Nature, origin and age of Holocene aeolian sand on Muckish Mountain, Co. Donegal, Ireland

Nature, origin and age of Holocene aeolian sand on Muckish Mountain, Co. Donegal, Ireland PETER WILSON

BOREAS Wilson, Peter 1989 06 01: Nature, origin and age of Holocene aeolian sand on Muckish Mountain, Co. Donegal, Ireland. Boreas, Vol. 18, pp. 15S168. Oslo. ISSN0300-9483.

Dissected sand sheets and flow-aligned sand shadows occur near the summit of Muckish Mountain, Co. Donegal. The sand is of medium to fine size and moderately sorted to moderately well sorted. Sand transport by northerly winds is indicated by the location and morphology of the deposits. The source of the sand is a series of friable quartzite beds immediately below the northern edge of the summit plateau. Minor additions of gravel and very coarse sand, derived from the disintegration of plateau clasts, were probably also incorporated within the deposits by aeolian action, although surface wash associated with heavy rain or snowmelt may have mobilized these particles. The absence of diagnostic aeolian transport textures on quartz grain surfaces reflects the short distance/duration of transport. I4C dates indicate two phases of sand sheet accumulation: onebetweenc. 5,300andc. 2,650B.P. andagainafterc. 1,91&1,760B.P. Thesandshadows have formed within the last hundred years in response to the construction of small cairns across the plateau. Sand sheet dissection suggests erosion is currently occurring, but this began prior to the late nineteenth century. Present-day sand accumulation is also apparent from the widespread scatter of grains trapped by surface vegetation. The sand represents the first recognition of aeolian deposition in the uplands of Ireland.

Peter Wikon, Department of Environmental Studies, Uniuersity of Ulster at Coleraine, Cromore Road, Coleraine, Co. Londonderry BT52 ISA, Nonhern Ireland, U . K. ; 1st September. 1988 (reuised 16th Decem- ber, 1988).

Aeolian deposits have not as yet been recognized in upland areas of Ireland whereas sand and/or silt, generally ascribed to aeolian transport and deposition in relatively cool upland environments during the Holocene, has been identified at several sites in the Scottish uplands (Ball & Goodier 1974; Goodier & Ball 1975; Birse 1980; Pye & Paine 1984; Ballantyne & Whittington 1987). The lack of information relating to these types of deposits represents a major gap in current knowledge of Irish upland environments. This paper reports the resultsof researchaimed at establishing thenature, origin and age of sand deposits found near the surn- mit of Muckish Mountain, Co. Donegal. The evidence presented indicates that the sand is locally derived, predominantly aeolian, and of Holocene age.

Study area Muckish Mountain (Lat. 55"06'N, Long. 8"OO'W; Fig. 1) is located in northern Donegal c. 8 km from the coast. It rises abruptly from the surrounding lowlands to a height of 670 m and forms an extensive, gently sloping plateau above c. 550-600 m.

Most of the mountain consists of Ards Quartzite (Precambrian), the main component of which is a thick and fairly uniform feldspathic sandstone of coarse to fine grain size (McCall 1954; Pitcher & Berger 1972). During the Pleistocene, ice from the granite mountains to the south apparently crossed the summit of Muckish (Charlesworth 1924); granite erratics occur on the lower ground and up to a height of c. 500 m on neighbouring mountains, but are absent from the plateau. Charlesworth (1924) considered the angular quartzite debris that now covers the plateau to result from Late Weichselian/ Midlandian frost action.

Present climate on Muckish can only be inferred from data for sites at lower levels. These data sug- gest an environment dominated by extreme wetness and strong winds (cf. Ballantyne 1987; Ballantyne & Whittington 1987). Precipitation on the lower ground (90-120 m) around Muckish is 1;380-1,760 mm y-l and at higher elevations (360- 440 m), 18 km southwest of Muckish, slightly in excess of 2,000 mm y-' is recorded (Fitzgerald 1984). The coast of north Donegal is one of the windiest parts of Ireland (Rohan 1975) with mean velocity of 7 m s-l. At Malin Head, 50 km north- east of Muckish, 54% of all winds during the period

160 Peter Wilson BOREAS 18 (1989)

Fig. 1. Muckish Mountain, Co. Donegal, northwest Ireland. 1. Very steep slopes. 2. Disused quarry in friable quartzite beds. 3. Depression in which sand deposits occur. 4. Dissected sand sheets. Contours in metres.

