Holocene alluvial-fan development in the Macgillycuddy's Reeks, southwest Ireland

E. Anderson School of Geography and Environmental Management, Middlesex University, Queensway, En®eld EN3 4SF, UK S. Harrison* Centre for Quaternary Science, Geography, NES, Coventry University, Priory Street, Coventry CV1 5FB, UK D.G. Passmore Department of Geography, University of Newcastle-upon-Tyne, Newcastle-upon-Tyne NE1 8ST, UK T.M. Mighall Centre for Quaternary Science, Geography, NES, Coventry University, Priory Street, Coventry CV1 5FB, UK

ABSTRACT probably a function of both anthropogenic British Isles, the majority of alluvial fans and and climatic forcing. A sequence of events debris cones are deeply incised, partially or Holocene alluvial landforms in the Mac- may have involved initial slope destabili- wholly vegetated, and show little sign of re- gillycuddy's Reeks, southwest Ireland, were zation due to overgrazing and removal of cent activity (Brazier and Ballantyne, 1989). investigated to determine the controls and vegetation that was followed by debris mo- Research on the chronology and paleoenviron- timing of postglacial geomorphic activity. bilization and fan aggradation during in- mental signi®cance of these landforms has Detailed geomorphologic analysis of three tense rainstorms associated with climate demonstrated widespread evidence of Holo- alluvial-fan and debris cones within high- change. cene surface aggradation and incision. This level cirque basins demonstrates evidence activity was particularly intense during the Keywords: alluvial fans, Holocene, south- of episodic phases of late Holocene surface tenth to twelfth centuries (Harvey et al., 1981; west Ireland. aggradation and incision. Radiocarbon Harvey and Renwick, 1987; Tipping and Hal- dates from peat horizons above and below INTRODUCTION liday, 1994) and sixteenth to nineteenth cen- inorganic units show that phases of aggra- turies (Innes, 1983; Brazier et al., 1988; dation cluster into two distinct periods, the Alluvial fans and debris cones are fan- Brazier and Ballantyne, 1989). ®rst after 230±790 calibrated (cal.) yr A.D. shaped bodies of sediment which form at the Despite the body of evidence cited previ- and the second from 1510 cal. yr A.D. to foot of mountain escarpments, slope gullies, ously, uncertainties remain regarding several the present. An additional phase of fan ag- and tributary valleys by rapid deposition from key research issues. First, discerning the re- gradation at one site is dated after 1040± channelized or uncon®ned water¯oods, tran- spective roles of climate and land-use changes 1280 cal. yr A.D. All three phases coincide sitional ¯ows, and debris ¯ows (Blair and Mc- (external forces) in controlling geomorphic ac- with episodes of enhanced late Holocene Pherson, 1994). They can be mutually distin- tivity remains a contentious issue (Ballantyne, valley-¯oor alluviation and debris-¯ow ac- guished on the basis of surface gradient and 1991a, 1991b). Alluvial records in upland ba- tivity from upland Britain. Alluvial fans bulk composition. Alluvial fans commonly sins are commonly incomplete, and materials consist of a variety of sediment facies types and debris cones have developed primarily suitable for dating are commonly lacking. and display surface gradients generally be- as a result of the resedimentation of late Consequently, these proxy records are seldom tween 2Њ and 12Њ (Blair and McPherson, Midlandian (Wisconsin) drift and talus of suf®cient resolution to isolate a de®nitive 1994). In contrast, debris cones are generally slopes, and mobilization of materials in- climatic or cultural signal. Second, studies steeper, with average gradients typically in the volved ¯ooding, transitional-¯ow, and de- range of 12Њ±25Њ (Brazier et al., 1988; Blair have focused on single sites where geomor- bris-¯ow processes. Pollen analysis of peat and McPherson, 1994), and include predomi- phic responses to ¯uxes in external boundary horizons interbedded with alluvial-fan and nantly superposed debris-¯ow lobes (Data Re- conditions may be dif®cult to differentiate debris-cone sediments indicates that land- pository Table DR11). In upland areas of the from internal stochastic or complex response use changes were an important factor in (Schumm, 1973). lowering the threshold for local slope ero- 1GSA Data Repository item 2000112, description In the Macgillycuddy's Reeks, alluvial fans sion. Phases of aggradation also coincide and schematic pro®les of alluvial-fan depositional and debris cones are a common feature at the with well-documented episodes of climate facies, is available on the Web at http:// base of incised-valley and cirque slopes and change, and, hence, fan development is www.geosociety.org/pubs/ft2000.htm. Requests re¯ect widespread slope destabilization during may also be sent to Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301; e-mail: the late Quaternary drift and talus covers. *E-mail: [email protected]. [email protected]. Three of these alluvial-fan and debris-cone

