Setting the scene: the first part of a series of articlesthat examines why a remarkably high number of newspecies seem to originate where the Cape Floristic

Region meets the Karoo

The Cederberg-Tanquatension zonetension stretching, being stretched, stress by which bar, cord,etc. (read ecosystem) is pulled when it is part of a system in

equilibrium or motion.(Concise Oxford Dictionary)

Abseiling off aghost water-en after winter rain. In wetter

s waterfall must have flowed andded the cliff. Now Ficus cordata trees

growing from the cliff are getting their rootsinto the fractures and exerting enough forplaces to weaken and even crack thePhoto: Roger Diamond.

TOP: Blinkberg - some time around 370 to 400 million years ago clay, silt and sand was washed off anancient coastline and deposited in a shallow marine environment to form the Bokkeveld Group. Layers of clayrich rocks that form the gentle slopes you see, were deposited in deeper water, while sandy layers that formsmall cliffs today, were once ancient sand bars or channels in a delta. The capping quartzite band is the baseof the later geological succession, the Witteberg Group, and probably represents ancient beaches or shallowsand bars, formed in storm events. Photo: Roger Diamond.ABOVE: Doring River Canyon - the tranquil water of the Doring River here belies the turbulence that comeswith winter floods. The desert slopes in the background, almost totally devoid of any vegetation, contrast withthe thick riverine flora. Photo Roger Diamond.

One of the driving forces behind plantspeciation (the creation of new species)in the Cape Floristic Region is the jux-taposition of contrasting habitats overshort distances. Simply put, it is theshifts in rainfall - amount and season-ality - and variation in geology, thathave prepared the Cape FloristicRegion for commanding global centrestage as a biodiversity 'hotspot'. Thespeciation process is more evidentwhere the Cape Floristic Region abutsthe Karoo: where the roughly L-shapedform of the Cape Fold Belt forms awedge between the coastal lowlands ofthe western and southern Cape, andthe vast interior expanse of the Tanquaand Great Karoo.

The Cederberg-Tanqua Karoo transi-tion is our area of focus. This is thedomain of the seasonal river - here theGroot, Riet, Matjies, Tra-Tra andBiedouw all conspire to join tIt Doringwhich becomes the conduit for thewater flowing from the easternCederberg and much of theSwartruggens.

The building blocksBut let us ftrst examine the underly-

ing geological building blocks of thismagJ;liftcent system. The Cape Fold Beltis the erosional remnant of a mountainrange that was formed some 215 mil-lion years or more ago, whenGondwanaland underwent a ftnal com-pression, leading to the building andfolding of the progenitors of the moun-tain ranges one sees today. Our arealies on the very edge of these foldedmountains and geologically comprisesPalaeozoic sediments of the Bokkeveldand Witteberg Groups.

The Bokkeveld succession is domi-nated by clay-rich mudrock that

weathers relatively easily, forming val-leys or gentle slopes. Examples are themudrock-floored valleys of the Ceres,Agter Witzenberg (Visgat) andKouebokkeveld regions. By contrast,the Witteberg Group is dominated byquartzites that are highly resistant toweathering and therefore often formsteep scarps or small mountains; forexample, the Blinkberg (1456 m) andTierhoksberg (1410 m). The 'classic'Table Mountain-Bokkeveld-WittebergGroup sequence, outcropping succes-sively from west to east, characterizesthe drive northwards from the turn-offto Katbakkies Pass (Swartrug Road) toMatjies River, and onwards to nearWuppertal. The Swartruggens Range isan excellent example of the ruggedmountainous terrain that the WittebergGroup can create.

To complete the sequence, theWitteberg Group in turn dips east-wards under the Karoo Supergroup.

Here glacial Dwyka and marine Eccasediments form the geological base forthe Tanqua Karoo and Roggeveldescarpment on its eastern border.

Apart from folding, another feature ofimportance in moulding the landscapehere is the presence of major fracturesor faults in the hard quartzites. Thesehave arisen mainly through the processof crustal compression during moun-tain building and crustal stretchingduring the break up of Gondwana.Faults occur in three main orientationsthroughout the south-western Cape:NW-SE, NE-SW and E-W. A typicalexample of a NW-SE fracture is theAlgeria valley. These fractures consti-tute weak zones along which riverscarve their paths. The Doring is a goodexample of a fracture-controlled river,even undergoing a 90° bend upstreamof Elandsvlei.

Another interesting feature of thisarea i8 its dry valleys and 'waterfalls'.

September 2004 Veld&Flora 115

LEFT: Dead River Valley - the Matjiesfontein River Kloof is dry and erosionally dead, having been carved out intimes of wetter climates when rainfall was high and fynbos and even forest covered vast parts of what today isKaroo. There is not even a scrap of riverine scrub to indicate groundwater flowing under the channel.Photo: Roger Diamond.RIGHT: Fractures to valleys - the blocky nature of this outcrop of Witteberg Group is caused by the fracturesthat can be seen clearly in the wall of the gUlly leading down to the Doring River. This gully would itself havestarted as a fracture hundreds of thousands to millions of years ago and one day will widen to a ravine or val-ley. Photo: Roger Diamond.

The deeply incisedcanyons of the Riet(Kagga Kamma) andDoring rivers are evi-dence of a much wetterera (the Tertiary) whenrainfall was high andfynbos and even forestcovered vast parts ofwhat today is Karoo.