196244 were from between south and west (Meteorological Service pers. comm. 1987). Of these, 75% exceeded 5.5 m s-l and 24% exceeded 10.8 m s-l. In addition, the mean number of days with gales (i.e. >17.2 m s-'), over the ten-year period 1961-70, was 42.4 and the strongest monthly gusts, recorded over the same period, ran- ged from 28 to 50111s-' (Rohan 1975). Mean annual air temperature at Milford (49 m), 19 km east of Muckish, is 8.9"C (Keane 1985). Assuming a standard temperature-altitude correction of 1"C/ 150 m, the mean annual temperature at 650 m on Muckish is 4.9"C. Therefore, the climatic environment on Muckish is similar to the 'maritime penglacial regime' described by Ballantyne (1987) for the British uplands.

Characteristics of the Muckish sand deposits Location and morphology Most of the sand deposits on Muckish Mountain occur in a broad, shallow depression that extends across the plateau and occupies an area of c. 57,500 m2 (Fig. 1). The axis of the depression dips gently northwest at angles generally less than 5", although some slopes steepen to 16" directly above the escarpment. Within this area, several mor-

phologically contrasting types of sand accumulation have been identified. The most distinctive of these comprises a dissected sand sheet, c. 250 x 60 m, along the northern edge of the plateau (Fig. 1). Dissection of the sand has produced vegetation-free scarps (Fig. 2A) and isolated sand 'islands', which clearly indicate a formerly more continuous cover of sand (cf. Ball & Goodier 1974; Goodier & Ball 1975; Ballantyne & Whittington 1987). The surface of the sand sheet is completely vegetated. Species include Festuca ovina, Calluna vulgaris, Vaccinium myrtillus, Armeria maritima, Huperzia selago, Rhacomitrium lanuginosum, Rhytidiadelphus loreus and Polytrichum com- mune. Sand sheet topography is even or gently undulating although in a few places pronounced linear hummocks and narrow channels are present, suggesting modification by surface water flow. Maximum sand thickness of c. 0.8 m occurs near the plateau edge. The sand thins rapidly with increasing distance from the edge to a few centi- metres thickness at its southern limit, c. 60 m from the plateau edge.

Over most of the depression angular quartzite clasts predominate. However, sand occurs in hol- lows between clasts, where it is intermixed with humustoformsoil'pockets'up to0.2 mdeep. Such areas are sparsely colonized by Festuca ovina and possess a veneer of white sand grains and grit. These, together with the white quartzite clasts,

BOREAS 18 (1989) Holocene aeolian sand, Ireland 161

Fig. 2. Sand deposits on Muckish Mountain. 0 A. Dissected sand sheet with vegetation-free scarps. The ranging pole is marked at 0.2 m intervals. 0 B. Excavated profile in dissected sand sheet showing alternating units of organic-poor and organic-rich sand. Ranging pole as before.


render the depression prominent on air photographs as an area of markedly paler tone. Sand is also present beneath surface clasts but contains only minor quantities of humus. Intermixed sand and humus accumulations also occur as 'tails' behind both individual boulders and small cairns (Wilson 1988). Seven cairns, consisting of several boulders heaped together, are aligned along the depression on a bearing of 160-340". On the south (upslope) side of each is a 'tail' of vegetated sand. The 'tails' are up to 3.2 minlength, 0.45 minheight and 1.6 min width. Whitesand has penetratedinto the void space of the cairns.