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

1834 HOLOCENE DEVELOPMENT, SOUTHWEST IRELAND complexesÐlocated in the Coomloughra distinct feeder channels incised into Younger downstream washout debris from transitional Glen, Hag's Glen, and Curraghmore cirque ba- Dryas till and is overlain by talus (Fig. 2). The ¯ows that deposited a series of stacked lobes sins (Fig. 1 and Table 1)Ðcontain buried peat proximal reach consists of superposed vis- and lobes on fan A. Fan aggradation was fol- horizons and hence facilitate an opportunity to cous-debris and transitional-¯ow lobes and lowed by renewed peat development, although address some of the research issues previously splays, which grade proximally into poorly surface stability was interrupted brie¯y by the outlined. Accordingly, the principal aims of sorted channel ®lls. Distally, dilute debris deposition of channelized sandy gravel in the this paper are to (1) establish the geomorphic ¯ows and ¯uvial boulder lobes replace these distal reach (LFA CG4, section 2). The initial development and chronology of alluvial fans facies, and the distal portion is made up of an timing of this episode of peat formation is un- and debris cones and (2) determine the envi- expansive assemblage of superposed ¯uvial certain, but the thickness of the peat layer in ronmental controls responsible for periods of cobble bars and sheet¯ood deposits. Five section 1 (0.28 m; Fig. 3) indicates a pro- landform stability and accelerated geomorphic -cut exposures (sections 1±5; Fig. 3) re- longed period of surface stability prior to the activity. The methodology employed in this veal that this surface assemblage, referred to return of alluvial deposition after 1510±1930 study is described in the Appendix. here as lithofacies association (LFA) CG5, cal. yr A.D. Renewed slope instability after consists of clast-imbricated, gravel-cobble and 1510±1930 cal. yr A.D. caused the deposition STUDY AREA cobble-boulder deposits, which ®ne distally of fan B and possibly transitional-¯ow depo- into a sequence of laminated silts and sands sition on fan A. At this time, the principal The Macgillycuddy's Reeks is a high-relief and are overlain by sand-and-cobble gravel. basin stream ¯owed around the outer margin massif, consisting of Devonian sandstones and Interstrati®ed peats and clastic units underlie of fan B (Fig. 2). Following the cessation of shales (Fig. 1). The legacy of Pleistocene gla- LFA CG5 in sections 1 and 2 (Fig. 3). Radio- fan aggradation, however, by fan B ciations has left the massif deeply dissected carbon dating of peat samples from LFAs surface and localized cobble-bar de- by several glacial troughs and numerous, well- CG2 and CG4 produced dates of 1510±1930 position appear to have caused the avulsion of developed cirque basins. The last phase of val- cal. yr A.D. and 650±790 cal. yr A.D., re- the principal basin stream, leading to the for- ley glaciation occurred during the late Mid- spectively (Table 2). Uprooted vegetation in mation of the present-day channel. landian Glenavy Stadial (26±13 ka). The feeder-channel deposits demonstrates that the Younger Dryas cooling (11±10 ka) also pro- fan is still partially active. The Hag's Glen Alluvial Fan moted the development of six small cirque Geomorphic relationships between fan A glaciers (Anderson et al., 1998). The present and fan B are inconclusive (Fig. 2), but strati- Geomorphology and Stratigraphy climate of this area is temperate maritime, and graphic evidence appears to indicate that at The Hag's Glen alluvial fan is located just the region is frequently affected by the pas- least part of fan A predates fan B. Paleocur- south of Lough Callee (Fig. 4) at the foot of sage of midlatitude low-pressure systems. rent analysis of the buried alluvial units (LFAs a gully cut into Glenavy Stadial till. The prox- Consequently, the weather is often stormy, CG3 and CGi) exposed in sections 1±5 dem- imal reach of the fan consists of several tran- and heavy rain is common throughout the onstrates that they were deposited by water- sitional-¯ow boulder lobes. Although the mid- year, particularly in the mountains, where an- ¯oods derived from the catchment area of fan fan area is heavily vegetated, close inspection nual precipitation probably exceeds 2500 mm. A and probably, therefore, represent the down- reveals an expansive assemblage of super- stream washout debris of transitional ¯ows posed stream¯ow boulder bars. Wells and Coomloughra Glen Alluvial-Fan and which formed fan A (Figs. 2 and 3). Presently, Harvey (1987) pointed out that ¯uvial boulder Debris-Cone Complex it is not clear at what stage during the devel- bars could be interpreted as sieve deposits (cf. opment of fan A that these alluvial units were Hooke, 1967). The boulder bars in the Hag's Geomorphology and Stratigraphy deposited because this fan probably consists Glen alluvial fan, however, overlie peat de- The alluvial-fan complex within Coom- of two temporally distinct assemblages of su- posits that would prevent the rapid surface- loughra Glen consists of two distinct fans, re- perposed transitional-¯ow lobes. The strati- water loss necessary to trigger sieve-like de- ferred to here as fan A and fan B, and a series graphic evidence outlined here, however, does position. The surface geomorphology of the of inset bars and channel lags along the length at least indicate that one of the transitional- distal reach is also largely obscured by surface of the main catchment stream (Fig. 2). Fan A ¯ow events probably occurred after 650±790 vegetation and organic topsoil. The facies is located at the base of an incised Younger cal. yr A.D. (Table 2). characteristics of depositional units and the Dryas till sheet (Anderson et al., 1998) and distal-reach stratigraphy, however, can be ob- consists of an assemblage of superposed, tran- Geomorphic Evolution served along stream-cut exposures (sections sitional-¯ow lobes and splays. Investigation of The presence of alluvial silts in the distal 6±13; Figs. 5 and 6). Cross-correlation of the the distal reach and marginal areas of the fan reach of the fan complex indicates geomor- upper distal-reach sheet¯ood splays exposed (fan Aii) reveals that component clasts display phic activity, and resedimented peat fragments between sections 7 and 10 (LFA HGe) and the a greater degree of iron oxide staining and in the complex demonstrate that they are Ho- cobble-bar deposits seen in section 6 (LFA vegetation cover than the clasts that form the locene (section 1). This geomorphic activity HGv; Fig. 5) is relatively straightforward be- lobes of the central fan axis (fan Ai). Such was followed by a period of surface stability cause sections 6 and 7 are only 8 m apart. contrasts may be explained by temporal and peat development. After 650±790 cal. yr They are also at the same elevation (1.5 m) weathering differences and duration of vege- A.D., surface aggradation was triggered by de- above the present stream channel and display tation development, indicating that fan A may stabilization of the upstream catchment slopes similar lithostratigraphy and paleocurrent di- have been formed by two distinct depositional of fan A. Initial activity commenced with the rections (Figs. 4, 5, and 6). These deposits are phases. deposition of distal-reach silts and sands and either equivalent in age or predate the super- Fan B is a debris-cone and alluvial-fan was followed by the sedimentation of gravel- posed ¯uvial boulder lobes and transitional complex that developed downslope of three cobble bars. These deposits are possibly ¯ow because the latter display onlapping re-

Geological Society of America Bulletin, December 2000 1835 ANDERSON et al. area. Figure 1. Location map of the alluvial-fan and debris-cone complexes studied in the Macgillycuddy's Reeks, southwest Ireland. Inset map shows study

