The geological evolution of East Africa - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 14

textured soils, whereas rocks of the basement
complex are generally rich in quartz and weather
to coarse-textured sandy soils of low fe rility.
These latter are highly porous, well-drained soils
and so less drought resistant than those derived
from volcanic rocks.
The soils of the mountainous areas of eastern
Africa have weakly developed profiles. They are
formed on recently eposed geologically young
surfaces, on either volcanics of Plio-Pleistocene
age or steep slopes on Basement where erosion by
gravity quickly removes the weathered products.
However, the low potenial or fe rility of the
parent rocks is offset in the mountains by higher
han average rainfall, which enhances the rate of
chemical weathering and sustains a prolific veg-
etation cover characterisic of the humid forests.
The resuling increase in organic matter in the
soil compensates for the lower nutrient status of
the parent material. This ferility is rapidly lost
when the vegetaion is removed and the area
culivated.

Superficial deposits and soils
Superficial deposits and soils found mantling vast
areas of the region's rocks, in all but high ground
and in stream secions where erosion rates are
high, are shown in the geological maps as being of
Neogene-Recent age. The soil characterisics
depend on the complex interacion of rainfall,
ime, topography, organic material and parent
rock. Space does not permit a detailed descripion
of the soils found in East Africa, but the following
general comments may help the reader assess the
possible soil type variations likely to be found.
Working in East Africa on Precambrian base-
ment areas in the 1930s, Milne (1935) observed
that topography greatly influences the soil proper-
ies. He termed the regular repeition of the par-
icular sequences of soil proiles in associaion
with paricular topographies, a catena. In simple
terms, soils fall into two main groups: the heavy,
black clay-rich muga or black cotton soils occupy
valleys and areas of impeded drainage; while the
other group includes reddish-brown sandy, fe r-
ruginous residual soils ofthe hillsides from which
the fine clays and silts have been washed out. The
first group are alluvial deposits, and the second
are eluvial.
Chemical weathering of rock proceeds rapidly
in the ropics because of the high temperatures
and humidity. At the prevailing high temp-
eratures, the rate of chemical weathering is
positively correlated with the amount of rainfall.
This in tum favours the release of nutrients from
the primary rock minerals. The chemical com-
posiion of the parent rock influences the ferility
of the soil by determining the amount of nurients
that are released on weathering. Lavas and rocks
which are poor in quartz give rise to fe rtile fine-

Sructure and evolution of the
landscape

The present-day landscape owes its origin to a
combinaion of geological processes essenially
operaing in opposiion. On the one hand,
weathering, erosion and deposiion have operated
coninuously since the uplift of the Mozambique
Belt, levelling the land to extensive plains. On the
other, sporadic faulting has elevated some por-
tions and lowered others (Figure 2.6). In some
places this fauling has been accompanied by
volcanism.
Across the enire face of Africa, there are wide,
level plains sometimes stretching for hundreds of

autt block F

� Kv:,':: : n ::: ::: n N : � ••• l N • � , ��� D

Rubeho Mis M kata-Wami Uluguru Mgelil- Ruvu Pare Mts Pilngani Udzungwil Mts Kilombero Mahenge Mis

-J 0 U/ .... \U/"\U[ ... � - ·.:�
Figure 2.6. Schemaic diagram to illustrate the consequences of verical tectonics on the
Precambrian rocks of Tanzania. Subsequent reacivaion of the Karroo aults has main-
tained the landforms to the present day.