quaternary glaciation of the bale mountains, ethiopia

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JOURNAL OF QUATERNARY SCIENCE (2005) 20(6) 593–606 Copyright ß 2005 John Wiley & Sons, Ltd. Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jqs.931 Quaternary glaciation of the Bale Mountains, Ethiopia HENRY A. OSMASTON, 1 * WISHART A. MITCHELL 2 and J. A. NIGEL OSMASTON 3 1 School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK 2 Department of Geography, University of Durham, Durham DH1 3LE, UK 3 Mott Macdonald & Partners, Demeter House, Station Road, Cambridge CB1 2RS, UK Osmaston, H. A., Mitchell, W. A. and Osmaston, J. A. N. 2005. Quaternary glaciation of the Bale Mountains, Ethiopia. J. Quaternary Sci., Vol. 20 pp. 593–606. ISSN 0267-8179. Received 4 February 2004; Revised 6 March 2005; Accepted 10 March 2005 ABSTRACT: Central Ethiopia comprises a high plateau at 2000–3000 m, formed from Tertiary lava flows and bisected by the Eastern African Rift. Ten volcanic mountains rise to altitudes of just over 4000 m, but on only three has Quaternary glaciation been substantiated by published field observa- tions. On the Bale Mountains (4400 m), a previous report based on limited evidence proposed an ice-cap extending to 600 km 2 . Based on aerial photographs and ground surveys, this paper reports evidence of a more complex situation. A wide spread of large erratic boulders on the plateau records a central ice cap of 30 km 2 , though ice probably extended for a further 40 km 2 . Further north two groups of deeply incised and clearly glaciated valleys contain moraines and roches moutonne ´es (60 km 2 ). On interfluves between them and on the open north slopes are moraines from an earlier stage of the same glaciation or from a distinct older event. Altogether about 180 km 2 may have been glaciated. Cores dated by 14 C from inside and outside the glaciated area suggest that at least the north- ern valley glaciers may date from the Last Glacial Maximum. Estimated equilibrium line altitudes for these glaciers and the ice-cap are 3750–4230 m. Copyright ß 2005 John Wiley & Sons, Ltd. KEYWORDS: Ethiopia; Bale Mountains; Quaternary; glaciation; air photographs. Introduction Interest in evidence of past and current climatic change has resulted in greater attention being paid to the apparent paradox of tropical glaciation (Kaser and Osmaston, 2002). Only high tropical mountains over about 4000 m were glaciated in the Quaternary and only the highest of these still retain glaciers. However, the extents of these glaciations are often well pre- served in the form of terminal moraines left by small to moder- ate-sized valley glaciers and this is true of some of the Ethiopian mountains. They present certain advantages for study. From glacier extent, changes of equilibrium line altitude (ELA) of the glaciers can be inferred, and in turn the changes in climate which may have caused them (Nesje and Dahl, 2000; Benn et al., in press). In contrast, the extents in higher latitudes of former continental ice sheets and large ice caps are also influenced by other factors and may generate their own weather systems. Quaternary glaciation elsewhere in Africa Numerous mountain ranges in East Africa (Fig. 1) contain evi- dence of Quaternary glaciation, though few have glaciers now and these are rapidly disappearing. In North Africa, the Atlas and other groups of mountains have evidence of glaciation but, in the absence of any dating, it is unclear when this occurred (Hughes et al., 2004) and, though Messerli (1967) estimated relevant ELAs, the available data were unreliable and incomplete. In South Africa geomorphic features resem- bling moraines and protalus ramparts have been described from the Drakensberg Mountains, but it is still unclear and dis- puted whether these features really indicate glaciation or merely periglaciation (Grab, 1996; Hall, 2004). The best-studied sites are in East Africa (Hastenrath, 1984; Osmaston, 1989a,b, 2004; Mahaney, 1990; Kaser and Osmaston, 2002; Osmaston and Harrison, in press). The slopes of the three highest mountains, Kilimanjaro (5894 m), Kenya (5199 m) and the Rwenzori (5108 m), are scarred by deeply incised glaciated valleys, where there are huge mor- aines 100 m or more high, that are conspicuous both on the aerial photographs and on the ground (Fig. 1). These record evi- dence of three to five major glaciations, dating back on Kili- manjaro to an estimated 400k yr BP from tills intercalated * Correspondence to: H. A. Osmaston, Thwaite End, Finsthwaite, Ulverston, Cumbria LA12 8BN, UK. E-mail: osmaston@clara.net

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Page 1: Quaternary glaciation of the Bale Mountains, Ethiopia

JOURNAL OF QUATERNARY SCIENCE (2005) 20(6) 593–606Copyright � 2005 John Wiley & Sons, Ltd.Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jqs.931

Quaternary glaciation of the Bale Mountains,EthiopiaHENRY A. OSMASTON,1* WISHART A. MITCHELL2 and J. A. NIGEL OSMASTON3

1 School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK2 Department of Geography, University of Durham, Durham DH1 3LE, UK3 Mott Macdonald & Partners, Demeter House, Station Road, Cambridge CB1 2RS, UK

Osmaston, H. A., Mitchell, W. A. and Osmaston, J. A. N. 2005. Quaternary glaciation of the Bale Mountains, Ethiopia. J. Quaternary Sci., Vol. 20 pp. 593–606.ISSN 0267-8179.

