Where trees turned into stones

Isdore Guvamombe

RECENTLY I took off for Mbire to find a forest where trees turned into stones. Strange as it may appear and sound, it was worthwhile excursion into an abyss of prehistoric happenings.

The road to Mbire leads due north of Harare. For over 300km it runs between vast farmlands, their homes set far back out of sight behind acre upon acre of intermittent maize and tobacco fields in Mazowe, Concession, Mvurwi and Guruve.

Thereafter, three times the road winds its way between communal lands dotted with clay-walled huts and tiny fenced-off fields, then it straightens and flows through, former commercial farms in Horseshoe then Bakasa on the edge of the Zambezi Escarpment.

On the side of the road, you would have noticed men hovering by the side of whitewashed shops and beerhalls in their faded formal clothes.

Women saunter through the grass verge, children at their heels, toddlers clad on their backs, firewood or water buckets balancing delicately on their heads.

On rare occasions you would have come across a spirit medium clad in black regalia that exuded an aura of sacred spirituality. Jam-packed buses, tractors, trucks, ox or donkey-drawn carts and cyclers dash along or crawl between the villages and, the business centres and farms.

At Bakasa, just as the road starts to descend it makes a wide sharp curve around the edge of the Zambezi Escarpment, and lo and behold! . . . the Zambezi Valley spreads out yawning flat and wide, north, north and further north, east and west. Vast!

On a clear day, the Zambezi River sparkles from a vintage point of the escarpment, some 150km away. And then, within 15km, the road altitude falls by a whopping 6 000 metres! The break in altitude is felt sharp and critical as it announces your arrival at Mahuwe Business Centre.

At Mahuwe, the gravel road begins and the journey slows down. Crawl! The heat is fierce, remorseless, baking and caking.

As the road runs north to Mashumbi Pools, the great, stout hunches of the mountains rise up blunt and grey behind. Four large rivers, Dande, Hunyani (Manyame), Kadzi and Angwa (Hangwa) drain from the escarpment into the Zambezi Valley and villages cling on to their banks and those of their tributaries. Away from these rivers and villages, Mbire is wild with pale sharp grass, dense with thin contorted trees and scattered with towering baobabs.

About 10km after Mashumbi Pools, small dotted homesteads pronounce Kemanzambara Village and on its eastern verge lies the petrified forest. Stone and not wood trees lie stubbornly on the ground. Villagers here say their trees’ story is older than history. Trees turned into stone, hard stones. If it was fossilisation it was a strange one.

Located just 11km to the north west of Mashumbi Pools, just off to the side of the road to Kanyemba via Angwa, one will see a sign directing you to the Petrified Forest. Follow the directions and you will soon find yourself being enthusiastically taken through something older than history, a multifarious array of trees that turned into stones. Yes, stones!

While the Petrified Forest is not a forest by the traditional definition of the word, the area is the location of a number of preserved tree trunks, one of which measures up to 4m in length. The Petrified Forest is situated on a huge savanna woodland forest dominated by mopane trees.

Here, a large collection of petrified trees, their stumps and trunks lie scattered as a reminder of pre-historic events that defy magazine description hyperbole. The trees-turned stones found in the site are of coniferous trees (muchenyaropa) which flourished in the area some 200 million years ago. These are used as environmental indicators of the type of climate ecology that prevailed during that aeon and how it has changed to the current scenario.

The coniferous trees that grew then are still found dotted in the area, some of them small, others extremely huge with trunk diameters of a metre. The current dominant vegetation is mopane woodland which is characteristic of hot areas and valleys.

The deduction therefore, is that the climate then was like that of temperate regions.

The Dande fossil-bearing sedimentary rocks have alternating bands of coarse and very coarse aggregates. That, coupled with annual growth, suggests that there were seasonal variations during the Karoo times. This is a most distinctive and rare specimen which would be a centrepiece of any petrified wood collections, and would serve as well as a handsome conversation piece for the more casual buyer.

The trees must have lived over 190 million to 200 million years ago and were ‘‘petrified’’ or solidified after being embedded in a thick alluvial sand deposit of the Zambezi Valley. As the tree trunks were buried under the sand they were cut off from oxygen and silicic acids began to build up and break the wood particles down, replacing them with quartz particles.

Today visitors will be able to view perfect replicas of these 200 million year-old-tree trunks, with the rings, bark and nodes of the branches perfectly preserved and turned to stone. Looking to travel to Petrified Forest of Mbire? Petrified wood there is almost solid quartz, weighing heavily. It is so hard, you can only cut it with a diamond tipped saw! Not chopping like wood.

The main rock type here is sedimentary: red pebbly or mature sandstones formed during the pre-historic times, others want to call it Jurassic times. These rocks show alternating coarse and fine sedimentary layers. Logs are characterised by narrow, horizontal vascular traces which traverse the entire radial width of the secondary xylum. In life, these vascular traces would have terminated on preventitious buds which were deeply embedded within the bark, but which typically were not silicified during the petrification process.

These buds would have had the capacity to develop into epicormic shoots when the crown foliage of the tree was damaged. Numerous shoot traces are readily apparent over the outer surface of well-preserved specimens.

The Dande petrified trees site lies within the Karroo super group of the coal-bearing Mid Zambezi Valley basin and partly in the Zambezi Mobile Belt of northern Zimbabwe. The trees are still in their original environment and many of them are in a well-preserved state. There are those that have barely been moved and those that are scattered.

The barely moved petrified trees consist of the well- preserved and unmoved fossil wood logs in the area. These logs appear to have maintained their position where they have been silicified after failing. Annular rings are also distinguished by colour.

These logs range in size from short blocks of intact lengths of about 1-1,2m to long trunks of 10 metres intact length.

The scattered wood logs appear to have been moved by water action and are mostly associated with low-lying areas such as streambeds. The fragmentation of fossils may have been caused by the smashing of the wood against each other. The area covered by both the barely moved and the scattered fossils is approximately 10 hectares. Scientists believe the fossils were formed from sandstones of the Escarpment Grit Formation of the Upper Karroo group.

Comparison with other similar properties
Similar fossil wood exists in the Karroo rocks of Southern Zambia and South Africa and the Arizona Petrified Fossil Forest National Monument in the United States of America.