In addition to these plateau sands, a sheet of sand and humus overlies peat on a slope of 22" immediately below the southeastern edge of the plateau (Fig. 1). This sand sheet extends downslope for c. 30 m and is c. 120 m in width. Dissection near the top of the slope reveals 0.5 m of sand above c. 1 m of peat, but sand thickness decreases rapidly downslope.

Stratigraphy and structures

profiles from the surface to the underlying regolith. In three profiles sand and humus were intermixed throughout the full thickness of the exposures. In two profiles (MS4-5), 150 m apart, a number of colour-differentiated sand units and peat layers were visible (Fig. 2B). These are depicted in Fig. 3 along with Munsell colours and percentage loss- on-ignition (which is assumed to approximate to organic matter content).

Profile MS4 displays a basal peat 5cm thick, overlain by six sand units totalling c. 75 an thickness. The lowermost sand unit (1) consists of light brownish grey (10YR 6/2) sand that is almost devoid of organic matter except for a few thin humus-rich streaks. The overlying sand units possess between 3 and 30% organic matter. In profile MS5 a 2cm thick, black (5YR 2.5/1), greasy, humus-rich sand covers the quartzite regolith. Above this layer are three visually distinctive sand units totalling c. 65cm thickness. Two of these units (1 & 3) possess negligible amounts of organic matterwhileunit2containsc. 5%. Alayer ofpeat, 6 cm thick, occurs near the base of unit 2.

Sedimentam structures are not atmarent in L 1

At five sites (MS1-5) vegetation-free scarps of the plateau sand sheet were excavated to reveal clean

either of the profiles examined. However, Fig. 2B shows that the gentle undulations of the sand sheet

Fig. 3. Stratigraphy, 14C dates, colours and % loss-on-ignition of Profiles MS4 and MS5.


surface follow a similar undulating subsurface horizon, suggesting a dune-like morphology although characteristic dune bedding has not been observed.

tosis values show the samples to be evenly divided between mesokurtic and leptokurtic classes.

These data imply a high degree of textural uni- formity for the sand components and between sample variability is therefore due to systematic differences in the gravel and silt fractions. Silt Particle size distribution

Twenty-four samples were collected from the various sand deposits for particle size analysis. Fifteen samples were obtained from the five profiles excavated in the dissected sand sheet of the plateau. Two samples each came from the surface veneer, the sand ‘tails’, the soil ‘pockets’ and from beneath surface clasts. One sample was taken from the sand sheet below the southeastern edge of the plateau. All samples were treated with hydrogen peroxide and 3N hydrochloric acid prior to dispersion in dilute sodium hexametaphosphate solution. Silt (+4to +9@)andclay(>+9@)fractionsweredeter- mined by pipette sampling and the sand (- 1 to +4@) component was subsequently dry-sieved at 0.25@ intervals. Gravel particles (<1@) were recorded without any further size subdivision. For the r6- sand fraction the textural parametersofmeangrain size ( M Z ) , sorting (aI), skewness (Ski) and kurtosis (KG’) were estimated graphically (Folk 1966).

Sand comprises 7698% by weight of the size distributions, the dominant components of which are fine sand (3450%) and medium sand (1% 45%). In only four samples does medium sand exceed fine sand. Very fine sand (519%) is generally more abundant than coarse sand (1-lo%), and the very coarse sand content is 0 .14%. A few samples possess significant quantities of gravel (1- 20 ~m I 14%) and/or silt (1-18%), but the clay fraction is

I

I I

aminor contributor (<3.5%) to all samples. Com- bined gravel, silt and clay contents only exceed 15% in five samples. All gravel particles are less than 15 mm in diameter but occasional quartzite clasts up to 10 cm in length have been found in the sand scarps.