Received 4 February 2004; Revised 6 March 2005; Accepted 10 March 2005

ABSTRACT: Central Ethiopia comprises a high plateau at 2000–3000 m, formed from Tertiary lavaflows and bisected by the Eastern African Rift. Ten volcanic mountains rise to altitudes of just over4000 m, but on only three has Quaternary glaciation been substantiated by published field observa-tions. On the Bale Mountains (4400 m), a previous report based on limited evidence proposed anice-cap extending to 600 km2. Based on aerial photographs and ground surveys, this paper reportsevidence of a more complex situation. A wide spread of large erratic boulders on the plateau recordsa central ice cap of 30 km2, though ice probably extended for a further 40 km2. Further north twogroups of deeply incised and clearly glaciated valleys contain moraines and roches moutonnees(60 km2). On interfluves between them and on the open north slopes are moraines from an earlierstage of the same glaciation or from a distinct older event. Altogether about 180 km2 may have beenglaciated. Cores dated by 14C from inside and outside the glaciated area suggest that at least the north-ern valley glaciers may date from the Last Glacial Maximum. Estimated equilibrium line altitudes forthese glaciers and the ice-cap are 3750–4230 m. Copyright � 2005 John Wiley & Sons, Ltd.

KEYWORDS: Ethiopia; Bale Mountains; Quaternary; glaciation; air photographs.

Introduction

Interest in evidence of past and current climatic change hasresulted in greater attention being paid to the apparent paradoxof tropical glaciation (Kaser and Osmaston, 2002). Only hightropical mountains over about 4000 m were glaciated in theQuaternary and only the highest of these still retain glaciers.However, the extents of these glaciations are often well pre-served in the form of terminal moraines left by small to moder-ate-sized valley glaciers and this is true of some of the Ethiopianmountains. They present certain advantages for study. Fromglacier extent, changes of equilibrium line altitude (ELA) ofthe glaciers can be inferred, and in turn the changes in climatewhich may have caused them (Nesje and Dahl, 2000;Benn et al., in press). In contrast, the extents in higher latitudesof former continental ice sheets and large ice caps are alsoinfluenced by other factors and may generate their ownweather systems.

Quaternary glaciation elsewhere in Africa

Numerous mountain ranges in East Africa (Fig. 1) contain evi-dence of Quaternary glaciation, though few have glaciers nowand these are rapidly disappearing. In North Africa, the Atlasand other groups of mountains have evidence of glaciationbut, in the absence of any dating, it is unclear when thisoccurred (Hughes et al., 2004) and, though Messerli (1967)estimated relevant ELAs, the available data were unreliableand incomplete. In South Africa geomorphic features resem-bling moraines and protalus ramparts have been describedfrom the Drakensberg Mountains, but it is still unclear and dis-puted whether these features really indicate glaciation ormerely periglaciation (Grab, 1996; Hall, 2004).

The best-studied sites are in East Africa (Hastenrath, 1984;Osmaston, 1989a,b, 2004; Mahaney, 1990; Kaser andOsmaston, 2002; Osmaston and Harrison, in press). Theslopes of the three highest mountains, Kilimanjaro (5894 m),Kenya (5199 m) and the Rwenzori (5108 m), are scarred bydeeply incised glaciated valleys, where there are huge mor-aines 100 m or more high, that are conspicuous both on theaerial photographs and on the ground (Fig. 1). These record evi-dence of three to five major glaciations, dating back on Kili-manjaro to an estimated 400k yr BP from tills intercalated

* Correspondence to: H. A. Osmaston, Thwaite End, Finsthwaite, Ulverston,Cumbria LA12 8BN, UK. E-mail: [email protected]

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between dated lavas (Downie, 1964). On each a younger andmore complete set of moraines probably dates from the LGM;two other mountains, Elgon (4320 m) and Aberdare (4001 m),have similar moraine systems that are probably of LGM age.Until recently the most reliable evidence for an LGM agewas on the Rwenzori where there is a date of >16 000 14C yrBP (not 14 750 as often misquoted) from the base of an inter-morainal lake (Livingstone, 1967), showing that glacier retreatmust have started shortly before this. On the three major peaksthere are also small moraines left by advances since the LGM.

On Mt. Kenya and Kilimanjaro, cosmogenic dates on mor-aine boulders (Shanahan and Zreda, 2000) confirm previousideas about moraine ages on Kilimanjaro but conflict withthose on Mt. Kenya. The latter are disputed (Mahaney, 2004)and we await further and more rigorous sampling to resolvethe matter.

Quaternary glaciation in Ethiopia:previous work

Central Ethiopia comprises a high plateau at 2000–3000 m,formed from successive lava flows and bisected by the EasternAfrican Rift. Ten volcanic mountains rise to altitudes of justover 4000 m (Fig. 1) but the evidence of glaciation herepresents fascinating differences, new problems and challengescompared with East African Mountains. Though reports of var-ious workers are not in complete agreement, it appears that five

of these mountains have been glaciated; only on three hasthis been substantiated by published field observations(Umer et al., 2004) and only on two is there evidence of a prob-able LGM age. On none of these three Ethiopian mountains isthere clear evidence for a lesser, later advance of the glaciersthan the main one, as there is in East Africa, but on all threethere is slight enigmatic evidence suggesting that there mayhave been an earlier and more extensive one.

The Simen (¼ Simien, Semyen)* Highlands to the north ofthe rift, are the highest mountains (4543 m), consisting of a ringof residual peaks around a deeply eroded central crater. Thepresence of small cirques, moraines, and glacial striationsdemonstrate the former existence of a number of small Qua-ternary glaciers, surveyed and mapped with great detail andprecision by Hurni (1982, 1989). Though evident on theground, many moraines have little relief and are difficult todetect on the available aerial photographs. The total glaciatedarea spread around on several different peaks is estimated tohave been about 13 km2, with ELAs of about 4100 m on thenorth and 4400 m on the south.

In central Ethiopia, on the Arsi (¼Arussi, Galama) Moun-tains, Mt. Bada (4210 m) has a complete array of large lateralmoraines at 3200–3700 m left by a series of valley glaciers des-cending east and west of the long north–south main ridge,which are conspicuous both on air photographs and on theground. The glaciated area is estimated to be 85 km2 (Potter,1976; Street, 1979a,b; but both reported a larger, incorrect,area due to scale error) with ELAs of 3700 m on the east and3900 m on the west. Moraines are reported less authoritativelyon Mts Kecha (¼Kaka), Enguolo (¼ Filfo) and Chilalo from aer-ial photographs (cf. Hastenrath, 1977).