Further readingMacRae, C. 1999. Life etched in stone. Fossils ofSouth Africa. Geological Society of South Africa,Johannesburg.Slingsby, P. & Coombe, E. 2002. Cederberg: Themap. Baardskeerder, Muizenberg.White, M. E. 1986. The greening of Gondwana.Reed, Frenchs Forest (NSW).

116 September 2004 Veld&Flora

Past climates, together with sporadiccrustal uplift of the whole region, sawimmense erosion that produced thedeep valleys (excavated to depths ofover 350 m) we see today. Thus thedeeply incised canyons of the Riet(Kagga Kamma) and Doring rivers areevidence of a much wetter era (theTertiary) when rainfall was high andfynbos and even forest covered vastparts of what today is Karoo. Dry cliffsand canyons with cliffs at their lipshave developed where there is a partic-ularly thick or hard layer of quartziteoverlying relatively softer layers. Thishard layer can act as a barrier to ero-sion of the landscape, but eventually,given enough water and time, the hardlayer is undermined and breaks so thata gully or valley can work its way intothe landscape. This process ofupstream 'eating' into the landscape bya watercourse is called headwarderosion.

Prehistoric lifeEmbedded within the alternating

mudrock and sandstone formations ofthe lower Bokkeveld Group is a richvariety of marine fossils, predominant-

ly the shells of invertebrates such astrilobites, brachiopods (lamp shells),crinoids (sea lilies) and molluscs. Thisfauna inhabited the shallow seas alongthe margins of the supercontinentGondwana (Africa plus the othersouthern continents of today) some400 million years ago in the earlyDevonian Period. The presence of otheranimals is indicated by a wealth oftrace fossils in the northern Bokkeveldoutcrops. Many Mid Devonian (c. 375million years ago) fossil fish, landplants, molluscs and trace fossils haverecently been collected from the upperBokkeveld rocks of the easternCederberg.

The fossil record of the WittebergGroup is much sparser. It is dominatedby a limited range of trace fossils andlocally-abundant fragments of primi-tive vascular plants, mainly lycopods orclub mosses, that lined the banks ofrivers in Devonian times and werewashed offshore into coastal seas dur-ing floods. Unusually, these fossils arefrequently found in quartzites ratherthan the finer-grained siltstones andmudrocks associated with quiet-water

deposition, which generally favoursfossil preservation. The most strikingand abundant Witteberg fossils are theintersecting, downward-spirallingtraces called Spirophyton (because theywere initially thought to be the remainsof fossil algae). When an organism isnot itself preserved but rather leaves atrace of its activity, it is called a tracefossil, and such an example is found inthese intersecting Spirophyton bur-rows. These were generated at somedepth below the sediment/water inter-face and probably over a considerableperiod of time. It seems they were con-structed in a fairly pure sandstone orquartzite (metamorphosed sandstone)which in itself would not have repre-sented a rich food source for a deposit-feeding organism. They probably repre-

sent the intricate faecal depository ofan animal which was 'gardening' mutu-alistic microbes on its own dungheap(like termites do).

The uppermost beds of the Witteberg(Waaipoort Formation), which crop outalong the western margins of thesouthern Tanqua Karoo, have yieldedthe well-preserved remains of freshwa-ter bony fish and vascular plantremains (mainly lycopods) of EarlyCarboniferous age (345 million yearsago). The fish corpses are sometimesfound packed densely together on bed-ding planes suggesting mass mortality- caused perhaps by sudden overturn-ing of the water body, an influx offreez-ing water, or poisoning by planktonblooms.

The glacial sediments of the Dwyka

Group are - not surprisingly - poorlyfossiliferous, although concretionscontaining plant and fish fragmentsare known from the Tanqua region. Theimmediately overlying, post-glacialmudrocks of the lower Ecca Groupbeds in the western Tanqua are ofpalaeontological interest mainly fortheir well-preserved fossil fish, crus-taceans and mesosaurid reptiles (theoldest known sea-going reptiles) of MidPermian age (270 million years ago).

In a forthcoming article, we will focuson the plants, animals and humancolonists of this special transition area.

If you are interested in finding out more abut thisfascinating area, and would like to join the Rivers

Dancers on an expedition, please contact Barrie orUshchi at 021-685 5445, 082 579 7040 or email usat coastec@mweb.co.za.

LEFT: Faecal cesspits? Two samples of Spirophytontrace fossils, perhaps faecal storage burrows madeby sediment dwellers around 370 million years ago.Photo: Roger Diamond.

BELOW Giant or club mosses living 400 million yearsago were amongst the first vascular plants (asopposed to algae and fungi) on land and have leftremains in the form of a spiky stem (lower mould inthe pic) and atough, needle-like leaf (below lens fil-ter). The left mould, also of a spiky stem, still hassome of the internal cast material, showing howsome fossils, after decomposition of the soft pith,are 'in-filled' with the same material that surroundsthem. Photo Roger Diamond.

LEFT: These leafy looking, fern-like things are indeedgrowths, but of minerals, not plants. The yellow isiron oxide and the black edge is manganese oxide.Formed in fractures by precipitation from high tem-perature fluids, these dendrites grow relatively fast, arefreshing change from the millions of years of manyother geological processes.Photo: Roger Diamond.