1836 Geological Society of America Bulletin, December 2000 HOLOCENE ALLUVIAL FAN DEVELOPMENT, SOUTHWEST IRELAND

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

Laboratory Catchment Basin Relief Feeder Altitude Surface Apex-distal Max area relief ratio channel (m) area reach length thickness (m2) (m) gradient (m2) (m) (m) Coomloughra A 0.337 529 0.47 0.49 500 4600 101 4 Coomloughra B 0.095 481 0.73 0.72 500 13350 175 2.5 Hag's Glen 0.132 470 0.57 0.49 340 7640 120 5.5 Curraghmore 0.676 670 0.51 0.33 300 12000 215 3.5 lationships (Fig. 4). Therefore, the prominent represent the overbank ®nes from a channel- setting suggests that energetic water¯oods depositional landform assemblages that form ized ¯ood or washout debris transported dis- formed them immediately after the deposition the central mass of the alluvial fan postdate tally following the sedimentation of a depo- of the transitional-¯ow sediment assemblage. the upper peat unit (LFA HGiv) in section 6 sitional lobe. Sometime after 1520 cal. yr The interbedded sequence of peats and allu- and were deposited after 1510±1930 cal. yr A.D. but before 1930 cal. yr A.D., a thick and vial units probably covers much of the distal A.D. (Table 2). Radiocarbon dates from sec- laterally extensive assemblage of transitional- portion of the alluvial fan, although its exact tion 11 demonstrate the greater antiquity of ¯ow lobes and splays, stream¯ow boulder surface extent and thickness is not known be- the sediment sequence exposed between sec- lobes and sheet-¯ood gravel was deposited. cause the sediment sequence is only partially tions 11 and 13. Comparison of the radiocar- Morphostratigraphic relationships between exposed along one stream-cut section (section bon dates from the base of LFA HGii (1040± these depositional landforms indicate that they 14; Fig. 9). This section exhibits six distinct 1280 cal. yr A.D.) and from the top of LFA probably represent the geomorphic response lithofacies units. Peat units bracketing the HG4 (1040±1270 cal. yr A.D.) indicates that to a single ¯ash ¯ood. lower alluvial layer have been dated to 1505 LFA HGi and LFA HG5 are probably the Ϯ 40 yr B.P. and 705 Ϯ 40 yr B.P. (Table 2). same depositional unit. In turn, LFA HGi Pollen Diagrams However, the younger date may be erroneous- probably correlates with the lower sheet¯ood Pollen diagrams and zone descriptions of ly old because the overlying contact is ero- facies (LFA Hga) exposed in sections 9 and peat sampled from depositional units LFA sional. The upper alluvial units are dated to 10 (Fig. 6). Consequently, the sedimentary de- Hgii and LFA HG1v from section 6 and from after 190 Ϯ 90 yr B.P. posits exposed in sections 6±13 represent a LFA HG2 and LFA HG4 from section 11 are A crosscutting relationship 7 m upstream semicontinuous vertical pro®le (Fig. 7). presented in the Data Repository (see text from section 14 demonstrates that the cut-and- footnote 1). These data are shown as Figures ®ll units postdate the distal-zone sequence of Geomorphic Evolution DR1 and DR2, respectively. interbedded alluvial layers and peats (Fig. 8). Formation of the Hag's Glen alluvial-fan Consequently, the bulk of the proximal- and complex commenced with the deposition of Curraghmore Alluvial Fan distal-zone sedimentary landforms appear to sheet¯ood gravel and vertical accretion of be younger than the upper peat unit exposed laminated silts in a low-energy environment. Geomorphology and Stratigraphy in section 14 (Fig. 9) and were therefore de- Resedimented peat fragments within the silt The Curraghmore alluvial fan is located im- posited after ca. 1640 cal. yr A.D. This hy- unit demonstrate that they are Holocene in mediately west of Lough Curraghmore (Fig. pothesis, however, rests on the assumption age. After 1760±1520 cal. yr B.C., a period of 1) at the foot of a gully cut into a Glenavy that the transitional-¯ow deposits and adjacent surface stability and peat deposition occurred Stadial till (Fig. 8; Anderson et al., 1998). The cut-and-®ll units are equivalent in age. Al- (LFA HG2), although it is not clear whether fan consists of three distinct landform assem- though the geomorphic evidence is compel- inactivity extended across the entire fan sur- blages: (1) proximal- and medial-zone, tran- ling, it is not conclusive. face at this time because the peat layer may sitional-¯ow lobes and splays; (2) a distal- be a localized channel ®ll. Alluvial activity zone sequence of interbedded alluvial units Geomorphic Evolution recommenced after 550±680 cal. yr A.D. with and peats buried beneath a surface cover of Alluvial activity commenced after 440±630 the deposition of channelized cobble bars and peaty ; and (3) a series of ¯uvial, boulder cal. yr A.D. with the deposition of distal-reach overbank sand and gravel. Following a brief lobes and cobble bars set between the proxi- sands and gravels, which may grade upstream period of possibly localized surface stability mal transitional-¯ow deposits and a ¯anking into buried boulder lobes or bars (section 15). between 890 and 1270 cal. yr A.D., sheet¯ood till sheet. After 1390±1510 cal. yr A.D., peat formation gravel was deposited across a wide area of the Inset around and between the frontal mar- occurred, although fan stability was interrupt- distal reach. The planar-bedded, sheet¯ood gins of the transitional-¯ow lobes are chan- ed brie¯y by the sedimentation of a localized couplets imply that the ¯oodwater was asso- nelized alluvial ®lls (Fig. 8) consisting of sand layer. Assuming that the stratigraphic ciated with autocyclic development and wash- clast-imbricated gravel cobbles with sparse scheme discussed above is correct, the evo- out of standing-wave antidune bedforms. boulders and sandy matrix (sections 15 and lution of the fan after ca. 1640 cal. yr A.D. These couplets, in turn, suggest that the whole 17; Fig. 9). These deposits are interpreted as may be outlined as follows. After ca. 1640 cal. unit may have been deposited during one cut-and-®ll deposits. These ®ll deposits un- yr A.D., geomorphic activity commenced with ¯ash-¯ood event. After 1040±1280 cal. yr conformably overlie a buried sequence of ¯u- the deposition of distal-reach sands and grav- A.D., a period of surface stability and peat vial boulder lobes or bars in section 15. The els and sedimentation of transitional-¯ow de- deposition occurred across the entire fan. Peat paleocurrent direction of the upper alluvial posits. The transitional-¯ow deposits may formation was interrupted brie¯y between unit in section 15 is parallel with the general have been formed either by two or more geo- 1400 and 1630 cal. yr A.D. by the deposition orientation of the transitional lobes and aban- morphic events or were possibly deposited of a localized drape of sand. This layer may doned interlobe channels. Their geomorphic during the same ¯ood with each lobe repre-