In southern Ethiopia, the Bale Mountains reach a maximumelevation of 4400 m in Tullu Dimtu, the highest summit on theSanetti Plateau (Fig. 2). Smeds (1959) was the first to recordevidence of glaciation in the Togona valley, a major north-draining valley from the plateau followed by Mooney(1963), a forester who reported moraines on Mt. Batu(4370 m) and also at 3350 m in the steep gorge above Rirabelow the Harenna Escarpment (Fig. 2). Messerli et al.(1977) on the basis of limited reconnaissance inferred thatthe Bale Mountains had been covered by an ice cap of600 km2 with valley glaciers descending from it and an ELAat about 3700 m (cf. Messerli and Winiger, 1992). Despite thisremarkable claim in a single-page abstract proposing that thisice cap exceeded in extent those of the higher East Africanmountains, no detailed geomorphological studies have beenundertaken, apart from brief reports on the glacial landformsin a major ecological text (Miehe and Miehe, 1994) and peri-glacial features (Grab, 2002).

Methodology

In lowland Britain and similar countries, the surface expressionof Quaternary features is often subdued and their recognition

*Map names and altitudes. Transliteration of the Ethiopian to the Roman alphabetis not yet standardised, and there is considerable variation in names on maps andbetween them and current local usage. Spot altitudes on maps also show discre-pancies of a few tens of metres from our GPS data. The geoid/ellipsoid correctionfor GPS altitudes on the Bale Mountains requires the addition of 14 m to the GPSaltitude.

The highest peak is incorrectly named Dimtu Guda (Bata) on the current Ethio-pian Mapping Authority 1:50 000 map (0639 B2, ed. 1EMA 2000). The altitude isshown as just over 4380 m by the contours and as 4377 m according to othersources, but careful GPS observations by the authors indicate that the altitudeis 4400�10 m, plus the local ellipsoid/geoid correction of 14 m. Thus we take4400 m as a minimum estimate. The road to the summit is wrongly shown onthe 1:50 000 map (EMA, 2000), but correctly on our Fig. 10.

Figure 1 Map showing East African and Ethiopian mountains wherethere is geomorphological evidence for former glaciers during theLGM (Last Glacial Maximum) and other mountains at similar altitudeslacking such evidence (after Osmaston, 2004)

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on aerial photographs masked by human changes to the land-scape and vegetation, so studies concentrate on the detailedsedimentary analysis of sections exposed naturally (cliffs), inci-dentally (road cuttings, quarries) or deliberately (pits, cores).On large tropical and subtropical mountains the converseapplies. Landscape-forming processes have been vigorousand features such as terraces and moraines are commonly lar-ger, more conspicuous and more reliably identifiable both fromthe air and on the ground than in Britain and are of ‘textbook’form. The landscapes and vegetation have not been, or at leasthave only partially been, modified by human intervention (e.g.fire), so the ecology of the natural vegetation often emphasisesthe landforms.

These factors are complemented by logistic ones. Where aremote area of several hundred or thousand square kilometresof steep, high (>3000 m), mountainous country is being inves-tigated, with access depending on several days of difficultwalking and camping and where little or no prior researchhas been done, aerial photographs are of prime importancein recognising the distribution of significant types of features,mapping them and identifying key areas to visit. An efficientfield trip can then be planned and executed to provide groundchecks on these features. Such checks will be primarily visualnot sedimentological; their interpretation will depend onexperience (though quantitative sedimentological analysismay appear more ‘objective’ the rationale of its interpretationoften has a similar dependence on experience) and their justi-fication to the wider scientific community will depend on mor-phological description and above all on publication ofphotographs, taken from both the air and the ground.

The authors have had experience of similar work for manydecades in East Africa and the Himalaya. This particular inves-

tigation was preceded by detailed study of the aerial photo-graphs for 2 years, frequent use in the field in 2003, and againrepeatedly afterwards. We submit that the features describedand illustrated below cannot be other than glacial in origin.The northern valleys conform to many others worldwide withan appropriate suite of associated features such as valley-floormoraines and roches moutonnees, demonstrating a glacialorigin. On the plateau we can envisage no other process, non-volcanic and non-meteoric, which will strew very large erraticboulders over a wide near-horizontal summit surface.

The north slope moraines conform closely both on aerialphotographs and in the field to accepted classic moraines else-where, though their positions on interfluves present problemsin interpretation. Floods may also form long, curvilinear,boulder ridges, such as are the subject of intense controversyover the possible glaciation of the Drakensberg, South Africa(Grab, 1996; Lewis and Illgner 2001; Hall, 2004), but theseoccupy valleys, gullies or outwash fans. Landslides can alsobe confused with moraines but a source area, lacking for theproposed moraines here, is essential and we had no difficultyin discriminating one large landslide both on air photos and theground.

Geology and general geomorphologyof the Bale Mountains

The Bale Mountains consist of a plateau formed of horizontallybedded late Neogene basaltic and trachytic lava flows(Gobena et al., 1996, 1998) The uppermost part of this

Figure 2 Map of the Bale Mountains, Ethiopia, showing the locations of the glacial and periglacial features described in the text

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low-relief surface is the Sanetti Plateau covering about600 km2 at about 4000 m altitude (Fig. 2). To the north andnortheast are deeply dissected valleys descending the northernslope, while to the west lava flows form spectacular bluffsabove the Web valley (3800 m; Fig. 2). The southern edge ofthe plateau is defined by the actively back-cutting HarennaEscarpment. Across the plateau are numerous volcanic plugsand cinder cones of Tertiary and Quaternary age (none ofthese ages are radiometrically determined). The most impor-tant of these form the highest summits of the area: Tullu Dimtuitself (4400 m; marked Dimtu Guda on the EMA 1:50 000 map)which is a cone of mixed cindery ejecta and closely jointedlava, while a very slightly lower peak 1 km to the east is

composed of more massive trachytic lava (Fig. 3). The similar-ity of many of the rocks and the lack of detailed mapping andlithological information make it difficult to identify the prove-nance of boulders in the moraines.

