Geology

Geology

 
The oldest rock formation in the broader study area is a metamorphic coarse-grained granite layer, of the Cape Peninsula Pluton. This pluton underlies almost the entire Peninsula, and is the result of the collision of the African and South American continental plates during a mountain-building period known as the Saldanian Orogeny. This collision folded and forced downward the Malmesbury Group sediments at the point where the continents collided, which under extreme pressure and temperatures, then melted and rose back toward the surface as magma, or molten rock. The magma crystallized and solidified into granite plutons some 540 Million Years ago, at a depth of about 8km beneath the surface (HTCGIS)

 
Over a period of some 40 million years, some 8 to 10 km of the surface was weathered down to a vast, nearly flat surface known as a peneplain. This exposed and flattened the Cape Peninsula Pluton onto which the Table Mountain Group sediments were then deposited between 500 and 450 million years ago. This process began with the deposition of the Graafwater formation (Og), which is reddish-purple in colour due to the presence of oxidised iron, and has distinctly thinner bedding than the rest of the Table Mountain Group. The cleaner sand stones of the Peninsula formation (Op) sandstone above are typically beige-white and form thicker, more massive units. Other sedimentary layers were deposited above the Table Mountain Group, but these have long since been eroded away in the study area.

 
Some 250 million years ago a massive mountain building period known as the Cape Orogeny gave rise to the Cape Fold Belt. The Cape Peninsula escaped much of the intense folding that occurred during this period, but it was warped and uplifted to its current position by this event. Around 130 million years ago the Gondwana supercontinent split just west of Cape Town, South America and Africa separating to give rise to the South Atlantic Ocean. During this period faulting (cracks in the earth's surface) and vulcanism (intrusion of molten rock), gave rise to a network of faults and Dolerite dykes. Since this period, there has been a relatively calm period marked by slow uplift and erosion, which has given rise to the topographical features that we see today (HTCGIS).

 
Large sections of the study area are covered by shell-bearing dune sand of varying coarseness (Qsr). Some dunes in the Dido Valley area possibly date from prehistoric times. On the southern side of the river in the Glencairn area light to pale grey sandy soil (Qg) is found. This sand consists of particularly pure silica, and is even older than the dune sands. Scree and gritty sand (Qt) is found along the mountain slopes. 

The upper and middle part of the catchment consists of rocks of the Table Mountain Series, while the lower part runs through an area of windblown sands (Huaghton, 1933)

Soils

The soils of the Else River catchment form part of the "Da Gama Zone" of Smith-Baillie et al. (1976), who describe these soils as shallow, stony, infertile and usually acid with a pH of less than 5. The aeolian (wind-blown) soils are deeper, very sandy and seldom become waterlogged.