Textural parameters indicate the sand component has a mean grain size range from + 1.85 to +2.38@; within the medium and fine sand cate- gories. However, for 19 samples the mean size is greater than +2@. Sorting coefficients range from 0.66 to 0.93@ and classify the sands as moderately well sorted and moderately sorted, although most (19)arein thelattergroup. All butsixsampleshave skewness values within the -0.1 to +O. 1 limits that define nearly symmetrical size distributions. Five of the six samples beyond these limits are nega-

Fig. 4. Quartzgrainsurfacefeatures. 0 A. Subangulargrainwith ‘blocky’ surface features. 0 B. Chemically etched surface with small area of grain breakage (right of centre). 0 C. Subrounded grain with ‘blocky’ surface features resulting from chemical etch-

tively skewed, the other is positively skewed. Kur- ing


shows a strong positive relationship with organic matter content, while gravel is generally less abundant in samples taken from the dissected sandsheet than in those from elsewhere within the depression.

Quartz grain surface features Twenty quartz grains, from the sieved fractions of medium and fine sand, were selected from each of eight samples for scanning electron microscope (SEM) examination. The analyses reveal that, both within and between samples, all grains have a remarkably similar, rather limited assemblage of surface features and the abundances of these features is also uniform.

The majority of the sand grains examined are subangular (Fig. 4A). A few angular and subrounded grains occur but no rounded grains have been recorded. All grains possess substantial areas dominated by an irregular ‘block-like’ surface feature resulting from both intensive and extensive etching. Etching has produced ‘blocks’ of various size and shape separated by triangular and rectangular pits and parallel steps (Fig. 4A, B). On a small number of grains euhedral quartz overgrowths also form a series of ‘micro-blocks’ . In addition, some grains display fresh cleavage faces and conchoidal fractures with arcuate and semi- parallel steps (Fig. 4B). These breakage surfaces truncate the ‘blocky’ surface features and lack evidence of subsequent modification by etching. They therefore postdate the etched surfaces.

The extensively etched grain surfaces and the paucity of mechanically produced features does not enable sand transport processes to be identified. The absence of etch features across the cleavage and conchoidal fracture surfaces suggests that etching is not a function of postdepositional chemical alteration but is related to chemical processes affecting the source material (see Discussion). Sand transport was probably of short distance/ duration and/or of low energy, preventing the development of diagnostic grain transport features and preserving previously acquired features.

Discussion Origin of the sand deposits

1. Immediately below the northern edge of the plateau occurs a series of friable quartzite beds (Fig. 1) c. 25 m in stratigraphical thickness (Bishopp & McCluskey 1948), but not extending to the plateau surface. The lateral extent of the dissected sand sheet along the plateau edge corresponds with the lateral extent of the friable quartzite along the escarpment. Elsewhere, escarpment cliffs are composed of non-friable quartzite and the plateau areas above lack sand deposits. The friable quartzite was quarried as a source of glass sand from the late nineteenth century until the mid-1950s. Since quarrying ceased, sand accumulations on the quarry floor (alluvial fans below gullies, talus ramps below free faces and small dune-like mounds) testify to the ease with which the quartzite disintegrates. Abandoned machinery at the outer edge of the quarry has been partially buried by up to 1 m of sand.

Eight sand samples were collected from the quarry for particle size analysis, following the pro- cedures outlined above. Four samples derive from sand deposits on the quarry floor and four represent in situ friable quartzite. Medium sand (12- 63%) and fine sand (21-52%) dominate the size distributions. Very fine sand (1-29%) exceeds coarse sand (2-14%) in all but two samples. Com- bined amounts of very coarse sand (O-O.S%), silt (1-9%) and clay (0-2%) are generally less than 3% of the total; gravel particles are absent. Textural parameters (Mz: 1.58-2.62@; q: 0.55-0.74& SkI: -0.10 - +0.11; KG’: 0.49-0.53) indicate that the sand component has similar characteristics to the plateausands, although thequarry samples display slightly better sorting.

Sand grains from these samples were also examined with the SEM and revealed a suite of surface features identical to that recorded on grains from the plateau sands (Fig. 4C). There is less evidence of grain breakage, in the form of cleavage and conchoidal fracture, on the quarry grains, but all other features are present in corresponding abundances to those of the plateau grains.

The presence of extensively etched surfaces on grains from the friable quartzite beds strongly sup- ports the assertion that equivalent etch patterns observed on grains from the plateau sands result from diagenetic and/or in situ weathering processes at source, rather than from postdepositional chemical alteration.