Geological Society of America Bulletin, December 2000 1837 ANDERSON et al. senting a distinct sediment pulse formed as Kingdom (Macklin and Lewin, 1993; Mack- DR2). Charcoal values from this assemblage, new supplies of debris were entrained up- lin, 1999) have argued that river response to however, suggest that ®re was not involved in stream. Floodwaters associated with each anthropogenic activity and climate change woodland clearance (Fig. DR2). This reduc- pulse caused localized cut and ®ll of the pre- should be viewed in terms of a continuum be- tion in woodland coincides with the rise in existing fan surface adjacent to lobes and tween these factors. In particular, they sug- Calluna values at 65 cm (ca. 2480 yr B.P.), splays and incised to the present position of gested that land-use changes might be critical indicating an expansion of acidic, organic the distal-reach channel. In addition, the tran- for sensitizing river systems to changes in . Plantago lanceolata, Melampyrum, Lac- sitional-¯ow landform assemblage appears to ¯ood frequency and magnitude. It is common- tucae, Fabaceae, Brassicaceae, and Pteridium have blocked and de¯ected the ¯ow of the ly the case, however, that geomorphologic in- are also recorded in low percentages just be- principal fan channel to the south, which in vestigations lack either chronological controls fore and/or during this expansion, suggesting turn prevented further distal-reach surface ag- and/or climate and land-use data at appropri- that pastoral agriculture in the local vicinity gradation. The boulder lobes and cobble bars ate spatial and temporal scales (e.g., Tipping, may have created conditions conducive for inset between the transitional-¯ow deposits 1994; Macklin, 1999). Intercalated peat hori- peat growth by removing woodland cover and and late Midlandian till represent the last sig- zons in the alluvial-fan complexes provide a modifying the local hydrologic regime. Re- ni®cant episode of geomorphic activity and rare opportunity to address some of these is- covering arboreal percentages in the middle of were deposited by channelized ¯oodwaters. sues by facilitating investigations of the chro- LPAZ HG11c may re¯ect reduced agricultural They may have formed immediately after the nology of fan development and local vegeta- activity, although Betula and Salix never at- diversion of the channel or may re¯ect more tion history. tained their former values (Fig. DR2). recent ¯ood events. The boundary between LPAZs HG11c and Alluvial-Fan and Debris-Cone HG11d also coincides with a reduction of DISCUSSION Development During the First Millennium woodland cover associated initially with de- A.D. clining Salix values between 52 and 48 cm Consideration of the external causes of Ho- (ca. 1615 cal. yr B.P.). Betula and Corylus av- locene fan-alluviation and debris-¯ow activity Accelerated geomorphic activity is re¯ected ellana±type brie¯y increase at this depth, pos- in upland areas of the British Isles has focused by the occurrence of thin clastic units in the sibly as a successional response to the loss of on the relative roles of human activity and cli- distal reach of all fans. Pollen spectra from the willow, but this is followed by a drop in per- mate change (e.g., Innes, 1983; Ballantyne, Hag's Glen alluvial fan indicate that the local centages of both taxa. The fall in the arboreal 1991a). Similar debates also have concerned vegetation cover after 550±680 A.D. consisted pollen values during the ®rst part of LPAZ geomorphologists and archaeologists working of acidic grasslands and woodlands character- HG11d is associated with a rise in herbaceous on valley-¯oor alluvial sequences in upland ized by birch, willow, pine, hazel, and oak pri- taxa, including Poaceae, Cyperaceae, Potentil- and lowland Britain (e.g., Macklin and Lewin, or to fan aggradation (Data Repository Fig. la-type, Ranunculaceae, Succisa pratensis, 1993; Macklin, 1999). The anthropogenic hy- DR1). The pollen record also re¯ects the im- and Plantago lanceolata, suggesting that both pothesis recognizes that vegetation distur- pact of human activity on the landscape at this pastoral activity and the areal extent of acidic bance (especially by soil tillage and grazing) time. This is not unexpected, and several other grasslands had increased. Again charcoal val- lowers the threshold for soil erosion, leading authors have reported evidence of Bronze Age ues remain low, suggesting that ®re was not to debris mobilization and gullying during pe- and Iron Age land-use changes from pollen involved in woodland clearance (Fig. DR2). riodic high runoff. Conversely, the climatic records from southwest Ireland (Dodson, The 14C date within HG11d (550±680 cal. yr hypothesis is based on the apparent coinci- 1990; Monk, 1993). Initial evidence includes A.D.) implies that this phase corresponds with dence between evidence of geomorphic activ- high values of Poaceae pollen and a suite of the early Christian period, which was marked ity and established periods of climate change, herbaceous taxa that is normally associated in Ireland by increasing agricultural activity although Ballantyne (1991b) proposed that in- with pastoral use. These herbaceous taxa in- and widespread forest clearance (Edwards, creased frequency of intense rainstorms as op- clude Plantago lanceolata, Rumex acetosa/ 1985). posed to general climatic deterioration might acetosella, Lactucae, Asteroideae, Vicia-type, The evidence from the pollen record dem- be the key factor. Indeed, in a recent review Brassicaceae, Trifolium-type, and Pteridium onstrates that catchment vegetation prior to of European alluvial records for the period within local pollen assemblage zones (LPAZs) the deposition of an inorganic layer (LFA 1300±1900 A.D., Rumsby and Macklin HG11a and HG11b (Fig. DR2). Because the HG3) had been modi®ed by human activity (1996) concluded that periods of enhanced arboreal pollen curve in LPAZs HG11a and between 1760±1520 cal. yr BC and 550±680 river aggradation and incision coincided with HG11b ¯uctuates around 25% to 30% of total cal. yr A.D. Therefore, it is possible that fan phases of climatic transition, whereas the most land pollen, it appears that agricultural activ- aggradation may have been facilitated, at least severe phases of climatic cooling were asso- ities were not associated with signi®cant in part, by the gradual removal of local veg- ciated with reduced ¯uvial activity. woodland clearance. The ®rst noticeable loss etation and possibly overgrazing, which desta- Recent reviews of Holocene alluvial valley- of woodland cover took place at the beginning bilized soil and sediment cover on the slopes ¯oor development at various scales (Passmore of LPAZ HG11c when Pinus, Betula, Quer- adjacent to the fan. The phase 230±790 cal. and Macklin, 1997) and throughout the United cus, and Salix pollen values all decreased (Fig. yr A.D., however, also coincides with evi-

Figure 2. (A) Map of Coomloughra Glen alluvial-fan and debris-cone complex. MNÐmagnetic north. (B) Key to sediment types for geomorphic maps shown as Figures 2A, 4, 8. M

1838 Geological Society of America Bulletin, December 2000 HOLOCENE ALLUVIAL FAN DEVELOPMENT, SOUTHWEST IRELAND

Geological Society of America Bulletin, December 2000 1839 ANDERSON et al.

