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SUBSURFACE EROSION AS A GEOMORPHOLOGICAL AGENT IN NATAL
by
B. H.
DOWNING
ABSTRAcr
Subsurface erosion occurs in all provinces of South Africa. In parts of Natal subsurface
erosion appears to be responsible for producing landscape features of unusual form without
the necessity of land movement. On steep hillslopes, valleys already in existence are extended
and also some entirely new, somewhat funnel-shaped, valleys are formed. The extension and
formation of these valleys is evidently as a result of the collapse of subsurface erosionproduced tunnels, the appearance of sink-holes and dongas, and sloping off of the donga banks.
In bottom lands (vleis) occupied by river flood-plains, cone-shaped mounds of soil are
produced by subsurface erosion. Coalescence and superposition of many cone mounds
aggrades the flood-plains in certain parts and it seems that river courses are forced, at
present, to run in positions away from the aggraded parts.
The mechanism of subsurface erosion depends mainly on the conditions of climate, soil,
bedrock, and possibly vegetation cover.
CONTENTS
Page
I INrRODUcnON
II MECHANISM ••
m
131
LANDscAPB FEATURES
South- and West-facing Hillslopes
North- and East-facing Hillslopes
The Vlei Surface
IV
CoNCLUSIONS..
•.
132
132
133
133
134
134
ACKNOWLEDGMENTS
134
Rl!FBRENCBS ••
134
••
I. INTRODUcnON
The presence of subsurface erosion in South Africa was first recorded when
Henkel. Bayer and Coutts (1938) described and illustrated its occurrence in the
Natal Midlands near Rosetta. Its presence has been noted more recently by
King (1942. p. 69) and by Bosazza (1950a. 1950b) in the Natal Midlands. in parts
of the Eastern Free State near Marquard. in the Transvaal at Wakkerstroom.
and in the Eastern Province. Subsurface erosion appears then to extend over
parts of areas where sediments of the Ecca. the Beaufort and possibly the Stormberg
Series of the Karroo System outcrop.
This paper confirms the description of subsurface erosion in the Natal
Midlands given by Henkel et al (1948), and gives some additional observations
made during 1964 and 1965 in the districts from Underberg through Mooi River
to Bergvi1le in Natal. as well as in the Wakkerstroom area of the Transvaal.
Attention is drawn to the significance of subsurface erosion in the formation of
certain valleys. and in the aggradation of flood-plains.
Subsurface erosion is shown by the following features:
(a) in mature soils on hillsides underground tunnels are present. sink-holes
and dongas may be present;
(b) in bottomlands. cone mounds and alluvial fans are present.
•
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132
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
II. MECHANISM
Subsurface erosion is dependent on the following factors:
(a) A deep, mature soil profile consisting of (i) a decidedly water-permeable
subsoil, (ii) a rather impermeable, clayey subsoil that shrinks, when dry,
to form columnar or prismatic peds separated by vertical cracks; the
subsoil must rest on the sloping surface of a relatively impermeable,
weathered or fresh bedrock.
(b) A seasonal climate having alternating rainy and dry seasons.
(c) Possibly, vegetation cover conditions in so far as they may affect
retention or loss of soil moisture, infiltration and runoff.
During a dry season, the subsoil tends to dry and shrink so that individual
soil peds are separated by vertical cracks. A preceding, lengthy drought period.
and perhaps some change in vegetation cover that promotes dry soil conditions.
may be necessary before cracks can form.
Early in the following rainy season. rain-water soaks through the topsoil
and trickles down the ped surfaces between the cracks before the peds have
been able to absorb sufficient water. swell. and so close the cracks. As water
trickles over the peds. fine clay particles that form the matrix of the peds are
taken into suspension. At the same time coarser particles held in the clay matrix
are released. This process reduces the volume of the peds and widens the cracks
(Plate Ia). Once enlarged. the cracks never close completely with subsequent
wetting and swelling of the peds. The cracks become permanent. and the removal
of soil particles takes place at any time of the year during and after rainstorms.
