- © 2008, The Clay Minerals Society
Canteen Koppie, a national monument of historical value, occurs next to Barkly West on the north bank of the Vaal River. Diamonds were discovered there in 1869 and it became the first alluvial diamond diggings in South Africa. The digging continued, albeit on and off in the years leading up to 1948 when the site was proclaimed a national monument. The sediments occur in a structurally controlled and glacially modified depression within the andesitic lavas of the Archaean Ventersdorp Supergroup. The fluvial gravels were deposited, and locally mixed with the colluvium, in the downstream end of a palaeo-loop of the Vaal River as a splay deposit where the channel abruptly widens as it exits this narrow loop.
The gravel accumulation has been described as the 12 m to 16 m terrace package linked to the Younger Gravels of the Vaal Basin and correlated with the Pleistocene Rietputs Formation. There are two gravel facies associations and one sand facies within the splay unit. Colluvial facies are dominant particularly in the upper part and are composed of large andesite fragments which are mostly sub-angular and lacking obvious abrasion features suggesting that these are of local derivation. The gravel of the fluvial facies are crudely cross-bedded and consist of small to medium sized exotic sub-rounded pebbles that have been mixed with the local andesite boulders in the toes of the scree deposits. These facies are more prominent in the lower part of the succession. The red sand facies occurs as thin cover particularly in the distal part of the gravel units and increases in thickness in the lee of the gravel splay. The exotic clasts in the fluvial gravel are derived from the palaeo-Vaal, erosion of nearby Dwyka sediments which can still be found along the north bank of the loop, and by reworking of higher level and older gravels, remnants of which are still present on top of the hill at Canteen Koppie. The input of the coarse andesite clasts is linked to scree slope deposits fed by exfoliation of local bedrock from this hill. The upward coarsening trend of this infill reflects the gradual abandonment of the loop by the palaeo-Vaal and its inability to remove the coarse colluvium during those latter stages of its occupation of this palaeo-loop. A climatic change to more arid periods might have had some influence.
Canteen Koppie has also produced an abundance of Acheulian Stone Age artefacts. These are present in both sedimentary facies suggesting that this splay deposit is at least Late Pliocene to Lower and Middle Pleistocene in age. Recent dating of the overlying sands indicates that these are at least 125 000 years old. Finally an analysis of the mining records suggests that this splay deposit might have produced between 10 000 carats and 15 000 carats of diamonds which would have expressed itself in the region of three to five carats per hundred tonnes. The oversize clasts of the scree deposits would have acted as important traps for the diamonds.