These hills have their origin in the Jurassic and Cretaceous periods (the age of the dinosaur!) spanning a period of 180 to 70 million years ago. This is fairly recent if it is borne in mind that the age of the earth is estimated at 4 500 million years. Until the commencement of the periods mentioned above, Africa, with South America and Antarctica, formed a unified landmass known as Gondwanaland. During these periods southern Africa split from Antarctica (to which Australia was still attached) and South America. The importance of this split for present purposes is that it created stress fractures in southern Africa.
At the stage these stress fractures were produced, the ancestors of the Cape Fold Belt (The major mountain ranges of the Cape such as the Swartberg, Outeniqua and Tsitsikamma ranges) already existed. This belt of mountains contained faults (that is, fractures in the earth's crust) associated with its own origin. The stress of the continents shearing away caused basins to develop along the already existing faults.
The area around Outshoorn is associated with the hidden Cango fault extending east-west for about 90 kilometers along the foot of the Swartberg mountains (corresponding roughly to the area between Calitzdorp and De Rust). This area, during the periods under discussion, became a landlocked basin as a result of the stress forces produced by the drifting continents. It is one of several near-identical basins of various sizes strung out along the three main northern boundary faults which can be traced intermittently from Worcester to Plettenberg Bay, from Touwsriver to the Gamtoos Valley (the Oudtshoorn basin belongs to this set of basins) and from Willowmore to Alexandria. The creation of these basins involved the dramatic sagging of the landmass closest to the fault. In time this basin was filled with sediment from higher areas (mainly the Swartberg range) primarily by means of alluvial fan deposits. The sediment is more than 3500 meters thick along the Cango fault. The sediment deposited initially along the Swartberg range was later, to a substantial degree, swept in the De Rust direction.
The main type of sediment filling the basin is called Enon Conglomerate (named after the mission station at Enon (near Kirkwood) where a prime example thereof can be found, and consists of sandstone, mud and quartzite in a matrix - basically a mix of pebbles suspended in a sand "cement". The courser material (quartzite and mudstone boulders derived from the Table Mountain and Bokkeveld groups), up to 2 meters in diameter, is deposited at the base of the conglomerate. Courser material was deposited first because of the initial large difference in elevation between the rim of the basin and its base - boulders were swept to the bottom of the basin before they could be eroded to finer material. In time, as the elevation between the top and the bottom decreased, the material swept down was subjected to more erosion, leading to decreased grain size and smoother pebbles. The lack of distinctive beds, fossils and the poorly developed soil suggest short transport distances and percussion marks on these boulders indicate extremely violent flow conditions and flash storms. Two fractures in the Enon conglomerate are found near the northern rim of the basin: This seems to indicate movement along the Cango fault during sedimentation.
Traditionally the Enon formation in the Oudtshoorn basin is divided into four distinct layers:
- lower (base) Enon conglomerate
- white small pebble conglomerate
- buff Enon sandstone
- upper (top) Enon mudstone and conglomerate
In view of the complex circumstances at the time of formation it is unlikely that these layers will be found intact. The characteristic red colour is due to oxidation of iron particles in the formation. The red (upper) Enon conglomerate almost encircles the basin suggesting sedimentation from both north and (to a lesser extent) south. The age of the Enon formation is not the same everywhere. Very old pieces of rock (Precambrian - that is, from the first 4000 million years of the earth's history) are found in the conglomerate.