There are two potential sources for the Muckish sand deposits: friable quartzite beds and clastic debris.

2. The summit plateau of Muckish consists of non- friable clastic quartzite debris with limited areas of


thin, peaty soils. Two categoriesofclasts have been recognized: one composed of angular clasts with slight edge-rounding and minor amounts of surface pitting, representing the majority, and one of subrounded clasts displaying heavily pitted surfaces. These intensively pitted surfaces may be due to granular disintegration, but decomposition of constituent feldspars is probably a more likely cause. Moreover, although sand occurs beneath clasts in the plateau depression, there is no notice- able difference in the proportions of clast types between the depression and those areas of the plateau lacking sand. This suggests that the majority of quartzite clasts are not susceptible to granular disintegration and the production of sand.

However, the gravel and very coarse sand particles found within the sand deposits probably do originate from clast disintegration. These fragments are not easily broken by finger pressure, as is the friable quartzite of the quarry. The absence of gravel and the negligible quantities of very coarse sand in those samples obtained from the quarry indicate a clastic source for this material.

Therefore, themajor sourceof theMuckish sand deposits is considered to be the friable quartzite beds, with coarser size fractions resulting from clast disintegration.

Mode of sand accumulation Sand accumulation on Muckish resulted from predominantly aeolian deposition of grains trans- ported from the friable quartzite of the escarpment by north and northwesterly winds. This is sup- ported by the location and morphology of both the plateau sand sheet and the sand sheet immediately below the southeastern edge of the plateau and by the sand ‘tails’.

Although the dominant winds in northern Co. Donegal are from between south and west, northerly airflows are not uncommon and may be enhanced around Muckish by topographic modification of prevailing winds. Mean wind velocity along the Donegal coast exceeds that required to transport sand with an average grain size of c. 24 (Warren 1979) and at higher elevations on Muck- ish, mean wind velocity will be significantly greater (M. T. Tullett pers. comm. 1988). Thus, the nec- essary wind direction and velocity needed to lift sand from the escarpment to the plateau occur.

The patterns of airflow and sand movement over windward-facing escarpments have been outlined recently by Bowen & Lindley (1977) and Marsh &

Marsh (1987). Maximum flow velocity occurs near the brow of an escarpment; at some distance beyond the brow flow separation takes place and velocity decreases to produce a lee area. Sand blown over an escarpment is deposited in this zone. The bulk of the sand is deposited near to the brow and thins with distance away from the brow. Marsh & Marsh (1987:385) regarded the model devel- oped by Bowen & Lindley (1977) as providing a gross approximation of airflow over escarpments with rounded brows. Sand deposition was observed to begin at some distance away from the brow, never at the brow itself. Only along the crest of a sandstone escarpment were sand deposits found close to the brow, due to the steep, angular configuration of the crest. An analogous situation occurs on the summit plateau of Muckish; sand deposits occur within a few metres of a steep north- facing escarpment with an angular crest and thin rapidly away from the crest.

The nature of sand accumulation associated with winds blowing from off a plateau was described by Bagnold (1954). When sand-bearing winds sweep across the sharp break of slope at the plateau edge, deposition of grains into the relatively sheltered area below takes place. Maximum sand thickness develops adjacent to the plateau edge and decreases rapidly downslope (cf. Ballantyne & Whittington 1987). The sand sheet immediately below the southeastern edge of the Muckish plateau conforms to this pattern and testifies to aeolian deposition by northerly winds. Moreover, there are no outcrops of friable quartzite on the southeastern slopes that could have acted as a source for this material. The sand probably represents material eroded from the plateau sand sheet.

The sand ‘tails’ correspond to the sand shadows of Bagnold (1954) and the current shadows of Allen (1982) and represent flow-aligned sand accumulation leeward of an obstacle. Such deposits have been reported from both aeolian and fluvial environments. The location of the Muckish shadows on the south (upslope) side of the cairns indicates deposition by northerly winds.

