KNYSNA RIVER: Topography, geology and soils

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Map of the KNYSNA River

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TOPOGRAPHY
    GEOLOGY                                    
The catchment of the Knysna River lies within the Cape Fold Belt with its long faults and folds that strike east-west.  These old features provided a framework for the Mesozoic and Cenozoic fabric.  Knysna is a true drowned valley, more or less silted up (Krige,1927).  (Ref2)
The oldest rocks in the area (of late Precambrian age) are found south of the Outeniqua Mountains to the west of Knysna.  They consist mainly of contorted bands of schist, phyllites and feldspathic quartzites of the Kaaimans Formation.  The adjoin outcrops of intrusive gneissic granite further to the west, but these old rocks are not in the Knysna River catchment. (Ref2)
Most of the catchment lies in rocks of the Table Mountain Group, including the Peninsula, Cedarberg, Tchando and Kouga Formations.  These rocks are supermature quartz sandstones which constitute the mountain ranges of the Cape Fold Belt and are believed to be mainly of marine origin.  Geologists familiar with this area stress that this heavily faulted area is potentially seismically active and point out that this may be important when considering building developments on steep unstable slopes.  The Baviaanskloof Formation, the youngest of the Table Mountain Group and the overlying Gydo Formation of the Bokkeveld Group, lies beyond the Knysna catchment.  (Ref2)
Deposits that are believed to be Cretaceous or Early Tertiary age appear at Knysna (Miller, 1963;DuToit,1966;Butzer and Helgren,1972;Toerien,1979).  The Enon pebble conglomerates represent torrential deposits in an arid climate under strongly oxidising conditions which give these deposits their characteristic reddish colour.

These deposits are part of the Uitenhage Group with the terrestrial Enon conglomerates succeeded upwards by estuarine deposits.  The conglomerates consist predominantly of rounded Table Mountain Group quartzite clasts (fragments of pre-existing rock) set in a sandy matrix which laterally, interfinger with finer sandstones and mudstones.  (Ref2)

A sequence of estuarine deposits occur in the Knysna Formation north of the town and contain lignites up to 1,5m thick (Du Toit,1966;Thiergart et al.,1963).  Plant fossils include Podocarpus and Widdringtonia and pollens indicate Knysna forest tree species but work by Thiergart et al. (1963) on Early Tertiary pollens reveals that the flora differed from that of the modern forest and include a palm.   (Ref2)
    SOILS                                    
The town of Knysna and parts of Thesen's Island are formed of colluvial slope deposits of Tertiary to Quaternary Age.  Tyson (1971) pointed out that in the Knysna area three categories of soil can be distinguished including youthful shallow azonal soils with imperfectly developed horizons, brown or grey soils forming under present day conditions and palaesols (relict from the past) including laterites and soils with Terra Rossa affinities.  Butzer and Helgren (1972), discuss these palaeosols in greater detail while Helgren and Butzer (1977) use their observations in this area to revise what is meant by term laterite.  The 'Formosa', Knysna and Brakkloof soils and their development are discussed.  Soils are generally acid, pH 4,5 - 5,5, and podsolization is extensive.   (Ref2)
Azonal soils are found on all steep slopes, on recent dunes and in wetlands. Except in areas liable to flooding where deep, dark, organic-rich soils develop with annual increments of silt, most azonal soils are shallow, rarely exceeding 30m in depth and are sandy in texture.  The  brown and grey soils forming under present day conditions are most extensive on the forested interfluves of the foothills zone.   (Ref2)
Lateritic palaeosols cover extensive areas over a wide variety of parent materials including on the 180-240m surface.  Laterites are considered to result from intensive leaching under seasonally warm wet conditions on level surfaces close to the water table.  The bleached lower horizon is the product of reduction under saturated conditions and the mottled intermediate layer results from alternative oxidation and reduction under conditions of a fluctuating water table.  Often it is only the presence of relict ironstone gravels which indicates the presence of the former soil processes.  The deep lateritic soils on the coastal platform are indicative of a long period of weathering and leaching in a stable environment.  (Ref2)
Soils of the coastal belt tend to reflect past climates more than that of the present, and consequently they vary considerably.  Generally the topsoils are fine-medium sand, having originated from the Table Mountain Group quartzites and sandstones, or being blown deposits from the littoral zone and coastal embayments.  The A-horizons are generally humus rich and of low pH.  Effective rooting depth varies widely from very shallow on the mountain midslopes to over a metre on some of the colluvial and plateau soils.  (Ref2)
A good deal of deep depositional clays occur on the coastal platform creating strong duplex profiles. Groundwater fluctuations produce widespread gleying.  Sandy-loam soils are associated with the granite plutons, as well as the Bokkeveld shales, and the pre-Cape schists and phyllites.  (Ref2)
The foothills zone exhibits gravels and sands related to ancient alluvial fans and more recent colluviation.  Shallow azonal A-C soils occur on rocky outcrops in the foothills and on the mountain mid-slopes.  Sand dunes show recently formed soils, whilst the upper plateau comprises a relic land surface preserved by extensive ferricretes and silcretes.  Laterisation is evident in many areas of the coastal Southern Cape.  (Ref2)
  References                                      

Ref2:  ESTUARIES OF THE CAPE-Report NO.30-KNYSNA(CMS13)
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