What is now a continent covered with deserts and grasslands was once an ice sheet comparable to today's Antarctica. Glacial deposits and striations mark the southern portion of the African continent and are evidence of a time when global sea-levels were lower and cold temperatures persisted throughout much of the planet.
African Ice-Sheets of the Late Paleozoic
African ice-sheets existed in the southern portion of the continent 250-300 million years ago, or during the late Paleozoic era. The ice-sheets extended from Antarctica into the African continent as global cooling dominated. The stratigraphic record of this era is incomplete in southern Africa - due to geological processes, mainly erosion - but in North America, there is evidence of repeated marine invasions. These sea-level fluctuations can be attributed to a period of cyclical ice build-up and decay of continental glaciers world-wide during the Pennsylvanian and Permian periods of the Paleozoic.
The cyclical fluctuation of sea-levels worldwide led to the repeated burial and reburial of Pennsylvanian and Permian swamps, which today is the source of large deposits of hydrocarbons (fossil fuels) in North America. During this same period, the African continent was experiencing tectonic drifting, causing the land mass to slowly drift northwards away from the cold-weather influences of Antarctica. Currently, this continental drift is still active, making it unlikely that the African continent will ever experience a period of glaciation comparable to that of the late Paleozoic.
Evidence of Glaciation in Southern Africa
Geologists can always tell when their has been glaciation in an area because of the clues left behind. In Africa, these clues include:
- Striated bedrock
- Tillite formations
- Meltwater deposits
Striated bedrock forms as a result of the glacier scraping over the bedrock. As glaciers mover over continents, they collect and deposit a variety of rocks along the way. Rocks at the bottom of the glacier scrape against the bedrock and leave striations in the direction of the scraping. This kind of glacial evidence is unique to glacial movement and provides information such as rate and direction of movement to glaciologists.
Tillite forms as a result of the compaction and cementation of glacial till deposits. Till deposits form when glaciers drop the rocks that they are carrying as they retreat. Like a bulldozer, glaciers break and shove rock over land as it moves forward. When warmer temperatures ensue, the glaciers retreat, leaving behind unsorted piles of rock. These rocks are of different sizes and types - characteristic of a glacier that picks up everything and anything as it moves forward.
Meltwater deposits, also referred to as meltwater stream sediments, form as a result of sand and gravel being transported by streams carrying water from a melting glacier. These sediments can also include a variety of rock type, but are generally better sorted than till deposits so that the most prominent size of rock is coarse sand and gravel.
Today, glaciers on the continent of Africa are restricted to the summit regions of a few African volcanoes, and even those are disappearing due to climate change.
Related Articles:
Source:
Skinner, Brian J., Porter, Stephen C, and Park, Jeffrey. Dynamic Earth - An Introduction to Physical Geology. NJ: John Wiley and Sons Inc., 2004.
 |
