page_title: Marine geology of the Cape Peninsula and False Bay
section_title: Cape orogeny
hierarchical_section_title: Marine geology of the Cape Peninsula and False Bay / The geological structure of the region / Cape orogeny
context_page_description: The Cape Peninsula is a peninsula that juts out into the Atlantic Ocean at the south-western extremity of the African continent. At its tip is the Cape of Good Hope. The peninsula forms the west side of False Bay. The three main rock formations are the late-Precambrian Malmesbury group, the Peninsula granite, a huge batholith that was intruded into the Malmesbury Group about 630 million years ago, and the Table Mountain group sandstones that were deposited on the eroded surface of granite and Malmesbury series basement about 450 million years ago. The sand, silt and mud deposits were lithified by pressure and then folded during the Cape Orogeny to form the Cape Fold Belt, which extends along the western and southern coasts. The present landscape is due to prolonged erosion having carved out deep valleys, removing parts of the once continuous Table Mountain Group sandstone cover from the Cape Flats and leaving high residual mountain ridges.
  At times the sea covered the Cape Flats and Noordhoek valley and the Cape Peninsula was then a group of islands. During glacial periods the sea level dropped to expose the bottom of False Bay to weathering and erosion.
context_section_description: The formation of the Cape Fold Belt is the result of a collision of tectonic plates that ended over 200 million years ago The accumulated strata of the Cape Supergroup and the older granites and Malmesbury group were raised and deformed by the pressure of the South American, Antarctic and African continental plates slowly moving together. The resulting fold mountains have been eroded to their present state over the ensuing period, and what exists today are the remnants of a much larger and higher mountain chain.
  Faults cut across and displace the rock layers. These more easily eroded zones are marked by ravines; cross-cutting faults separate multiple peaks of the Twelve Apostles. Some fault zones of crushed rock (breccia) are re-cemented by dark brown coloured iron and manganese oxide minerals.
  The present landscape is due to prolonged erosion having carved out deep valleys, removing parts of the once continuous Table Mountain Group sandstone cover from the Cape Flats and leaving high residual mountain ridges.
caption: Possible evolution of the Cape Town landscape: The nearly horizontal Table Mountain sandstones represent the trough and the steeply dipping sandstones of the same formations of the Hottentots Holland Mountains to the east the limb of a large fold that has since eroded away to expose the underlying shale. Thus the modern landscape may represent an inverted version of an earlier landscape (dashed lines) characterised by a large mountain where the Cape Flats are now. (after Compton 2004)