Quaternary glaciation on theBale Mountains—overview

The evidence of glaciation on the Bale Mountains presentsmajor differences from the glaciated East African mountains

Figure 3 Sanetti Plateau and the summits of Tullu Dimtu (4400 m) and un-named second peak to the right. The vegetation is dominated by Heli-chrysum bushes and giant Lobelia. This figure is available in colour online at www.interscience.wiley.com/journal/jqs

Figure 4 Togona valley looking south towards upper plateau showing glaciated valley morphology with truncated spurs. The remarkable rochemoutonnee is on the east (left) valley side in mid-distance. This figure is available in colour online at www.interscience.wiley.com/journal/jqs

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and from other Ethiopian ones. However, there is clearevidence to propose that this area was glaciated and probablyon more than one occasion. Lateral moraine ridges can beidentified at a number of locations (Fig. 2), but since they lieon open slopes and at the edges of interfluves there is doubtwhether they were formed by the same event which producedthe nearby deeply incised valleys.

The major rivers flowing north from the Sanetti Plateauoccupy valleys with broad bottoms and steep side-slopes, asillustrated by the Togona (Fig. 4) whose cross-profile (Fig. 5)clearly shows a valley form with possible truncated spurs(Fig. 4) that can be best explained as having been formed byglacier erosion. Examination of the aerial photographsalso shows that there is a clear down-valley limit to this glacialvalley profile below which the river occupies a narrow V-shapegorge (Fig. 6); this transition is interpreted as the down-valleylimit of the former valley glaciers.

The Sanetti plateau presents a completely different type ofrelief and geomorphic evidence of glaciation. Around the sum-mit peak of Tullu Dimtu there is a wide and variable scatter oflarge angular boulders (Fig. 7); these are conspicuous on theground but scarcely visible and impossible to delimit on theaerial photographs. These are interpreted as glacial erratics

composed of trachyte overlying basalt which forms most ofthe plateau surface and can be traced across the higher partof the plateau, centred on the summit peaks of Tullu Dimtuand particularly the un-named peak (Fig. 2).

Conversely a disrupted drainage pattern, both within andexternal to the bouldery area, is inconspicuous on the groundbut clearly evident on the aerial photographs as a strong indi-cation of glaciation. Negative evidence is also important, suchas the presence of volcanic plugs and lava flows on the plateauwhich have clearly not been glaciated (Fig. 8) and so constrainthe area of possible glaciation.

These observations show that the earlier conclusion ofMesserli et al. (1977) was both an overestimate and anoversimplification. On the one hand the possible extent ofice is more limited because of the presence of various ungla-ciated features. On the other hand there is a variety of glacialfeatures which seem to suggest variable styles of ice cover ofpossibly different ages. Since their temporal relationships arenot clear, we describe assemblages of similar features in parti-cular regions as units, a noncommittal term which must sufficeuntil radiometric dates can be obtained. We have identified anumber of distinct glacial geomorphological units across theplateau and the northern slopes.

Figure 5 Section across the Togona and Kaficha valleys through the moraines of the Northern Slopes Unit (Fig. 2), and marked M on Fig. 6. Note howthe moraines of the Northern Slopes Unit are elevated above those of the Togona Valley Unit. (Vertical exaggeration�10)

Figure 6 Aerial photo of the Togona and Kaficha valleys (No. VM1370 R.351 38133, 11 Dec 1967). The road which was made after the date of thesephotos has been transferred from the map and the 1984 photos. LS¼ landslide; L¼ lower limit of glaciated valley; M1, M2¼moraines of NorthernSlopes Unit (field checked); M?¼probably moraines of Northern Slopes Unit

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The Tullu Dimtu Unit

The scatter of erratics around the main peak of Tullu Dimtu hasbeen defined as the Tullu Dimtu Big Boulder Moraine (BBM) byMiehe and Miehe (1994). It should, however, be noted that theBig Boulder Moraines described by Lehmkuhl et al. (2002) inTibet are different in both form and genesis. Around TulluDimtu, this moraine comprises a spread of large massive, angu-

lar, lava boulders up to 7 m diameter and some low ridges of till(Fig. 9), occupying an area of about 30 km2 within a radius of3–4 km centred on Tullu Dimtu peak and nearby peak (Fig. 10).Table 1 gives the size counts of two randomly sited sampleplots of boulders at point B (Fig. 10). Around much of the pla-teau the erratics form a well-marked boulder limit, possiblydelimiting the former extent of an ice cap (Fig. 11).

The BBM boulders mostly lie on a surface of soil-coveredground without evident rock outcrops. They do not appear to

Figure 7 Tullu Dimtu peak and erratic boulders at the eastern edge of the Big Boulder Moraine, looking west about 3.5 km east of Tullu Dimtu. Thisshows their characteristic distribution, sometimes singly, sometimes in groups. A particular concentration of moraine boulders is just visible on thecrest of the ridge in the right middle distance. Taken from Point C, Fig. 10. This figure is available in colour online at www.interscience.wiley.com/journal/jqs

Figure 8 Aerial photograph of Wele volcanic plug, on southeast Sanetti Plateau (No. VM1370, R373 40675 17 Dec 1967). This is an example of theevidently unglaciated volcanic features which constrain the limits of the ice cap. Radial jointing is clearly visible

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have been long exposed to weathering, judging by the lack offractured weathering debris around their bases. When takingsamples, a thin weathering rind, only a few millimetres thick,was the only evidence of weathering. In contrast, apparently

older boulders from trachytic lava flows in the Web valleyappear to have been greatly attacked by solutional weatheringand one even had well-developed rillen-karren. In some placeson the plateau the boulders are so closely spaced that theyform small groups touching each other; elsewhere there areboulder-free spaces hundreds of metres across. There are someconcentrations of boulders on ridges and at the outer limit ofthe bouldery area, e.g. at its eastern limit, and near the roadto the summit they are locally constructed into a ridge.