If the bedrock surface slopes, the soil suspension including the coarser
particles flows down slope by moving laterally around the peds in the cracks.
close to the contact between subsoil and bedrock (Plate Ib). The soil suspension
flows at a velocity partly determined by steepness of the bedrock surface. Corrasion
of peds by the soil suspension now takes place. A small tunnel is formed (Plate
Ie). In time. the tunnel is greatly enlarged especially as small lumps of soil
break away from the peds and are disintegrated or carried away if the water
flows swiftly enough. When the ped bases have been eroded away. the undermined peds collapse and heighten the tunnel. Sink-holes appear in places (Plate
Id) and the bedrock may be slightly corraded. Dongas result when considerable
lengths of tunnel roof collapse. Dongas discharge at the margin of a bottomland
where alluvial fans are deposited.
Underground tunnels drain downwards to bottomlands where they erupt
upwards to the surface. At the surface. cone shaped mounds of soil (Henkel et al.
1938) are slowly built up and are stabilised by vegetation (Plate IIa). Cone mounds
may be up to 50 cm high and 10 metres in diameter. but are usually smaller.
There are one or two openings in the cone mound through which water erupts
and deposits more soil on the cone mound. Should the tunnel become blocked
near the cone mound. a new cone mound or mounds are apparently formed in
the vicinity.
m.
LANDSCAPE FEATURES
The landscape features of subsurface erosion are most clearly and comprehensively shown in and about plain vleis. These features will be described and
discussed for a plain vlei and its surrounds near Estcourt. Natal. as seen by
an observer standing in the middle of the vlei.
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SUBSURFACE EROSION AS A GEOMORPHOLOGICAL AGENT
133
South- and West-facing Hillslopes
The vlei is bounded on its northern and eastern margin by steep southand west-facing hillslopes. On these hillslopes, sink-holes are numerous and
several dongas are present. A number of valleys of unusual form occur on the
hillslopes. Examination of aerial photographs shows that these valleys are distinct
from other, normal valleys that arise less abruptly from vleis or seepages and
which appear to be older.
The unusual valleys fall roughly into two types.
(a) Longer valleys (Plate lIb) which capture water from fair-sized catchments; a stream or rill is present which runs in a distinct channel in
the valley bottom. Above the head of such a valley a line of sink-holes
and small dongas indicates where the valley will extend uphill. Tributary
tunnels, sink-holes and dongas are also present on the sides of such
a valley.
(b) Shorter valleys (Plate lIe) take the form of a small, somewhat funnelshaped depression whose narrow end points uphill and often stops
below a scarp or where a stratum of resistant rock crops out. There is
no stream or rill in such a valley although tunnels and sink-holes may
be present.
It seems as though these valleys have resulted from subsurface erosion through
formation of tunnels and dongas, and without the necessity of land movement.
Apparently. where subsurface erosion occurs in and about a normal valley. a
longer. unusual valley is formed. This happens as dongas extend the normal
valley headwards. and as dongas add tributaries to the sides. Also, the rather
sinuous. shallow stream in the normal valley shifts into a subsurface erosionformed donga situated a few metres away to the side. The new donga-formed
stream channel is deeper and runs straighter than the older water-course.
Additional run-off from the catchment area. which previously flowed fairly evenly
over the hillslope, is concentrated by the tributary dongas into the new. steepbanked stream channel. Incision of the stream bed. retreat and sloping-off of
the banks occurs.
Where subsurface erosion takes place away from normal valleys, the shorter,
unusual valleys are evidently formed. The catchment area surrounding such a
valley is too small to provide water for a perennial stream and subsurface erosionformed dongas are scoured only by storm-water. The donga banks retreat. are
sloped off and covered by vegetation to produce a shorter valley. The presence
of tunnels and sink-holes in some shorter valleys indicates that after suitable
soil conditions have developed in a shorter valley, the process of subsurface
erosion may be repeated.
