TABLE MOUNTAIN’S HISTORY INCLUDES EPISODES OF BOTH FIRE AND ICE By Deborah Painter

“Hoerikwaggo” is the old name of the mountain in the language of the Khoisan.  The old mountain is “Tafelberg” in Afrikaans and “Taboa do Cabo” in Portuguese.  Table Mountain is one of the most famous mountains on Earth.  It is a landmark not only to those who live in Cape Town, but the world.  When Europeans, Asians and Americans who have not visited the city are shown photos of Cape Town, the most recognizable image they associate with it is Table Mountain.

 

Table Mountain is the only mountain with a constellation named for it. Mons Mensae, “Table Mountain” in Latin, was the original name given by Nicolas Louis de Lacaille who was the first to name the keystone shaped constellation of dim stars now known as Mensa.   It is located in the portion of the Southern Celestial Hemisphere sky featuring the larger of the two Magellanic Clouds (also called Cape Clouds) that are actually small galaxies, not small star clusters surrounded by gas.  Lacaille was reminded by this satellite galaxy of the Milky Way of the clouds that he had often seen draped over the summit of Table Mountain.

As if this were not a lovely enough association with the famed mountain, there is also its origin that lends romance and interest to its history.  Very hot igneous rocks are involved (the fire) as well as ancient glaciation (the ice).  In addition, the Table Mountain story includes cool but not cold sedimentary deposits.

It is easy to see why astronomer Nicolas Louis de Lecaille associated Table Mountain with the cloudy appearance of the Greater Magellanic Cloud galaxy. Credits: Public Domain Pictures

The geology of the mountain is not unique but it is interesting because oldest layers of the sedimentary rocks are at the bottom of the mountain and the youngest are above them.  That seems the most logical progression, of course, but it does not always happen in a given area of strata because plate tectonics often cause sedimentary rock layers to be tilted and even in some cases folded over so that older layers can be above younger ones.  That is not the case here.  The oldest portion of the mountain is the late Precambrian Maimesbury Group of grey fine grained sandstone and shale, deposited 560 million years ago on an ancient continental slope.  Metamorphosis heated the layers and tectonic forces tilted them so they are now almost vertical.

 

About 540 million years ago, a great mass of intrusive igneous rock known among geologists as the Peninsula Granite melted its way into much of the sedimentary rock that now comprises this mountain.  The igneous rock can be seen at the Sea Point Contact.  One can sometimes see crystals of glassy quartz and white feldspar in this igneous rock that cooled and hardened without ever reaching the surface as lava.  Then, the sedimentary mudstone, sandstone and siltstone of the Table Mountain Group were deposited 500 to 440 million years ago.   The area was a broad river channel then.  Back then the continent was part of Gondwana and was far closer to the South Pole than it is today. The great Carboniferous Period Ice Age that lasted from 340 to 260 million years before present left the Pakhuis Formation, a glacial outwash deposit, on the very top of Table Mountain. In this deposit are glacial erratic boulders and cobbles taken from elsewhere and pulled and dragged along the bottom of the continental glacier to rest at this spot. Macclears Beacon, the highest point, is a man-made feature built using these erratics.

A view from the top. Credits: rhysara

The scenery around the mountain now looks as it does today because of long term erosion.  During the Pliocene and Pleistocene epochs between 2.6 million and 18,000 years ago the sea level rose and fell multiple times along this coastline.   The erosion carved out valleys into the softer rock groups and left Table Mountain and nearby ridges as ridges of more resistant hard rocks.  Ores were exposed such as the manganese ores mined decades ago.  The biodiversity of the mountain is due to the diverse origins of the eroded and weathered soil here.