Figure 3. Coomloughra Glen, sections 1 to 5. LFAÐlithofacies association. dence of a major shift to wetter climatic con- climatic change during the fourth to sixth cen- neously with the fall in arboreal-pollen values ditions (Fig. 10). In blanket peats across up- turies A.D. was also proposed by Wendland between 48 and 44 cm in HG11d (Fig. DR2). land areas of the British Isles, climatic change and Bryson (1974). Therefore, climate change This increase combined with a decrease in is re¯ected by increased surface wetness, and, possibly associated with increased storminess Calluna pollen support a shift to locally wet in west Ireland, this phase has been dated to may also have been a contributing factor in hydrologic conditions. Fan aggradation, at after 550±740 cal. yr A.D. (Blackford and promoting geomorphic activity between 230 least in the Hag's Glen, therefore, is most like- Chambers, 1991). Furthermore, climatic de- and 790 cal. yr A.D. Indeed, some evidence ly to be related to a combination of human terioration at this time in Ireland is recorded in the pollen record supports a climatic inter- activity and climate change and is primarily a by narrow tree-ring growth, especially around pretation for fan aggradation. Cyperaceae pol- response to external forcing. A sequence of 541 A.D. (Baillie and Munro, 1988). Global len and Sphagnum spores increase simulta- events may have involved initial slope desta-

1840 Geological Society of America Bulletin, December 2000 HOLOCENE ALLUVIAL FAN DEVELOPMENT, SOUTHWEST IRELAND

TABLE 2. RADIOCARBON DATES OF PEAT SAMPLES FROM ALLUVIAL-FAN AND DEBRIS-CONE bilization due to overgrazing followed by de- COMPLEXES bris mobilization and fan aggradation during Laboratory number Location and depth Uncalibrated age Calibrated age an intense rainstorm or series of rainstorms (yr B.P.) associated with climate change. Coomloughra Glen Marked channel incision and/or aggradation SRR-5957 Section 1: 0.66m 240 Ϯ 40 A.D. 1510±1930 SRR-5958 Section 1: 0.93m 235 Ϯ 40 A.D. 1510±1920 during the third through eighth centuries A.D. SRR-5959 Section 1: 1.22m 1305 Ϯ 40 A.D. 650±790 has also been recorded in several tributaries Hags Glen and trunk reaches of the south Tyne basin, SRR-5960 Section 6: 0.68m 240 Ϯ 40 A.D. 1510±1930 northern England (Fig. 10; Macklin et al., SRR-5961 Section 6: 0.80m 450 Ϯ 40 A.D. 1400±1510 SRR-5962 Section 6: 0.84m 445 Ϯ 45 A.D. 1400±1630 1992a, 1992b; Passmore and Macklin, 1997). SRR-5963 Section 6: 1.14m 820 Ϯ 45 A.D. 1040±1280 It is concluded herein, however, that climate SRR-5964 Section 11: 0.23m 835 Ϯ 40 A.D. 1040±1270 change cannot have been the sole cause of val- SRR-5965 Section 11: 0.28m 1020 Ϯ 45 A.D. 890±1160 SRR-5966* Section 11: 0.36m 1410 Ϯ 45 A.D. 550±680 ley-¯oor destabilization, because hydroclima- SRR-5967 Section 11: 0.74m 3360 Ϯ 45 1760±1520 B.C. tologic changes of a similar if not greater Curraghmore magnitude had occurred in the Tyne basin ear- BETA-117295* Section 14: 0.30m 190 Ϯ 90 A.D. 1640±present lier in the Holocene without resulting in sig- SRR-5955 Section 14: 0.60m 705 Ϯ 40 A.D. 1390±1510 SRR-5956* Section 14: 0.72m 1505 Ϯ 40 A.D. 440±630 ni®cant upland erosion. Instead, it is suggested Note: Peat samples extracted from beneath erosive contacts. that extensive deforestation during Iron Age and Roman times had lowered the threshold for valley-¯oor erosion by indirectly aug- menting runoff rates and stream powers. Evi-

Figure 4. Map of Hag's Glen alluvial fan showing the location of sediment sections. See Figure 2B for explanation of sediment types. MNÐmagnetic north.

Geological Society of America Bulletin, December 2000 1841 ANDERSON et al.

Figure 5. Hag's Glen alluvial fan, sections 6 and 7. LFAÐlithofacies association.

