Introduction

Investigation of three rock glaciers on Mount Kenya developed out of a study of Quaternary strati-graphy and soil morphogenesis. Rock glaciers of the valley-side type (Reference Outcalt and BenedictOutcalt and Benedict, 1965) are found in Teleki Valley (Naro Mom River) above 4000 m on Mount Kenya (Fig. 1). Avalanche couloirs deliver trachyte-tuff agglomerate and porphyritic phonolite debris across 30° talus slopes into the three rock-glacier systems. In this paper the origin and age of the rock glaciers are discussed.

Fig. 1. Map of Mount Kenya showing Naro Mora drainage basin.

Field Area

Mount Kenya is a prominent strato-volcano of late Pliocene/early Pleistocene age, located approxi-mately 200 km north of Nairobi, and rising to 5 185 m above the high plateau of Kenya (Fig. 1 (Reference BakerBaker, 1967). Twelve glaciers are found above 4600 m; prominent moraines, outwash trains, alluvial fans, talus cones, and rock glaciers of late Pleistocene/early Holocene age mantle the flanks of the mountain.

Vegetation in the vicinity of the rock glaciers has been described by Reference HedbergHedberg (1964) as belonging to the Upper Alpine group consisting of giant groundsel (Senecio keniodendron), Helichrysum, carex sedge and Agrostis grass, Carduus platyphyllus and Lobelia telekii.

Climatic data for Teleki Valley are incomplete but Reference HedbergHedberg (1964) estimated the mean monthly temperature for August to be 3.1–6°C. Generalized isohyets for 1961 (Reference Coetzee and BalkemaCoetzee, 1967) approximate precipitation to be 108 cm. The wettest slopes are on the south-east flank of the mountain where precipitation reaches 375 cm.

Teleki Valley Rock Glaciers

Lobate-shaped rock glaciers in Teleki Valley consist of poorly sorted and angular debris, principally composed of porphyritic phonolite, agglomerate, and trachytic porphyry derived from nearby bedrock and moraine sources.

No interstitial ice was observed in the three rock-glacier systems but minimum temperatures taken at the base of a 78 cm open soil pit (site TV₁, Fig. 2) averaged −4.1°C (n = 5, in August 1976). The lack of interstitial ice is more the rule than the exception according to numerous workers (Reference WhiteWhite, 1976). These three systems have numerous concave depressions behind the outer lobe, a number of lateral, and a few longitudinal furrows and “drunken stands” of S. keniodendron and L. telekii (Fig. 3), suggesting that previous ice had melted out.

At least three lobes that are generally wider than they are long have developed below avalanche talus on the north side of the valley. The thicknesses of the rock glaciers are estimated at 100–150 m, and two of the outermost lobes appear to be re-activated (Fig. 2). However, the lobes on all three systems have c. 80% lichen cover (e.g. Rhizocarpon geographicum, Lecanora spp.?, Umbilicaria haumania, Umbilicaria africana and Usnea), suggesting that movement is exceedingly slow. Isolated boulders and large blocks were found in front of the two re-activated fronts (Fig. 2).

Fig. 2. Map showing distribution of rock glaciers in Teleki Valley at approximately 4 100 m. Late Pleistocene/early Holocene soil profile (TV_I) is on the third lobe of the middle system. Soils mantle approximately 20% of surface; remaining 80% is an open network of boulders.

Fig. 3. Talus with Lobelia telekii and Senecio keniodendron sparsely distributed on stones feeding into the middle rock-glacier system.

Weathering criteria

The degree of weathering increases from lobe IV to lobe I as shown in Table I.

The data indicate that the several lobes have similar ages and correlate closely with late glacial moraines described by Reference Mahaney and MahaneyMahaney (in press).

Table I. Weathering Criteria Used to Differentiate Rock-Glacier Lobes

Soil Development

A discontinuous distribution of fine sediment supports a moderately developed Inceptisol as well as numerous grasses and sedges. A thin 1–3 cm mat of partly decayed black organic material (10YR 1/1, 2/1 m) overlies a pedon (TV₁ soil profile) with the following horizonation:

Particle-size de terminations indicate that two parent materials are present (e.g. rock-glacier debris and loess). The data are shown in Table II.

The upward-fining sequence where silt is higher in the solum and lower in the sub-soil compares closely with soils on nearby end moraines of late Würm age (Reference Mahaney and BalkemaMahaney, 1979) in Teleki, Mackinder, and Liki North Valleys, and with late-glacial soils in the Rocky Mountains (Reference Mahaney and MahaneyMahaney, 1974). The higher clay and silt quantities in the solum of this soil further substantiate the observations of Reference ZeunerZeuner (1949), who described aeolian materials in soils on the mountain.

No clay minerals were detected in the < 2 μm grade-size material. However, rock-forming minerals in the fine clay-grade size include plagioclase and quartz. Feldspars exist in trace quantities throughout the soil, while quartz ranges from moderately abundant (>50% on diffractograms) in the A11, A12, and IICox horizons to traces in the B2ir horizon. The absence of clay minerals is noteworthy, especially since they occur in other nearby late-glacial soil systems (Mahaney, in press).

Table II. Particle-Size Distribution^* for the Soil Horizons in Profile Tv₁

The analysis by X-ray diffractometry of primary minerals in the silt-sized fractions (63–4 μm) indicates that quartz is nil in the solum, increasing to small amounts (30–50% based on peak height above background radiation) in the C horizon. Feldspar is found in trace amounts in the upper solum and increases to small and moderate amounts in the B and G horizons. The data correlate closely with mineral assemblages found in nearby soils forming on late glacial moraine systems and suggest that common rock-forming minerals weather quickly in soils above the timber line.

Conclusion

Three massive valley-side rock glaciers are found in Teleki Valley just above the 4000 m contour. Numerous criteria including transverse furrows, steep re-activated fronts, and “drunken stands” of giant groundsel and lobelia attest to differential movement, but no interstitial ice cores were observed in the field. Rainfall and temperature regimes are marginally sufficient to support the growth of ice in the rock lobes. Soil and weathering criteria suggest a late-glacial age for the rock glaciers, while re-activation as indicated by steep fronts may have coincided with the advent of glacial advance in the late Holocene (< 1000 years B.P.).