North- and East-facing Hillslopes
The vlei is bounded on its southern and western margin by gentle northand east-facing hillslopes. On these hillslopes. sink-holes. dongas and the unusual
valleys described above are absent. This is possibly because the gentle slope of
the bedrock does not permit rapid subsurface erosion. which does not here
seem to advance beyond the stage of small tunnels.
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134
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
The Vlei Surface
A conspicuous feature of the vlei is that a meandering river which has
formed the flood-plain, flows at the present time for most of its length close
to the northern and eastern margin of the vlei. Where the river is not too
close to this margin, alluvial fans are present at valley mouths. The largest alluvial
fans are present at the mouths of the longer, unusual valleys. Cone mounds
are rarely seen on the northern and eastern parts of the vlei surface.
There are numerous cone mounds situated on some parts of the vlei surface
towards the south and west, particularly where the vlei margin is poorly defined.
Close examination of these parts (easily done after the vegetation cover has
been burned off) reveals that they have a peculiar micro-topography and are
higher. from about 0.5 to two metres, than other parts of the vlei surface.
This appears to be the result of coalescence and superposition of numerous cone
mounds of varying ages. Cone mounds appear, then. to have built up considerable
areas of the flood-plain such that it tends to "merge" with the gentle hillslopes
to the south and west. Hence the poorly defined vlei margin in these parts.
It also seems that this up-building, or aggradation. of the vlei surface to
the south and west is partly the reason why the river flows for most of its length
in the northern and eastern parts of the vlei.
IV. CONCLUSIONS
1. Once started. subsurface erosion depends only on water supply for its
continued action. The steeper the slope of the bedrock surface. the faster the
rate of subsurface erosion.
2. On steep slopes subsurface erosion seems to be responsible for extending
and initiating a cycle of erosion without any land movement taking place. This
has apparently resulted in the enlargement and extension of valleys previously in
existence, and in the formation of new valleys.
3. Cone mounds produced by subsurface erosion are apparently responsible
for the aggradation of flood plains quite independently of river action. of land
movement. and of sheet or rill erosion at the surface.
4. Subsurface erosion is subordinate to stream-cycle erosion and pediplanation. but is an important geomorphological agent in parts of Natal. and possibly
in other parts of South Africa.
ACKNOWLEDGEMENTS
I thank Professor A. W. Bayer, of the University of Natal. for his interest
and guidance.
This work was done while I was employed by the Department of Agricultural
Technical Services.
REFERENCES
V. L., 1950a. The Erodability of Soils. Reprinted from Farm. May 1950.
--1950b. Cut a Donga Through your Vlei. Reprinted from Farm. August. 1950.
HENKEL, 1. S., BAYER, A. W. and COUITS. 1. R. H., 1938. Sub-Surface Erosion on a Natal
Midlands Farm. SA. Jour. Sci. 15, 236-243.
KING, L. C., 1942. South African Scenery. London.
BOSAZZA
NATAL PARKS. GAME AND FISH PRESERVATION BOARD,
P.o. BOX 662.
PIETERMARITZBURG.
Accepted for publication by the
SocielY on 16th Sep!embet. 1967
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2009).
LEGENDS FOR ILLUSTRATIONS
Plate I
A
B
c
D
A dark topsoil overlies a pale subsoil with
enlarged vertical cracks. Turbid soil suspension
flows in the donga.
'
There is no surface flow, but water flows close
to the contact between subsoil and bedrock.
A small tunnel about 35 em above the bedrock
surface.
Swiftly flowing water in a tunnel as seen through
a sink-hole.
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TRANS.GEOL SOC.SAFR .. VOL. LXXI
DOWNI NG. PLATE I
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2009).
Plate II
A
B
C
One figure stands on the margin of a small cone
mound, the other on the apex.
Tributary sink-holes and dongas (indicated -+)
extend a normal valley to produce a longer
valley.
A shorter vaIley with some sink-holes on its
sides.
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TRANS.GEOL'sOCS.AFR.. VOL. LX XI
•
VOWNI NG. PLATE 1/
'.
u