dence of valley ¯oor alluviation in Ireland, gests that the dominant vegetational commu- etation, such as Ericaceae. Harvey et al. dated to between 1500 and 1200 yr B.P., has nity was acidic grassland with alpine elements (1981) suggested that these pollen changes re- been reported by Glanville et al. (1997) from such as Jasione-type. Evidence of grazing is ¯ected a signi®cant increase in the area of de- the upper catchment area of the River Liffey demonstrated by the presence of Rumex ace- forestation and inferred that this occurred in in the Wicklow Mountains (Fig. 10). Although tosa/acetosella, Pteridium, Lactucae, Astero- response to the expansion of sheep grazing these authors acknowledged that ¯uvial activ- ideae, and Brassicaceae in the upper half of following the arrival of Scandinavian settlers ity coincided with sub-Atlantic cooling and LPAZ HG11e and the gradual increase of in the tenth century. Harvey and Renwick that local pollen records do not show land-use Plantago lanceolata throughout LPAZs (1987) also attributed tenth century alluvial- changes during this period of time, they did HG11e to HG11c to about 7% total land pol- fan aggradation in the Bowland Fells, northern not speculate on the probable causes of allu- len (Fig. DR2) just beneath the overlying clas- England, to increased human activity. Tipping viation. Evidence of valley-¯oor alluviation tic unit LFA HG5 (Fig. 6). Consequently, fan and Halliday (1994) demonstrated that the ag- has also been reported from continental Eu- aggradation appears to coincide with evidence gradation of a large tributary fan in the South- rope at this time. For instance, Becker and for more intensive grazing around the fan and ern Uplands extending onto the valley ¯oor of Schirmer (1977) documented evidence of re- was, therefore, an important factor in condi- the River Tweed occurred between 894±1045 newed ¯ooding after 350 A.D. in the Danube tioning slope response to rainstorm events. cal. yr A.D. and 1282±1397 cal. yr A.D. How- basin, Austria, and after 560±711 A.D. in the The rise in Cyperaceae values also suggests ever, they were unable to relate fan develop- Main Valley, southern Germany. In contrast, that soils become wetter during the last part ment with proxy records of climate change or however, ¯uvial basins in North America were of LPAZ HG11e, which may have lowered the anthropogenic activity. characterized by relative stability after 1800 yr threshold for soil movement. These episodes of geomorphic activity from B.P. (Knox, 1983). Several other authors have documented al- the British uplands together with the evidence luvial-fan and debris-cone aggradation in up- from the Hag's Glen demonstrate that the Fan Aggradation in the Second land Britain during the tenth and eleventh cen- tenth and eleventh centuries were associated Millennium A.D. turies (Fig. 10). Harvey et al. (1981) described with widespread slope destabilization. In turn, a debris cone, which occurs at the base of a this suggests that the geomorphic activity may Geomorphic activity in the Macgillycuddy's deeply incised soli¯uction slope in the How- have been promoted by regional climatic forc- Reeks after 1040±1280 A.D. is recorded sole- gill Fells, northern England. This cone is ing. However, other proxy records demon- ly on the Hag's Glen alluvial fan (LFA HG5; made up of debris-¯ow deposits overlying a strate that this period was associated with cli- Fig. 6). Pollen analysis of an earlier peat ho- buried organic soil horizon dated to between matic amelioration and reduced North Atlantic rizon (LFA HG4; LPAZ HG11e) indicates a 2580 Ϯ 55 yr B.P. and 940 Ϯ 95 yr B.P. Pol- storminess (Lamb, 1982), and alluvial records stable open landscape existed from 890±1160 len analysis revealed that the soil horizon co- from North America (e.g., Knox, 1983) and cal. yr A.D. to 1040±1270 cal. yr A.D. The incides with vegetational changes indicative Europe (e.g., Becker and Schirmer, 1977) in- occurrence of high percentages of Poaceae and of the rapid transition from a mixed landscape dicate that this period was associated with val- the presence of Potentilla-type, Campanula- dominated by Alnus and Rumex to a more ley-¯oor stability. Therefore, the high runoff ceae, Gentianaceae, and Ranunculaceae sug- open landscape dominated by moorland veg- rates that initiated debris mobilization proba-

1842 Geological Society of America Bulletin, December 2000 HOLOCENE ALLUVIAL FAN DEVELOPMENT, SOUTHWEST IRELAND

Figure 6. Hag's Glen alluvial fan, sections 8 to 13. LFAÐlithofacies association.

Geological Society of America Bulletin, December 2000 1843 ANDERSON et al.

Figure 7. Correlation and age of the Hag's Glen alluvial-fan lithofacies association. LFAÐlithofacies association.

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

1844 Geological Society of America Bulletin, December 2000 HOLOCENE ALLUVIAL FAN DEVELOPMENT, SOUTHWEST IRELAND

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

bly were caused by exceptionally intense fron- cality from 1040±1280 cal. yr A.D. to 1510± number of factors suggest that climatic forc- tal or convection storms, and the apparent co- 1930 cal. yr A.D. Indeed, these vegetation ing, however, triggered slope instability. First, incidence in the timing of fan aggradation is characteristics are very similar to the present- based on radiocarbon dating, fan aggradation probably a function of site-speci®c factors and day ¯ora. Agricultural activity, particularly after 1510 A.D. appears to have been broadly the poor data set on regional alluvial-fan grazing, is re¯ected throughout this time by synchronous. Second, the generation of mul- development. Rumex acetosa/acetosella, Urtica dioica±type, tiple transitional-¯ow lobes and water¯ood Artemisia, and Plantago lanceolata. There is deposits implies a number of high-discharge Fan Aggradation after 1510 Cal. Yr A.D. no evidence of increased grazing in the upper events in response to a distinct phase of in- part of HG5 (Fig. DR1), but a sharp charcoal tense rainstorms and increased wetness. Last, This period represents the most signi®cant peak just beneath the overlying clastic unit this period of geomorphic activity coincides phase of fan aggradation recorded in the Mac- LFA HGv indicates that vegetation burning with the well-documented phase of increased gillycuddy's Reeks. Historical records reveal may have affected slope stability. storminess during the Little Ice Age of the six- that the forest cover in upland areas of County Pollen evidence for wetter conditions (ei- teenth to nineteenth centuries (Fig. 10; Lamb, Kerry was substantially modi®ed in the six- ther climatic changes or re¯ecting hydrologic 1979, 1982). teenth to eighteenth centuries following the ar- changes) in the Macgillycuddy's Reeks during Increasing evidence exists that short-term rival of Elizabethan settlers. Woodland de- historical times is unclear. Sphagnum spore hydroclimatic changes during the Little Ice struction then resumed during the seventeenth values remain below 5% total land pollen, Age caused widespread slope destabilization and eighteenth centuries for the production of whereas Cyperaceae values are much higher and valley-¯oor alluviation throughout the charcoal for local ironworks (McCracken, compared to prehistoric times (Fig. DR1) but Northern Hemisphere (Grove 1988), including 1971; Watts, 1984). The pollen diagram for fall gradually throughout LPAZ HG4 prior to western and central Europe (cf. Rumsby and section 6 (Fig. DR1) of the Hag's Glen allu- the deposition of unit LGA HGiii. Higher Cy- Macklin, 1996), Scandinavia (cf. Matthews et vial fan, however, suggests that a stable open peraceae values during LPAZ HG5 (up to al., 1997), and the western United States (cf. landscape, dominated by wet acidic grasslands 30%), suggest that wetter soil conditions al- Meyer et al., 1997). In Britain, increased river and small amounts of birch and oak woodland lowed the spread of sedges, but these values ¯ooding in response to Little Ice Age storm- (ϳ15% total land pollen), existed in this lo- dip before the deposition of LFA HG5. A iness has been demonstrated in the Tyne basin

Geological Society of America Bulletin, December 2000 1845 ANDERSON et al. n and Ireland, and Figure 10. Episodes ofinvestigators late depicted Holocene by alluviation numbers and debris-¯ow and activity key. in upland areas of the British Isles. Study areas shown in map of Great Britai

