Why are South African mountains short and flat?

Question

I drove through South Africa from Cape Town to Mozambique and noticed that almost all heights and mountains were no higher than approx 200-400 meters and had a flat top, just like the iconic Table Mountain. Why is this the case? Thanks.

Answer 1

This is a great question! The simple answer is: Lithology and differential weathering!

To explain this in greater detail, let us simplify Table Mountain (and other similar features in the area) as having a "layer cake" stratigraphy. This means that layers of rock were deposited on top of each other in a flat orientation. When these layers of rock were deposited, we can assume that they were laterally continuous, meaning that they extended for miles (or kilometers) in all directions. Over time, these layers were weathered (broken down into sediment) and eroded (sediment was carried away); however, the rate of weathering and erosion depends on the type of rock. The layer that is currently at the top of Table Mountain (conveniently called the "Table Mountain Sandstone") is fairly resistant to weathering and erosion in the current climate of South Africa. This means that it takes longer for it to be worn away than other rocks in the "layer cake" (this process is called differential weathering). So, while other rock layers have been worn away, the Table Mountain Sandstone remains. This layer forms the top of most of these mountains, and they are all at about the same height (200-400m, as you pointed out) because the Table Mountain Sandstone was initially laterally continuous.

So, if the sandstone was laterally continuous, why are these tall and flat features not continuous in a single plateau? Well, just because the sandstone is *more* resistant to weathering and erosion, does not mean it doesn't break down at all! The breakdown of the sandstone can happen for a variety of reasons, even if it is much slower overall. The layer that sits right below the sandstone in most places is predominantly shale. (Remember that if a layer is lower than another in our layer cake, it is older.) Even though the sandstone forms a protective cap for the shale, the shale is much easier to weather and erode in South Africa's current climate. Therefore, as the shale breaks down underneath the sandstone, it can cause the sandstone on top to collapse from underneath. This leaves behind the table like features that dot the landscape.