Aeolian transport, however, has not produced sand deposits of finer size or with improved sorting values relative to the escarpment quarry samples, nor have diagnostic aeolian surface textures been impressed upon the quartz grains. These apparent inconsistencies with an aeolian mode of sand accumulation are probably due respectively to the addition of coarser sand from clast disintegration

BOREAS 18 (1989) 166 Peter Wilson

and the short distance/duration of sand transport. The increased number of breakage features (conchoidal fractures and cleavage faces) observed on some sand grainsmay have been caused by aeolian grain impacts.

The presence of gravel and very coarse sand in the deposits implies that during sand accumulation some of the clastic debris within the depression remained exposed and underwent disintegration. These size fractions may have been incorporated within the sand sheet by aeolian transport and/or by surface wash during periods of heavy rain or snowmelt. In some Late Weichselian and Holo- cene aeolian sands, evidence for limited fluvial/ nival deposition is represented by thin concentrations of gravel or coarse sand (Straw 1963; Buck- land 1982; Ballantyne & Whittington 1987) but such concentrations are absent from the Muckish sand scarps; gravel fragments are dispersed throughout. The inclusion of gravel within the sand shadows is also unlikely to result from surface wash. A minimum wind velocity of c. 10 m s-l would be required to transport these coarse components (Warren 1979). Winds in excess of this speed are not infrequent in north Donegal. Never- theless, surface wash cannot be excluded entirely as a transport/depositional process. Present-day wash, associated with heavy rain, mobilizes gravel and sand from the eroding scarps and soil 'pockets' to produce a surface veneer of white grains. The occasional quartzite clasts, found in the sand scarps, probably relate to phases of more intensive wash.

Detailed observations by Ballantyne & Whit- tington (1987) showed sand deposits at c. 600- 800 m on An Teallach, northwest Scotland, to be of predominantly niveo-aeolian origin. This form of deposition was heldresponsible for the generally unstratified and poorly-sorted nature of the deposits and the formation of sand sheets rather than dunes. The Muckish sands are similar in several respects to the AnTeallach deposits and therefore may also be niveo-aeolian. Although present- day snow-lie on Muckish (c. 20dyr-') is con- siderably less than on An Teallach (c. 160 d y ~ ' ) , periods of more prolonged snowcover during the recent past cannot be ruled out, but there is no direct evidence to support a niveo-aeolian mode of sand accumulation on Muckish.

The wide variation in organic matter content within profiles MS4 and MS5 (Fig. 3) indicates non- uniform rates of sand accumulation. Sand units with negligible amounts of organic matter (MS4( 1)

& MS5(1& 3)) were most likely deposited rapidly, while in other units vegetation growth probably kept pace with deposition and contributed humus to the slowly thickening sand. Vegetation would have assisted in trapping sand blown onto the plateau.

Age of sand deposition Four 1 cm thick samples of organic-rich material from within and below the plateau sand sheet were 14C dated at the Natural Environment Research Council (NERC), Radiocarbon Laboratory, East Kilbride. The dates are indicated on Fig. 3 and define two phases of sand accumulation.

Theearliestphasepostdates5,300 ? 70 B.P. but predates 2,650 * 50 B .P. and is represented by the basal unit (1) of clean sand in profile MS5. Accept- ing the postulated rapid deposition of this sand, the thinness of the unit ( s25 cm) suggests a relatively short period of accumulation within the defined time interval. Two interpretations for this phase of sand deposition can be envisaged:

1. Rapid sand accumulation, to an unknown thickness, occurred sometime after c. 5,300 B.P., followed by development of a stable vegetated surface. Destruction of the vegetation and erosion of the upper sand/humic horizons took place prior to peat formation c. 2,650 B.P. on the remaining sand.

2. Erosion of an unknown thickness of blanket peat leaving a thin, greasy, humus-rich sand that .had formed c. 5,300 B.P. Rapid accumulation of sand then occurred, immediately followed by peat formation c. 2,650 B.P.