1846 Geological Society of America Bulletin, December 2000 HOLOCENE ALLUVIAL FAN DEVELOPMENT, SOUTHWEST IRELAND

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

(Macklin et al., 1992b; Passmore et al., 1993; have demonstrated that the Little Ice Age con- was conditioned by land-use changes and Rumsby and Macklin, 1994) and inferred by sisted of a complex series of alternating and driven by increased storminess. A sequence of Tipping (1994) to interpret Holocene alluvial nonsynchronous, cold and warm episodes events may involve initial slope destabiliza- ®lls in the Tweed valley, southern Scotland (Bradley and Jones, 1995). Signi®cantly, tion related to overgrazing and removal of (Fig. 10). Brazier and Ballantyne (1989) attri- Rumsby and Macklin (1996) have demonstrat- vegetation and followed by debris mobiliza- buted accelerated debris-cone aggradation ed that phases of enhanced ¯uvial activity tion and fan aggradation during an intense since the ®fteenth century in the western across Europe coincided with periods of cli- rainstorm or series of rainstorms associated Cairngorm Mountains, Scotland, to climatic matic transition. Nevertheless, the impact of with climate change. Across the British Isles, forcing and were able to reject anthropogenic hydroclimatic forcing must have been condi- periods of late Holocene valley-¯oor alluvia- activity as a forcing agent because the slopes tioned to some degree by land-use changes de- tion and slope destabilization in upland areas above the debris cones were thickly wooded spite the strong association between debris cluster into three phases: (1) third to eighth and there was no evidence that this woodland mobilization and climate change during the centuries, (2) tenth to eleventh centuries, and cover had ever been disturbed. Instead, they Little Ice Age in catchments with a long his- (3) sixteenth century to the present (Fig. 10). suggested that debris mobilization might have tory of pastoral or arable farming. Such regional synchroneity of geomorphic ac- followed a rare and exceptionally intensive The evidence presented here suggests that tivity also implies that climatic forcing was rainstorm that stripped away surface vegeta- anthropogenic activity and climate forcing the primary driver of geomorphic responses. tion and lowered the threshold for subsequent cannot be viewed as mutually exclusive caus- That all three phases are associated with al- gullying and fan aggradation. Recently, a es of late Holocene alluvial-fan and debris- luvial development in the Macgillycuddy's number of high-resolution climate reconstruc- cone development in upland areas of the Brit- Reeks also suggests that this area was partic- tions based on proxy and historical sources ish Isles. Instead, it appears that such activity ularly sensitive to increased storminess. In-