This eastern part of the generally level Sanetti Plateau has anegligible east–west slope but slopes down slightly over a dis-tance of 20 km from 4100 m in the north to 3850 m in the south

Figure 9 Big Boulder Moraine (BBM) looking east from the lower slopes of the summit of Tullu Dimtu. Taken from Point B on Fig. 10. This figure isavailable in colour online at www.interscience.wiley.com/journal/jqs

Figure 10 Geomorphological map of the Sanetti Plateau, Bale Mountains, Ethiopia, showing the area covered by the Tullu Dimtu Big Boulder Mor-aine, and possible limits of glaciation where there is evidence that it extended beyond this moraine. This limit is uncertain along the edge of theescarpment to the southwest

Table 1 Size counts of boulders in Big Boulder Moraine

Max. diameter (m) 1–3 3–5 5–7 >7 Total Density

Plot 1 (0.50 ha) 10 5 1 2 18 36 ha�1

Plot 2 (0.25 ha) 12 3 0 1 15 60 ha�1

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to the edge of the Harenna Escarpment. To the east of TulluDimtu, the plateau has a disrupted drainage pattern withnumerous small lakes, while to the southeast is a series of lowtill ridges running north–south for up to 3 km, spaced at about100–200 m, and 5–10 m high. The ridges and the resulting drai-nage pattern of north–south streams between them are clearlyvisible on aerial photographs and on the 1:50 000 map (Fig. 10).

On the crests of two of these ridges crossing the road 3 kmeast of Tullu Dimtu, there is a dense alignment of boulders run-ning along their crests for at least 1 km, as well as a narrowband of boulders up to 4 m diameter crossing the ridges andintervening depressions perpendicularly in an east–westalignment. Although the general ground slope here is to thesouth, it must be remembered that the direction of ice flow isdetemined by surface ice slope. If there was a roughly symme-trical ice dome with radial flow from Tullu Dimtu and the sec-ondary peak, an easterly ice flow would have prevailed herewith the ridge lines transverse to it. However, it is not clearwhether these moraines were formed subglacially or at aretreating ice margin. Similarly this effect could have causedthe roughly circular distribution of big boulders; it is likely thatthey were entrained and deposited subglacially like some of theerratic trains left by the British ice sheets.

We infer that these erratic boulders and associated featuresmark a former ice cap centred on Tullu Dimtu but it is doubtfulwhether the limits of the boulders mark the outer limits of theice. The extent of the boulders is clear to the north, east andsoutheast (C in Fig. 10) but to the northwest and west thereare only occasional groups of boulders. In most of the areato the southwest and south of Tullu Dimtu there are noboulders, but here the ice would have flowed from the scoriac-eous and closely jointed Tullu Dimtu itself so carrying none.There are some periglacial blockfields here of unknown agebut the presence of an isolated group of large boulders beyondDimtu Tika (4130 m) on the edge of the escarpment by the Sidastream suggests that this area without boulders should beincluded in the BBM Unit To the southeast the boundarybetween the barren and the boulder-laden ice is clearly visiblewhere it crosses the road to the summit (marked B on Fig. 10).Just uphill from this point is a small moraine below the road,composed mostly of material less than 30 cm diameter butwith a few larger boulders; if there was a major ice cap, thismust postdate it.

Plateau areas possibly glaciated outsidethe Big Boulder Moraine Unit

The eastern half of the plateau is conspicuously differentiatedfrom the west by the presence of over 50 small shallow lakes,mostly without outflows, and the tortuous courses of the upperMireta and Gusa streams indeed appear to avoid them. In min-iature this resembles in appearance the lake-dotted surface ofthe glacially scoured Canadian Shield; some lakes seem to bein shallow rock basins, others may be dammed by till or volca-nic ash of which occasional deposits can be seen. Some liewithin the area of the BBM and are compatible with an originfrom irregularities in glacial erosion and deposition on an initi-ally nearly level surface. Along the northern limit of the BBM(GR 9350 to 9650) there is a conspicuous alignment of lakes,which suggests that the previous southerly drainage from thearea north of them has been blocked by deposition associatedwith the BBM (Fig. 10).

Other lakes lie outside BBM to the northwest on ground ofuneven relief, which may indicate a greater extent of ice in thatdirection at the same or an earlier time (Fig. 10). To the north-east and separated by a lake-free strip from the BBM areanother 25 lakes, aligned in a crescent along the slightly higherridge which runs east–west just south of the headwaters of theTogona valley and west of Badegesa hill. These are set indepressions between rocky hills, but do not appear to be con-sistent with a volcanic origin so their formation presents a pro-blem. However, a small isolated group of apparently erraticboulders, including one very large boulder (Fig. 12, markedX on Fig. 10), lies at about GR 596000 758000, 4 km northeastbeyond the BBM and just beyond the second belt of lakes; theremay also be others.

Limits of possible glaciation on the plateau

Apart from the apparently glaciated areas described abovethere are certain features, clearly unglaciated, which pro-vide negative evidence and ultimate constraints to the possibleextent of ice on the plateau. The maximum extent to the

Figure 11 Boulder limit to the north of Tullu Dimtu looking southwestwards across the Sanetti Plateau. The boulders cover the surface of a thin tillcover and can been clearly distinguished from the lower boulder-free ground to the west (right). This figure is available in colour online at www.interscience.wiley.com/journal/jqs

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southeast is unequivocally marked by the upstanding and evi-dently unglaciated volcanic plug of Wele (Fig. 8). Though it hasnot been dated, the time needed to remove all the slopes of vol-canic ejecta which must once have both surrounded it andreached a greater height is clearly greater than at least the per-iod since the LGM. The upstanding plugs of Tulu Sato and KaraTuluke mark a likely southern limit, especially as at their basesthere are important and unique features, Trenched BoulderSlopes (TBS), apparently of periglacial origin (Fig. 10) (to bereported in a separate paper). This inference is supported byevidence of periglacial conditions on Simen, probably at thetime of the last glaciation, indicating dry and cold conditions(Williams et al., 1978; Hurni,1982).

