Coast KZN

10 Jan 2024

Unveiling nature’s ancient story at Thompson’s Bay

James Anderson (North Coast Courer) Picture: The iconic 'Hole in the Wall' at Thompson's Bay is a roadmap to our past.

If you know where to look along our coastline, you can find the fascinating geological history of the world sketched on the faces of the rocks. Stories of ancient sandworms, petrified vegetation and millions of years of underground movement are all told in unison by nature itself.

The Courier recently joined expert nature guide John Roff for a tour of the geology of Thompson’s Bay and the wonderful history it reveals. Roff grew up in Zimbabwe but has been hosting nature tours across KwaZulu-Natal and South Africa since moving to Pietermaritzburg in 1989.

Thompson’s Bay is an excellent place to go rock spotting.

He has a particular interest in geology and sees it as a perfect way to tell a story about how our world came to be, illustrated by the rocks around him.

“I of course want my tours to be educational, but mostly I hope to create curiosity for nature,” said Roff.

“Nature is restorative and offers so much when you know what to look for.”

The tour group at Thompson’s Bay last week.

Taking off from the Thompson’s Bay parking lot last week, Roff led a group of 10 through the locally famous ‘Hole in the Wall’ to the bay on its southern side.

The walkway is itself a record of nature’s relentless consistency.

“The angle of the rock is such that it has caught the prevailing wind for thousands of years,” said Roff.

“Sand, water, salt and anything else carried by the wind battered the rock until it finally gave way and a gap was left.”

John Roff is an expert nature guide and has been taking tours around KZN since 1989.

If you check back in a few thousand years, that hole will have increased in size. Wait long enough and it will open to the sky.

“The world is a bit more flexible than we think. The surface of the Earth is not fixed.”

A good way to see the evolving nature of Earth is to look back towards the the hole once through on its southern side.

The holes in these rocks are modern in geological terms and were formed when salt got trapped and slowly expanded.

There is a grand rock wall that shows both sedimentary (sandy) and igneous (volcanic) rock and how they came to be combined. Sedimentary rock can be identified by layers. Consistent layers show a calm formation, while jagged lines show a turbulent period. Darker lines typically indicate a higher carbon content, which means they were likely formed by ancient vegetation becoming petrified.

This is a special piece of gneiss that has made its way to the surface after intruding the surrounding igneous rock.

Igneous rock is made up of solid pieces rather than layered lines, as it cooled from molten lava into individual formations. During the formation process, sedimentary and igneous rocks might become combined, like at Thompson’s Bay, when meeting at high pressures caused by shifting tectonic plates. Once linked, they slowly worked their way to the surface and become visible for us walking along the beach today.

Another good place to see the eras of movement is on Thompson’s Bay’s main beach.

Roff shows how consistent layers distinguish sedimentary rock.

A line of rocks jutting into the sea carry a special visitor. Set in the igneous rock formation are small deposits of gneiss, a metamorphic rock that rarely reaches the surface and normally has to be mined. Small deposits of it were caught by the igneous rock as it made its way up and are now on display for all beachgoers.

“This is one of my favourite geological sites in all of KZN. The temperature, viscosity and timing of the rock formation all had to be perfect for this to reach the surface.”

“They are called xenoliths, which means foreign rock, and are quite unusual.”

Next time you walk along the coast, take a moment to read this roadmap to our past. A guide certainly helps and Roff intermittently visits the North Coast.