Geoscience Reference
In-Depth Information
GLACIERS ABOVE 4200m
WINTER 4
o
- 8
o
C COLDER
HILLSLOPES UNSTABLE
SOLIFLUCTION
ACTIVE
FAN AGGRADATION
EROSION
TREE-LINE AT 3000m
SHORTER WET SEASON &
REDUCED SUMMER RAINFALL
ZERO FLOW IN WINTER
WINTER DEFLATION OF
CHANNEL SANDS TO
FORM SOURCE-
BORDERING DUNES
SAVANA GRASSLAND REPLACES WOODLAND
PEAK EROSION OF VALLEY SLOPES
DURING SUMMER
MAXIMUM SOUTHWARD AND UPWARD
EXTENSION OF DESERT SCRUB
NW
SE
Figure 10.12. The late Pleistocene Blue Nile. (After Williams,
2012a
.)
from glacial aridity (
Table 10.3a
) to interglacial and postglacial climatic amelioration
(
Table 10.3b
). The precise timing of the last glaciation in the Ethiopian Highlands
is still being investigated. Osmaston et al. (
2005
) considered that up to 180 km
2
of
the Bale Mountains of Ethiopia could have been glaciated at this time, with a central
ice cap of at least 30 km
2
. Glacial moraines and periglacial deposits in the Semien
Mountains near the sources of the Tekezze and Blue Nile/Abbai rivers are presently
being dated using cosmogenic nuclides. The few available radiocarbon ages point
to colder LGM conditions (4-8
C cooler), with a lowering of the upper tree-line by
about 1,000 m during the LGM (Williams et al.,
1978
; Hurni,
1982
).
°
EXPANSION OF
MONTANE FOREST
STABLE SLOPES PROVIDE
SUSPENDED - LOAD OF
CLAY AND SILT
WEATHERING OF
BASALTS & TUFFS
FORMATION OF RED AND
BLACK CLAY SOILS
CLAY DEPOSITION
ON GEZIRA FAN
EXPANSION OF LOWLAND FOREST
SAVANA GRASSLAND
NW
SE
Figure 10.13. The early Holocene Blue Nile. (After Williams,
2012a
.)