To the southwest it is likely that the ice reached the edge ofthe Harenna Escarpment. However, glacial evidence on suchan actively eroding scarp might be short-lived and a reconnais-sance down the escarpment disclosed no geomorphologicalevidence to suggest a further extension of ice, althoughMooney (1963) reported a moraine at 3350 m in a steep valleyabove Rira. The smooth surface and well-integrated dendriticdrainage patterns on the central and western Sanetti plateau,and the lava flows between that and the Web valley whichhave evidently not been glaciated, exclude the possibility thatthe ice cap extended there, though this still needs to be con-firmed by radiometric dates on the lavas to show that theyare at least older than the LGM. Including the area of the

Figure 12 An isolated group of erratic boulders including one very large one. This lies on the high plateau south of Garba Guracha at aboutGR596000 758000, about 4 km further north than the main spread of the Big Boulder Moraine and its associated lakes, and also beyond the secondbelt of lakes and rocky hills. Either the extent of the Tullu Dimtu Big Boulder Moraine Unit was more extensive than the rest of the evidence indicates,or this is a relic of an earlier extensive glacial stage. Taken from Point X, Fig. 10, looking north. This figure is available in colour online at www.interscience.wiley.com/journal/jqs

Figure 13 The remarkable roche moutonnee, middle Togona valley. This outcrops half-way up the southern side of the valley. This view is lookingdown-valley. In the foreground is a smooth glaciated surface littered with erratic boulders. This figure is available in colour online at www.interscience.wiley.com/journal/jqs

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BBM, these features constrain the maximum likely area ofglaciation on the plateau to about 70 km2.

The Togona Valley Unit

Ground observations by both Messerli et al. (1977) and theauthors have established that a valley glacier about 10 km longoccupied the Togona valley down to 3400 m. The markedtopographical change at the lower end of this glaciated valleywhich is clearly distinguishable both in the field and on aerialphotographs (Fig. 6) was not visited on foot but was carefullyexamined both on aerial photographs and from the northernlimit of the distinct lateral moraines that occur just at the valleyedge (M on Fig. 6). No sign could be seen of any substantialterminal moraines extending beyond the glaciated upper val-ley, a feature that seems to be characteristic of all the glaciatednorthern valleys.

The former presence of ice in the Togona valley is clearlyexpressed by a remarkable roche moutonnee on the south sideof the valley (Fig. 13). This clear evidence of glacier erosionconfirms that these valleys were once occupied by valleyglaciers. Just up-valley of this roche moutonnee, there are anumber of small moraine ridges in the valley floor suggestiveof ice retreat. In the cirque-like head of a tributary in thecatchment area, there is a lake, Garba Guracha (3945 m),which is dammed by a moraine which must postdate the mainglaciation of the valley (Fig. 14). The geological map (Gobenaet al., 1998) shows this valley head as a breached and erodedcrater, but we were unable to confirm this in the field.

In the central section of the valley there is evidence of a largelandslide (ca. 0.1 km3) from the northwest side, clearly visibleon aerial photographs (Fig. 6). Boulders associated with thisrock slope failure have stretched across the valley floor and lefta scar backed by precipices with a width of only tens of metresin the level-topped interfluve between the Togona and Kafichavalleys. The debris forms a series of ridges at the foot of theslope and across the valley floor clearly indicating a sourcefrom the valley slope. Their orientation, the presence of a back

scarp and the fact that they are all of the same lithology sup-ports their explanation as landslide rather than moraine. Thisrock slope failure clearly impounded the river, forming a lakethan has since infilled into swamp deflecting the river to theeast side of the valley (Fig. 15) and must postdate the last glacialevent in this part of the Togona valley.

The Kaficha valley runs parallel with the Togona valley, veryclose to it on the northwest side. Its form on aerial photographs(Fig. 6) closely resembles that of the Togona suggesting that ithas clearly been similarly glaciated. The catchments of thesevalley glaciers are not sharply defined as they tend to mergewith the main plateau. However, there is a line of steep groundat about 4100 m around the heads of the Togona and Kafichavalleys which may mark the southern upper limit of theiraccumulation areas. This runs round to the west to meet theMt. Batu–Buyemo ridge (4370 m), scarcely lower than TulluDimtu. These two valleys together with the inferred catchmentarea above them comprise the Togona Valley Unit, ca. 30 km2.

On the interfluves on either side of these valleys are mor-aines which must have been deposited before these valleyswere eroded. Since it is not clear whether these were formedat the initial stages of the valley glaciers or in a separate earlierglaciation they are provisionally grouped with other similarones as a separate Northern Slopes Unit.

The Wasama Valleys Unit

Further west is a group of five valleys rising on Wasama,Wergona and adjoining hills (4200–4300 m) whose upper sec-tions appear on aerial photographs to have been glaciated, andfor which there is some independent confirmation from groundobservations. According to G. Miehe (pers. comm., 2004) atleast one moraine can be identified, suggesting glaciation inthe Wasama valley. There is also evidence from the small Bativalley in the headwaters of the Danka River valley suggestingglaciation (A. Street, unpublished). We therefore define this asthe Wasama Valleys Unit and include it within the glaciatedarea, proposing contemporaneity with the Togona Valley Unit(ca. 30 km2).