Geological Society of America Bulletin, December 2000 1847 ANDERSON et al. deed, this is not unexpected considering the raphy of the fans was established by sediment log- Steering Committee provided ®nancial assistance. close proximity of this area to the North At- ging of exposed stream-cut sections. Depositional Erica Milwain, Coventry University, helped with facies (Data Repository Table DR1) were classi®ed the preparation of the diagrams. The paper is much lantic seaboard and highlights the potential of as debris ¯ows, transitional ¯ows, and stream¯ows improved thanks to the detailed and useful com- geomorphic records in southwest Ireland as on the basis of surface morphology, texture, sorting, ments made by Paul Bierman and Karen Jennings. sensitive indicators of late Quaternary climate and internal structures using the criteria set out by ¯uctuations driven by changes in North Atlan- Wells and Harvey (1987), Costa (1988), Blair and McPherson (1994), and Meyer et al. (1997). Debris REFERENCES CITED tic thermohaline circulation. The importance ¯ows are non-Newtonian ¯uids that possess high- of land-use changes in conditioning alluvial- yield strengths (Ͼ40 Pa), and consist of viscous Anderson, E., Harrison, S., Passmore, D.G., and Mighall, fan development also may explain why there slurries of sediment and water moving as a single T.M., 1998, Evidence of Younger Dryas glaciation in is no evidence of Holocene geomorphic activ- phase (Costa, 1988; Meyer et al., 1997). Deposition the Macgillycuddy's Reeks, southwest Ireland, in ity in the Macgillycuddy's Reeks before the occurs as the slurry thins out, resulting in the for- Lewis, L.A., ed., Mountain glaciations, Cambridge, mation of prominent depositional lobes character- UK, Quaternary Research Association, Quaternary rise of agriculture. For instance, ¯uvial re- ized by steep ¯anks and sharp lateral margins. Proceedings, v. 6, p. 75±90. cords from central Europe (Becker and Schmi- These lobes consist of very poorly sorted debris Baillie, M.G.L., and Munro, M.A.R., 1988, Irish tree rings, rer, 1977), North America (Knox, 1983), and with weakly developed strati®cation and clast fab- Santorini, and volcanic dust veils: Nature, v. 332, p. rics that strongly re¯ect compressional and exten- 344±346. lowland Britain (Macklin and Lewin, 1993) Ballantyne, C.K., 1991a, Holocene geomorphic activity in sional ¯ow regimes (Costa and Jarrett, 1981; Wells demonstrate episodes of landscape destabili- the Scottish Highlands: Scottish Geographical Maga- and Harvey, 1987; Costa, 1988; Meyer et al. 1997). zine, v. 107, p. 84±98. zation in response to early and middle Holo- In contrast, stream¯ows are turbulent Newtonian Ballantyne, C.K., 1991b, Late Holocene erosion in upland cene hemispheric climatic forcing. Therefore, ¯ows that have no yield strength and low sediment Britain: Climatic deterioration or human in¯uence?: the absence of corresponding aggradation loads (Costa, 1988). Facies types, therefore, consist The Holocene, v. 1, p. 81±85. Barber, K.E., 1976, History of Vegetation, in Chapman, units in an area that appears to be climatically of well-sorted and clast-supported deposits that form low-relief sedimentary landforms with inde®- S.B., ed., Methods in Plant Ecology: Oxford, UK, sensitive implies that the natural vegetation nite lateral margins (Meyer and Wells, 1997; Blair Blackwell Scienti®c Publications, p. 5±83. cover prevented debris mobilization. and McPherson, 1994). Transitional or hypercon- Becker, B. and Schirmer, W., 1977, Palaeoecological study on the Holocene valley development of the River centrated ¯ows represent the stage between non- Main, southern Germany: Boreas, v. 6, p. 303±321. CONCLUSIONS Newtonian and Newtonian ¯ows and possess a Bennett, K.D., Whittington, G., and Edwards, K., 1994, Re- small but measurable shear strength (10±40 Pa; cent nomenclatural changes in pollen morphology in the Costa, 1988). Facies types are, therefore, interme- British Isles: Quaternary Newsletter, v. 73, p. 1±6. Examination of two alluvial fans and an al- diate between debris-¯ow and stream¯ow deposits Blackford, J.J., and Chambers, F.M., 1991, Proxy records and form prominent landforms consisting of mod- of climate from blanket mires: Evidence for a Dark luvial-fan and alluvial-debris complex in the Age (1400 B.P.) climatic deterioration in the British Macgillycuddy's Reeks reveals evidence of erately to poorly sorted deposits, which may be both clast and matrix supported and display crudely de- Isles: The Holocene, v. 1, p. 63±67. episodic phases of late Holocene aggradation veloped strati®cation and clast imbrication (Wells Blair, T.C., and McPherson, J.G., 1994, Alluvial fans and their natural distinction from rivers based on mor- and incision. Radiocarbon dates from peat ho- and Harvey, 1987; Costa, 1988; Meyer et al., 1997). phology, hydraulic processes, sedimentary processes, rizons above and below inorganic units indi- A total of 14 peat samples, from in situ horizons and facies assemblages: Journal of Sedimentary Re- cates a high degree of synchroneity between exposed in stream-cut sections, were selected for search, v. A64, p. 450±489. radiocarbon analysis. Where possible, peat samples Bradley, R.S., and Jones, P.D., 1995, Climate since A.D. phases of aggradation, which cluster into two were extracted from horizons above and below non- 1500: London, UK, Routledge, 706 p. distinct periods: after 230±790 cal. yr A.D. erosive contacts with inorganic sediments to mini- Brazier, V., and Ballantyne, C.K., 1989, Late Holocene de- and 1510 cal. yr A.D. to the present. An ad- mize gaps in the timing of the onset and cessation bris-cone evolution in Glen Feshie, western Cairngorm Mountains, Scotland: Transactions of the Royal Society ditional phase of fan aggradation is dated from of ¯uvial activity and peat accumulation. Samples were analyzed at the Natural Environment Research of Edinburgh, Earth Sciences, v. 80, p. 17±24. after 1040±1280 cal. yr A.D. (Fig. 11). The Brazier, V., Whittington, G., and Ballantyne, C.K., 1988, Council Radiocarbon Laboratory, East Kilbride, interstrati®ed peat horizons imply that these Holocene debris-cone evolution in Glen Etive, West- Scotland, and Beta Analytical, Miami, Florida, ern Grampian Highlands, Scotland: Earth Surface Pro- episodes were separated by phases of relative USA. All radiocarbon dates were calibrated using cesses and Landforms, v. 13, p. 525±531. surface stability. Evidence of land-use changes OXCAL (version 2.18; Stuiver and Kar, 1986). Pol- Clark, R.L., 1982, Point-count estimation of charcoal in prior to aggradation from the Hag's Glen al- len and charcoal analysis was carried out on sam- pollen preparations and thin sections of sediments: ples (sections 6 and 11) collected in aluminum Pollen et Spores, v. 24, p. 523±535. luvial fan suggests that agricultural activity monolith tins (15 cm ϫ 50 cm) from the Hag's Glen Costa, J.E., 1988, Rheologic, geomorphic, and sedimento- was an important factor in lowering the alluvial fan. Subsamples of 0.5 cm thickness and 2 logic differentiation of water¯oods, hyperconcentrated g wet weight were prepared for pollen analysis us- ¯ows, and debris ¯ows, in Baker, V.R., Kochel, R.C., threshold for slope erosion. Conversely, epi- and Patton, P.C., eds., geomorphology: New sodes of fan development also coincide with ing the procedure described by Barber (1976). One York, USA, Wiley, p. 113±122. well-documented phases of climate changeÐ Lycopodium clavatum spore tablet was added to Costa, J.E., and Jarrett, R.D., 1981, Debris ¯ows in small each subsample to calculate charcoal concentrations mountain stream channels of Colorado and their hy- climate deterioration after 550±740 cal. yr using the point-count estimation procedure outlined drologic implications: Association of Engineering Ge- A.D. (Blackford and Chambers, 1991; Baillie by Clark (1982). At least 500 land-pollen grains ologists Bulletin, v. 18, p. 309±322. and Munro, 1988) and the Little Ice Age of were counted for each subsample. Pollen grains Dodson, J.R., 1990, The Holocene vegetation of a prehis- torically inhabited valley, Dingle, County Kerry: Pro- the sixteenth to nineteenth centuries (Grove, were identi®ed using the keys of Faegri et al. (1989) and Moore et al. (1991) and by comparison with a ceedings of the Royal Irish Academy, v. 90B, p. 1988). Fan aggradation is therefore, probably modern type-slide collection. Nomenclature follows 151±173. a function of both anthropogenic and climatic Edwards, K.J., 1985, The anthropogenic factor in vegeta- Stace (1991) and includes nomenclature changes as tional history, in Edwards, K.J. and Warren, W.P., eds., forcing. recommended by Bennett et al. (1994). The Quaternary history of Ireland: London, UK, Ac- ademic Press, p. 187±215. APPENDIX. METHODS ACKNOWLEDGMENTS Faegri, K., Kaland, P.E., and Krzywinski, K., 1989, Text- book of pollen analysis: Chichester, UK, John Wiley and Sons, 328 p. Depositional landform assemblages and mor- We thank Aideen Cullen, Simon Wathen, Natalie Glanville, P., Thorp, M.B., and Gallagher, C., 1997, Bio- phostratigraphic relationships were determined by Williams, and Jason Murray for ®eld assistance and climatic change and the response of rivers in Irish up- geomorphic mapping at scales ranging from 1:1000 Eileen and Joe Cronin for their hospitality. Middle- land regions during the late Quaternary: With partic- to 1:4000 using enlarged aerial photographs as base sex University, Coventry University, and the Natu- ular attention to the upper River Liffey Catchment, maps and by topographic surveying. Lithostratig- ral Environment Research Council Radiocarbon County Wicklow, in Sweeney, J., ed., Global change

1848 Geological Society of America Bulletin, December 2000 HOLOCENE ALLUVIAL FAN DEVELOPMENT, SOUTHWEST IRELAND

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