Geoscience Reference
In-Depth Information
the importance of the forest cover and watershed role of the high African mountains
from the equator to the Mediterranean in the context of changing climate. Mark and
Osmaston (
2008
) provided a comprehensive and thoughtful review of Quaternary
glaciations in Africa, with the main focus on East Africa, which is where the late
Henry Osmaston had worked most intensively. They concluded that although existing
radiocarbon ages only yield minimum ages for the Last Glacial Maximum advance
(21
2 ka), those ages appeared to show that such advances had been synchronous
across Africa. However, direct ages of moraines from cosmogenic radionuclides
(CRN), in this case
36
Cl, are only available for Mounts Kilimanjaro and Kenya, so
this conclusion may be premature. Limited chronological evidence from these two
mountains suggests that the maximum ice extent since MIS 5 may have occurred close
to 30 ka rather than later. In southernAfrica, the evidence for late Pleistocene glaciation
remains controversial, although recent work in the highlands of eastern Lesotho does
provide a plausible case for a limited niche glacier during the LGM (Mills et al.,
2009
).
Once again, the age of themoraines is still not known, so reference to an LGMagemust
remain hypothetical. This stricture applies equally tomany of the periglacial landforms
mapped in the drier regions of Africa (Messerli,
1972
; Hastenrath,
1972
; Hastenrath,
1973
; Hastenrath and Wilkinson,
1973
). Until they have been dated directly using
cosmogenic nuclide dating methods, any attempts to derive information about late
Quaternary climates from such evidence must perforce be speculative. However,
there is a useful role for speculation in devising testable working hypotheses. In
this regard, the attempt by Hastenrath (
1972
) to compare inferred modern and late
Pleistocene snow-lines on a north-south transect across Africa, South America and
Australia-New Guinea led him to postulate an equatorward shift of the westerlies
during the Last Glacial Maximum, bringing higher rates of precipitation to uplands
in temperate latitudes in the Southern Hemisphere.
Besan¸on et al. (
1973
) have provided a detailed and comprehensive account of
the difficulties involved in efforts to reconstruct former glacial and periglacial pro-
cesses and associated altitudinal limits in Lebanon. After reviewing all previous
work, they concluded that there was a complete lack of accordance, and so pro-
ceeded to examine every putative glacial cirque in the country. The result of this
detailed fieldwork by three highly experienced geomorphologists is sobering: all of
the features claimed as glacial, whether cirques or moraines, appear to reflect vari-
ations in the limestone bedrock lithology and structure, and owe little to glacial
processes. Nivation hollows formed by corrosion beneath snow patches are evid-
ent at high elevations where the aspect or wind regime allowed snow to accumu-
late and persist. Periglacial landforms are likewise poorly developed and could not
be used to determine former temperature changes. Their final conclusion is worth
noting, for they considered that evidence from pollen analysis, faunal studies and
prehistoric archaeology, buttressed by absolute age control, would all be needed to
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