Figure 14 Garba Guracha, upper Togona valley, looking northwards. This lake lies at the head of a small tributary in a well-developed glacial cirque.It is dammed by a terminal moraine and cut by the outflow stream which then descends into the main valley. A 16-m core from this lake is beinganalysed by M. Umer. This figure is available in colour online at www.interscience.wiley.com/journal/jqs

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The Northern Slopes Unit

On the northern slopes of Bale, outside the glaciated valleysdescribed above, the evidence of glaciation is definite, butenigmatic. A key site is a moraine on the open slopes, 2 km eastof the incised Togona valley and close to the road, which isassociated with a scatter of erratic boulders of various litholo-gies on the intervening ground and possibly lower down theslope (Fig. 2). We identified, examined and sampled this mor-aine in the field. It is not large, about 30 m across, 5 m high and200 m or more long, but the surface has many upstanding erra-tic boulders up to 2 m in diameter. It is covered in shrubbyheather and only just distinguishable on aerial photographs.We did not have time to explore the area further on the groundand were unable to see on the aerial photographs any further

lateral moraines of this type. Clearly it was formed by ice thatwas different from that which cut the deep valleys; it seems thatthese lateral moraines are more likely associated with outletglaciers from a former plateau ice cap.

This problem is accentuated by a group of three similar butlarger lateral moraine ridges, closely parallel but over 1 kmlong, which also lie on the open slope but at the very edgeof the steep southeast side of the Togona valley and are invisi-ble from the valley bottom (Fig. 2). Large erratic boulders ofvarious igneous rocks, quite different from the bedrockexposed at the valley edge, can be observed along the moraineridges (Fig. 16). which are clearly identifiable on aerial photo-graphs and on the ground, and are easily accessible from theroad. There is also an expanse of apparently ice-smoothed rockwith erratic boulders scattered on its surface. Looking north-westwards across the valley there appear to be corresponding

Figure 16 View northwest over the Togona valley showing a well-defined lateral moraine ridge (M1) associated with the Northern Slopes Unit. Thisfigure is available in colour online at www.interscience.wiley.com/journal/jqs

Figure 15 Togona valley rock slope failure. The debris from this landslide occupies the valley floor in the mid-distance. It dammed the valley whichhas since infilled into a swamp forming the light tones within the valley floor. This figure is available in colour online at www.interscience.wiley.com/journal/jqs

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moraines on the interfluve between the Togona and Kafichavalleys. These moraines are grouped into a Northern SlopesUnit.

The close relationship of these moraines to the Togona valleysuggests that they are related to a glacier which occupied thatvalley. However, they lie at 3800 m, some 250 m above thevalley floor, only 3 km from the glacier terminus. It is unlikelythat the glacier was that thick at this point so it seems that thesemoraines were deposited before the valley was substantiallyincised. In that case, there could be a considerable differencebetween their ages and the smaller recessional moraine ridgesmapped within the valley.

Similar moraines on an interfluve between the Denka andWasama valleys (GR 583769, altitude 3900 m) were identifiedby Street (pers. comm., 2004) on the aerial photographs andconfirmed by us, though it was not possible for either party tocheck them on the ground.

Between the Togona and Wasama Units, the Shayia Tika andShayia Guda rivers run either side of a broad valley, with steepsides and a flat floor 1 km wide, down to about 3400 m, belowwhich the rivers join in a single narrow fluvially eroded gorge.The vegetation patterns on aerial photographs suggest that thebroad valley floor is covered with till or outwash not solid rock,but the marginal rivers are incised deeply below it, unlike riversin the glaciated valleys described above, suggesting that a longertime has elapsed since the valley floor was formed. We wereunable to visit this valley but these features suggest that it toowas glaciated, possibly at an earlier time than the other valleys,so provisionally it is grouped with the high interfluve moraines.

Together all these features represent the Northern SlopesUnit and are interpreted as being associated with a plateauice cap extending down the northern slopes. This occurred atan earlier date than the Togona and Wasama Units, before thevalleys were incised, and prompts comparison with old inter-fluve moraines on the major East African mountains, whichalso appear to represent an ice cap type of glaciation (Kaserand Osmaston, 2002). Further fieldwork is required beforeany reliable estimate can be made of its ELA or extent (whichoverlapped other units but probably covered at least 50 km2 inaddition), though analysis of the boulder samples already takenwill give an indication of its age relative to the valley glaciersand the BBM.

The ages of the glaciation Units

The only local radiometric ages available are as follows:

(a) A 16-m core from Garba Guracha lake in 6 m depth ofwater is currently being analysed but a preliminary 14C datefrom near the base has given a date of 13 950 14C yr BP(Umer and Palmer, unpublished). This lake is dammed bya terminal moraine which is thought to record recessionfrom the main period of glacier development within the val-ley; it therefore indicates the latest possible time for the val-ley to have become ice-free.

(b) Within the glaciated part of the Danka valley, a 2.4-m corefrom a bog at 3830 m gave a base dated at 7500 14C yr BP(Hamilton, 1982, Bonneville and Hamilton, 1986), provid-ing a minimum age for deglaciation here.

(c) A 2.6-m core taken from a swamp at ca. 4000 m near theeastern edge of the plateau just north of Badegesa Hill, out-side the area of the BBM, has given a basal date of15 600� 255 cal. yr BP at 2.55 m (W. Zech, unpublished),i.e. ca. 13 000 14C yr BP. This core, which may be just out-

side the limit of the extent of the ice on the east of the pla-teau, should yield important information.

(d) There is a basal date of ca. 14 000 14C yr BP for the base of ashort pollen core 1.8 m long in a swamp at Tamsaa, about39 � 43’ E 7 � 07’ N, at 3000 m. This is 10 km outside and400 m altitude below the lowest glaciation in the TogonaValley. However, there is pollen of high-altitude vegetationat the base (Umer and Bonnefille, 1998) and they suggestedthat deglacierisation started at about this time.