The second phase of sand accumulation post- dates 1,910 * 60 B.P. in profile MS4 and 1,760 * 60B.P. in profile MS5. Despite this similarity of dates and close proximity of profiles, the nature of sand deposition between profiles clearly differed, as indicated by the variations in organic matter content (Fig. 3). Sand accumulation in either profile may be a more recent event than implied by these dates, especially if peat erosion was active prior to sand deposition.

In the absence of palynological data, these 14C dates can only be regarded as a first approximation to the age of sand accumulation. Ballantyne & Whittington (1987) obtained anomalously young 14C dates that were attributed to in situ con-


tamination by downwash of organic material through the permeable sand deposits. The possi- bility that this has also occurred in the Muckish sands cannot be ignored.

There are no 14C dates associated with the sand sheet below the southeastern edge of the plateau. If the sand was eroded from the plateau deposits it must postdate at least the earlier phase of plateau sand accumulation.

The sand shadows are thought to have formed within the last hundred years. The cairns, to which the shadows are attached, were probably con- structed as navigational aids for quarry workers. Bishopp & McCluskey (1948) record that trial pits were excavated on the southern slopes of Muckish in the search for friable quartzite. The cairns are aligned directly across the plateau from the top of the quarry to the top of the southeastern slopes.

Present erosion and accumulation The dissected nature of the plateau sand sheet and the veneer of sand and gravel issuing from the vegetation-free scarps suggests that erosion is currently taking place. That this erosion was initiated prior to the commencement of quarryingin the late nineteenth century is indicated by two shallow drainage channels dug close to the plateau edge to direct surface water into the quarry. The channels were dug into quartzite regolith, across extensively eroded areas. Channel excavation may have led to enhanced sand erosion during the present century.

On Scottish mountains, sand sheet erosion has been attributed to climaticdeterioration during the ’Little Ice Age’ of the seventeenth and eighteenth centuries and overgrazing by sheep during the eighteenth andnineteenth centuries (Ball & Good- ier 1974; Pye & Paine 1984; Ballantyne & Whit- tington 1987). Although similar causes of erosion could be advocated for Muckish, the exact mech- anisms remain to be established.

Present accumulation of sand is apparent from the widespread scatter of grains trapped within the surface vegetation of both the sand sheets and sand shadows. These additions may represent localized aeolian reworking of sand eroded from the vegetation-free scarps as well as material derived from the escarpment.

Conclusions The similarity of the climatic environment on

Muckish to that described for the British uplands (Ballantyne 1987) warrants application of the term ‘maritime periglacial regime’. Extreme wetness and strong winds characterize these areas rather than severe cold and frequent freeze-thaw cycles. In the past, contemporary periglacial activity on Irish mountains was linked with the limited development of features resulting from frost action (Lewis 1985), but no detailed reports to substanti- ate their current activity have been published. Implicit in adopting the term ‘maritime periglacial regime’ for the Irish uplands is recognition of a broader range of processes and landforms, not all of which involve frost action. Some of these processes (e.g. debris flow, aeolian activity) have long been regarded as important forms of mass transport in arctic and alpine periglacial environments, but their status in Ireland is unclear and requires careful consideration. The sand accumulations described in this paper represent the first recognition of aeolian deposition in the Irish uplands and testify to the geomorphic importance of wind action when coupled with conducive lithological characteristics.

Acknowledgements. -The author is grateful to: the Department of Biology, University of Ulster, and in particular Dr. Steve Lowry, for provision of and assistance with SEM work; Mike Tullett for valuable discussion of aidow characteristics in upland regions; Kilian McDaid and Nigel McDowell for producing the diagrams and photographs; the NERC and Dr. Douglas Hark- ness for the I4C analyses.

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Wilson, P. 1988: Recent sand shadow development on Muckish Mountain, Co. Donegal. Irish Naturalists’Journal22,52P-531.

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Nature, origin and age of Holocene aeolian sand on Muckish Mountain, Co. Donegal, Ireland

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