29/01/2024
An interesting article by the late Tony Ardington on the Rocks at Zinkwazi beach
The author of the following article is Tony Ardington. Tony is a third generation local sugar farmer. He was educated at Michaelhouse, attained honours in Geology at Rhodes and spent three years at Oxford reading Politics, Philosophy and Economics. Thereafter he and Libby, his attorney wife, farmed at Mandini where they brought up their family of three. Tony served the Sugar Associations for 32 years – 8 of these as Chairman. He was also Chairman of Cane Growers for 14 years. His great interest however was geology. Although the Ardingtons retired to Cape Town, they retained a share in a beach front property where Libby remarked “Tony can do what he loves most – Look at his beloved rocks” !!
The remarkable origin of Zinkwazi’s rocks.
At low tide beaches are rewarding places to study rocks. Zinkwazi beach is no exception. The rocks exposed at low tide East of the Ski Boat Club are most unusual rocks worthy of closer inspection. All the rocks south to the Nonoti Lagoon, other than Black Rock, south of our car park, reveal a large variety of stones and pebbles embedded in a matrix that is made of very small clay particles. These rocks look like a raisin bun with the raisins all of different sizes, shapes and colours. The rock type is Dwyka Tillite.
How did they come about?
Rocks are classified according to their origins. There are three major types:
Sedimentary – matter deposited on the land by water, wind or ice;
Igneous – molten rock which cools and solidifies and lastly;
Metamorphic – rocks which have changed under conditions of great pressure and temperature.
The Zinkwazi rocks are Sedimentary. Deposits by water always have two characteristics – the rocks and stones are reduced and rounded, as they make their way over rapids from the mountains to the sea, and sifted by weight and density. As the velocity of the water falls the larger stones are deposited, then smaller pebbles followed by sand and finally the tiny clay particles that stay in suspension until they reach the sea. .
Deposits by wind are likewise rounded and sifted according to weight and size. Deposits by Glaciers have stones and pebbles of all shapes and sizes and from different areas embedded in the matrix. The Zinkwazi rocks are of glacial origin.
As the Glacier travels slowly down the valley, soil, sand, rocks, boulders, pebbles, trees and other plants fall onto the slowly moving ice. Rocks jutting out in the valley are plucked by the glacier and scrape against the sides as the glacier inexorably moves down the valley. Rocks from the bottom of the valley are embraced by the ice and gouge out the valley floor.
Where the glacier melts it deposits this hotchpotch of solids – no grading according to weight and density. As glaciers travel over huge distances rocks from different places are all deposited where the glacier melts. Time, pressure and heat turn the hotchpotch into rock.
When you go at low tide to inspect the Zinkwazi rocks you will see chunks of white granite, red granite, black granite or dolerite, sandstone, mud stone, shale, etc. – some large, some small, some smaller than your little finger nail, shaped just as they were when they fell into the glacier,
The Dwyka Tillite was deposited 220 million years ago into the huge Karoo basin which covered much of Southern Africa when South Africa was closer to the South Pole. Subsequently these glacial rocks were covered by further deposits of shale, mudstone, sandstone, and igneous rocks such as ‘Black Rock’. These in turn were capped by sand blown deposits, the Clarence Sandstones, best illustrated by the Cave Sandstones forming the cliffs of the little berg in the Drakensberg.
160 to 150 million years ago continuous volcanic activity created one of the largest out pourings of lava in the world. It covered much of Southern Africa. As the lava solidified it became Basalt. Over the millennia it has mostly eroded away. However 1000 to 2000 metres of Basalt still remain capping the top of the Drakensberg.
