris mobiliza-tion and fan aggradation during an intenserainstorm or series of rainstorms associatedwith climate change. Across the British Isles,periods of late Holocene valley-floor alluvia-tion and slope destabilization in upland areascluster into three phases: (1) third to eighthcenturies, (2) tenth to eleventh centuries, and(3) sixteenth century to the present (Fig. 10).Such regional synchroneity of geomorphic ac-tivity also implies that climatic forcing wasthe primary driver of geomorphic responses.That all three phases are associated with al-luvial development in the Macgillycuddy’sReeks also suggests that this area was partic-ularly sensitive to increased storminess. In-


ANDERSON et al.

deed, this is not unexpected considering theclose proximity of this area to the North At-lantic seaboard and highlights the potential ofgeomorphic records in southwest Ireland assensitive indicators of late Quaternary climatefluctuations driven by changes in North Atlan-tic thermohaline circulation. The importanceof land-use changes in conditioning alluvial-fan development also may explain why thereis no evidence of Holocene geomorphic activ-ity in the Macgillycuddy’s Reeks before therise of agriculture. For instance, fluvial re-cords from central Europe (Becker and Schmi-rer, 1977), North America (Knox, 1983), andlowland Britain (Macklin and Lewin, 1993)demonstrate episodes of landscape destabili-zation in response to early and middle Holo-cene hemispheric climatic forcing. Therefore,the absence of corresponding aggradationunits in an area that appears to be climaticallysensitive implies that the natural vegetationcover prevented debris mobilization.

CONCLUSIONS

Examination of two alluvial fans and an al-luvial-fan and alluvial-debris complex in theMacgillycuddy’s Reeks reveals evidence ofepisodic phases of late Holocene aggradationand incision. Radiocarbon dates from peat ho-rizons above and below inorganic units indi-cates a high degree of synchroneity betweenphases of aggradation, which cluster into twodistinct periods: after 230–790 cal. yr A.D.and 1510 cal. yr A.D. to the present. An ad-ditional phase of fan aggradation is dated fromafter 1040–1280 cal. yr A.D. (Fig. 11). Theinterstratified peat horizons imply that theseepisodes were separated by phases of relativesurface stability. Evidence of land-use changesprior to aggradation from the Hag’s Glen al-luvial fan suggests that agricultural activitywas an important factor in lowering thethreshold for slope erosion. Conversely, epi-sodes of fan development also coincide withwell-documented phases of climate change—climate deterioration after 550–740 cal. yrA.D. (Blackford and Chambers, 1991; Baillieand Munro, 1988) and the Little Ice Age ofthe sixteenth to nineteenth centuries (Grove,1988). Fan aggradation is therefore, probablya function of both anthropogenic and climaticforcing.

APPENDIX. METHODS

Depositional landform assemblages and mor-phostratigraphic relationships were determined bygeomorphic mapping at scales ranging from 1:1000to 1:4000 using enlarged aerial photographs as basemaps and by topographic surveying. Lithostratig-

raphy of the fans was established by sediment log-ging of exposed stream-cut sections. Depositionalfacies (Data Repository Table DR1) were classifiedas debris flows, transitional flows, and streamflowson the basis of surface morphology, texture, sorting,and internal structures using the criteria set out byWells and Harvey (1987), Costa (1988), Blair andMcPherson (1994), and Meyer et al. (1997). Debrisflows are non-Newtonian fluids that possess high-yield strengths (.40 Pa), and consist of viscousslurries of sediment and water moving as a singlephase (Costa, 1988; Meyer et al., 1997). Depositionoccurs as the slurry thins out, resulting in the for-mation of prominent depositional lobes character-ized by steep flanks and sharp lateral margins.These lobes consist of very poorly sorted debriswith weakly developed stratification and clast fab-rics that strongly reflect compressional and exten-sional flow regimes (Costa and Jarrett, 1981; Wellsand Harvey, 1987; Costa, 1988; Meyer et al. 1997).In contrast, streamflows are turbulent Newtonianflows that have no yield strength and low sedimentloads (Costa, 1988). Facies types, therefore, consistof well-sorted and clast-supported deposits thatform low-relief sedimentary landforms with indefi-nite lateral margins (Meyer and Wells, 1997; Blairand McPherson, 1994). Transitional or hypercon-centrated flows represent the stage between non-Newtonian and Newtonian flows and possess asmall but measurable shear strength (10–40 Pa;Costa, 1988). Facies types are, therefore, interme-diate between debris-flow and streamflow depositsand form prominent landforms consisting of mod-erately to poorly sorted deposits, which may be bothclast and matrix supported and display crudely de-veloped stratification and clast imbrication (Wellsand Harvey, 1987; Costa, 1988; Meyer et al., 1997).

A total of 14 peat samples, from in situ horizonsexposed in stream-cut sections, were selected forradiocarbon analysis. Where possible, peat sampleswere extracted from horizons above and below non-erosive contacts with inorganic sediments to mini-mize gaps in the timing of the onset and cessationof fluvial activity and peat accumulation. Sampleswere analyzed at the Natural Environment ResearchCouncil Radiocarbon Laboratory, East Kilbride,Scotland, and Beta Analytical, Miami, Florida,USA. All radiocarbon dates were calibrated usingOXCAL (version 2.18; Stuiver and Kar, 1986). Pol-len and charcoal analysis was carried out on sam-ples (sections 6 and 11) collected in aluminummonolith tins (15 cm 3 50 cm) from the Hag’s Glenalluvial fan. Subsamples of 0.5 cm thickness and 2g wet weight were prepared for pollen analysis us-ing the procedure described by Barber (1976). OneLycopodium clavatum spore tablet was added toeach subsample to calculate charcoal concentrationsusing the point-count estimation procedure outlinedby Clark (1982). At least 500 land-pollen grainswere counted for each subsample. Pollen grainswere identified using the keys of Faegri et al. (1989)and Moore et al. (1991) and by comparison with amodern type-slide collection. Nomenclature followsStace (1991) and includes nomenclature changes asrecommended by Bennett et al. (1994).

ACKNOWLEDGMENTS

We thank Aideen Cullen, Simon Wathen, NatalieWilliams, and Jason Murray for field assistance andEileen and Joe Cronin for their hospitality. Middle-sex University, Coventry University, and the Natu-ral Environment Research Council Radiocarbon

Steering Committee provided financial assistance.Erica Milwain, Coventry University, helped withthe preparation of the diagrams. The paper is muchimproved thanks to the detailed and useful com-ments made by Paul Bierman and Karen Jennings.

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