Together these suggest that some of the glaciated featuresmay date from the LGM, which accords with evidence on otherEthiopian mountains. On Bada a date of 11 500 14C yr BP wasobtained at the base of a core in a glacial cirque (Street1979a,b; Hamilton, 1982). On Simen there are only some datesof about 4100 14C yr BP on peat inside the moraines (Hurni,1982, 1989). An LGM age would also conform to that of mor-aines on the East African mountains (Osmaston, 2004).

The ages of the different glaciation units will only be clarifiedby the application cosmogenic radionuclide dating, which hasproved successful in dating moraines (see Cockburn and Sum-merfield, 2004). Rock samples for this purpose were collectedfrom the features described above to provide critical informa-tion for discriminating between the hypotheses discussedabove; these now await funding and processing.

Equilibrium Line Altitudes

The lowest point at which we found the BBM was by the SidaRiver at 3860 m near the edge of the Herenna escarpment; butit is possible that ice may have descended further. On the west,north and east sides on the main plateau the lowest limit is atca. 4100 m.

With the very low plateau gradient and probably very coldconditions, it is likely that a considerable thickness of icewould have built up on the main peaks, but taking this conser-vatively as only 100 m would give a top altitude of 4500 m.There is not sufficient evidence to plot contours of the icesurface, so neither area ratio nor balance ratio methods of esti-mation can be used. However, for a conical or domed ice cap,which has a large lower extent relative to its upper extent, theappropriate value for the THAR (toe-headwall-altitude-ratio)

Table 2 Comparative summary of glaciated mountains in Ethiopiaand East Africa

Bale Mountains Summit Glaciated ELA (m)altitude (m) area (at LGM?)

1. Big Boulder Moraine Unit 30 km2 4070–42302. Uncertain areas bordering 40 km2

BBM3. Togona Valley Unit 30 km2 3750–39004. Wasama Valleys Unit 30 km2 40005. Northern Slopes Unit <50 km2

(additional to 3 and 4)Possible total for Bale 4400 <180 km2 3750–4230MountainsSimen Mountains 4543 13 km2 4100–4400Bada Mountains 4210 85 km2 3700–3900Kilimanjaro 5895 160 km2 4250–5200Mt. Kenya 5199 230 km2 4100–4500Rwenzori Mountains 5108 500 km2 3500–4250

Sources: Kaser and Osmaston, 2002; Osmaston, 2004; Umer et al.,2004.

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method is about 0.33 (Osmaston, 1975); this would indicate apossible ELA of about 4070 m on the south slope, rising toabout 4230 m on the other aspects (cf. Benn et al., in press).

In the northeast valleys, the lowest limits of glaciation are atca. 3400 m and highest point in their accumulation areas is Mt.Batu (4370 m). Using a THAR of 0.5 appropriate for valley gla-ciers, this indicates a maximum ELA of about 3900 m, thoughmost of the headwalls were lower than Batu at 4100 m whichwould suggest a lower ELA of about 3750 m. In the northwestvalleys the headwalls are about 4350 m with termini at 3600–3800 m, indicating an ELA at about 4000 m. In view of thesmall vertical extent of the glaciers the estimates of their ELAscan have only small errors and they are probably correctto� 100 m. These estimates are roughly in accord with thosefor other Ethiopian mountains (Table 2). This table showsclearly that the practice of some authors of giving a single alti-tude for the ELA on a given mountain is misleading, particularlyon a large mountain where there are big differences in the ELAof glaciers on different regions or aspects of it.

Conclusions

The Bale Mountains show clear evidence for a former glaciercover with a spatial extent of about 180 km2. This took the formof a plateau ice cap, centred over the high summits of TulluDimtu, which was probably separate from deeply incised val-ley glaciers confined within the northern valleys, such as theTogona. Estimates of the ELA range from 3750 m to 4230 mfor different units and aspects (Table 2). In addition there is evi-dence from lateral moraines on the northern slopes for an ear-lier period of glaciation covering up to a further 90 km2.

Radiocarbon dating shows that various key sites werefree from ice by 14 000–7500 14C yr BP. These dates wouldbe compatible with an ice cover at the LGM or earlier. Cosmo-genic dating of samples already obtained will elucidate someof these uncertainties, but further field investigation is alsorequired.

Since there are no existing glaciers on any of the mountains,it is impossible to estimate the change in ELA since the last gla-ciation. Any current virtual ELA over the Sanetti plateau mustbe higher than 50 m below the summit of Tullu Dimtu (thehighest potential glacierisation level), i.e. 4350 m. This impliesthat the change in ELA could be as little as 4350–4230¼ 120 mand so due to a temperature change of less than 1 �C (assumingan environmental lapse rate of ca. 6 �C km�1). However, corre-sponding ELA changes on currently glacierised peaks in EastAfrica are several hundred metres, indicating crude tempera-ture changes of 3–5 �C (Kaser and Osmaston, 2002).

Acknowledgements We are grateful to Professor Alayne Street forproviding the 1967 aerial photographs which were the original basisfor the study; Professor Francesco Dramis (University of Roma Tre)and Dr Bekele Abebe (Dept. of Geology and Geophysics, Addis Ababa)for the invitation to the IAG Symposium at which an earlier version ofthis paper was read; Ato Amenti Abraham and Dr Tarekegn Tadese(Ethiopian Institute of Geological Surveys) for help with informationand for gaining official approval; Ato Mohammednur Abachebsa(Natural Resources and Environment Division, Oromiya Government)for permitting the fieldwork; Ato David Haile (Ethiopian MappingAuthority) for providing further aerial photographs; Dr and Mrs StuartWilliams (Ethiopian Wolf Conservation Project) for help and hospital-ity. We also thank Dr Philip Hughes and an anonymous referee forcogent criticisms of an earlier draft, which have been very helpful.Thanks are also due to the Design and Imaging Unit, Durham Univer-sity for preparing the maps.

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