Geology Reference
In-Depth Information
isms (Andrews & van Couvering, 1975). The
central highlands of Africa were not formed unil
the Pliocene and Qu atenary and so the Atlantic
rainfall system would have penetrated further
eastward than at present. For example, a late
Miocene (12.2 Myr BP) leafflora from the Kenyan
rift valley, in what is now dry Acacia wooded grass-
land, contains fossils of the moist forest grasses
Humbetochloa and Leptaspis Jacobs & Kabuye,
1987). At the end of the Miocene major topo-
graphic changes in East and Central Africa
dramaically altered drainage pattens and so
limited dispersal along rivers.
the forest gap at Makambako
the southen
Uzungwa Mountains.
From the early Pliocene, climaic history
becomes clearer following analysis of deep-sea
sediments and the development of models based
on changes in the earth's orbit and orientaion
(Hays, lmbrie & Shackleton, 1976; lmbrie &
lmbrie, 1980). The evidence and proposed cli-
maic changes are reviewed by Hamilton (1988).
From the beginning ofthe Pliocene (c. 9 Myr BP)
to the mid-Pliocene (6.4 Myr BP) the climate was
warm and humid, an observaion supported by
fossil evidence from northwest Ethiopia, suggest-
ing a moist orest with westen Guineo-Con-
golian afiniies around 8 Myr BP (Yemane,
Bonnefille & Faure, 1985). Off the southwest
African coast deep-sea drilling indicates that
strong, persistent upwelling of cold waters (the
present Benguela system) began in the early late
Miocene about 10 Myr BP (Siesser, 1980). After
6.4 Myr BP, southward movement of Antarcica
enhanced the cold Benguela current. After 5 Myr
BP a major ice sheet developed on Antarcica
(Shackleton & Kennett, 1975), strengthening the
cold current, bringing further aridity to the south-
west coast ofAfrica and compressing the southen
Guineo-Congolian forests. Kalahari sands,
thought to be primarily of aeolian origin, dating
from the Teriary or early Qu aternary, have been
found as far north as the River Zaire and as far
east as Zimbabwe. Subtropical rain forest at the
Cape containing taxa such as Microcach ys, Win-
teraceae, Sarcolaenaceae and Chloranthaceae
which are no longer represented in Africa, was
lost at the close of the Teriary (Coetzee & Mul-
ler, 1984). Between 4.6 and 2.43 Myr BP the
climate was again warm and wet. Lake Turkana in
northen Kenya possessed a much richer riverine
forest than at present, with western Guineo-Con-
golian elements (Williamson, 1985; Dechamps &
Maes, 1985). During the warm, wet periods of9-
6.4 Myr BP and 4.6-2.43 Myr BP migraion across
the arid corridor via the northen and, more par-
icularly,. the southen migraion route would have
been possible for readily dispersed plants tolerant
of a range of moist forest types.
Uplift of the Cenral African Plateau
During the mid-Teriary the ancient peneplaned
surface of the central African plateau was flexed
and warped (Burke & Wilson, 1972). The East
African hinterland swelled upwards and the
Congo sank into a basin. These topographic
changes would have altered drainage patterns and
accentuated the coninental divide. At the end of
the Miocene, c. 10 Myr BP, further lifing raised
the cenral plateau to its present level and major
rifts developed. Rivers were rejuvenated and the
rise ofthe east-west continental divide limited the
western Guineo-Congolian climaic and bioic
influence, separaing it from the easten forests by
a north-south band ofdry seasonal country called
the arid corridor (reviewed by Werger, 1978).
The arid corridor is today seen in those areas
where, for at least three consecutive months a
year, there is less than 10 mm of rain a month. It
connects northeasten and southwestern African
arid areas, and comes east to the tropical Indian
Ocean in northen Mozambique west of Mada-
gascar. Called a corridor because of north-south
disjuncions of arid land species, it presents a for-
midable barrier to east-west migration or disper-
sal of moist forest organisms. The remaining
possible migraion routes between the western
and easten forests would have either been north-
wards over the Kenyan highlands and down the
rivers draining eastwards; or southwards via the
Lake Tanganyika rift, along the mountains and
valleys of the southern Rukwa rift, into the cli-
maic system created by Lake Nyasa and across
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