The Red Stone Hills of Oudtshoorn are red conglomerates and are part of the Buffelskloof formation. These formations are also known as Enon Conglomerates. These formations are called Breccia and this is a rock that is composed of broken fragments of minerals or rock that are cemented together.
The conglomerates were deposited by rivers and streams while the breccias represent scree deposits. The precise age of the Red Stone Hills are not known because of poor fossil preservation. Oxidation of Iron present in the soil is responsible for the reddish color of the rocks.
During the early Cretaceous Period the ancient super continent of Gondwana split up and Africa was finally separated from South America, Australia, India and Antarctica. This split created stress fractures in southern Africa along pre-existing fault lines. As the stress fractures developed, so basins were formed.
One such basin is the Oudtshoorn Basin of the Eastern Little Karoo which extends as far west as Calitzdorp. This basin developed mainly along a deep fracture running from East to West along the base of the Swartberg called the “Cango Fault”. The basin floor subsided mainly along its northern margin (a half-graben). Accompanying this, rejuvenated rivers on the new highlands of Gamkasberg and Rooiberg as well as from the uplifted area to the North, flowed into the Oudtshoorn basin.
They carried with them vast amounts of scree and rock, fractured from the Table Mountain sandstone and quartzite’s from the mountain chains surrounding the Little Karoo and deposited them on the emerging lake shores. This is why these basins are referred to as sedimentary basins.
Traditionally the Enon sediments in the Oudtshoorn basin are divided into four distinct layers: lower (base) Enon conglomerate, white small pebble conglomerate, buff Enon sandstone and upper (top) Enon mudstone and conglomerate. The upper (red) layer of mudstone and conglomerate almost encircles the basin.
Over the last 100 million years, erosion of the sediment within this basin has hollowed out the Riet Valley and has left these isolated koppies or mountains in a band stretching from De Rust almost to Calitzdorp. Initially eroding quite quickly, they subsequently formed an outer crust which was somewhat more resistant. Weathering has broken through some of this crust resulting in small caves and overhangs as is evident here in the Rooikoppe Mountains.