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Transactions of the Symposium on Photo Interpretation

The rocks in this area consist of Pre-Cambrian granites intruded by an
extensive dolerite sill which has an undulating upper surface and appears at
varying altitudes. The margins of the sill often have a massive blocky appear
ance and weather readily, whereas the inner parts have a “softer” photo-tone
and weather more smoothly. The granite in immediate contact with the sill
has undergone alteration and readily weathers. Because of this, the contact zone
of the two rock types can supply abundant weathering products which could
form lateritie soils.
The geology has previously been mapped in the field [5] and some contacts
were field checked. The different rock types may generally be distinguished
on the photographs; the similarity of the altered granite and the blocky dole-
rite is marked.
A summary of the airphoto characteristics of the rock types and soils follows:
a. Granites - light tones in outcrop and soils - blocky or sometimes smooth
outcrops - sandy soils - vegetation medium dense to sparse - light coloured
b. Dolerites - dark tones in outcrop and soils - blocky weathering with a
broken “soft” photo texture - dark red clayey soils - vegetation medium to
dense - dense over outcrop - dark coloured anthills.
The landform is predominantly mature to senile, having an altitude of
4500-4900 ft A.M.S.L. The granite and dolerite hills have a juvenile morphol
ogy. The drainage patterns and developments of soil catenae are well shown
by the dark vleis over the granitic areas. This darker tone is not seen so well
over the dolerite. Examination of the vlei pattern and of the watercourses
themselves shows that the senile part of the landform is being modified; the
watercourses are actively cutting down in the vleis and river capture has oc
curred in one place. This demonstrates that if laterite is to be found, not only
has the availability of parent materials to be considered in relation to present
landform but that its past availability related to the original landform is of
great importance. Thus, it is of little use searching for laterites purely on the
evidence of present drainage conditions related to the catenae. If, as was
originally thought, the laterites were restricted to the vlei margins, the higher
parts of the interfluves would have been rejected as being the highest parts of
the catenae. However, investigation of these areas has proved some large
laterite deposits demonstrating that their formation is coeval with that of the
original landform. The laterites developed in doleritic soils are too plastic for
use as base or sub-base and the elimination of barren areas can effect important
economies in prospecting.
The selection of potential laterite sites was based initially on criteria ob
served at a known laterite deposit just south of the area. The deposit underlay
a light-coloured sandy soil with sparse bush and tree cover, the border of an
adjacent vlei showing a white speckling. The sites marked A (fig. 1) correspond
to the deposits underlying sandy cover and the B sites correspond to speckled
vlei borders. Subsequent interpretation based on the criteria already described