Throughout the Barberton region, small ripple marks in some of the ancient sandstone bear testimony to the existence of tidal currents which migrated back and forth as the tides turned.
What looks like a bad erosion scar of white sand has a very different tale to tell. These sandstones tell the story of ancient beaches, tides and some of the earliest life.
The Moodies Group rocks are largely quartzites formed from beach sand that has been subjected to heat and pressure and are about 2.5 km to 3.5 km thick. The beach sand and marine sediments were deposited in layers and then tilted and uplifted over millions of years .
White tidal sands like these have also unlocked other new information – when this rock was formed the moon was much closer to earth, the tidal pull was stronger and the water rushing around the earth, with no land mass to slow it down, made for huge tidal water surges.
In some of these sandstones are the remains of very early life forms; layers of mats of microbes that grew in shallow water, using sunlight for their energy and producing oxygen as a waste product. It was organisms like these that over hundreds of millions of years helped produce Earth’s atmosphere.
The herringbone patterns in these sandstones tell us that currents flowed rhythmically back and forth over the sand, evidence of tides.
Herringbone cross-stratification is type of sedimentary structure formed in tidal areas, where the current periodically flows in the opposite direction.
During the conventional formation process of cross-stratification, sand grains saltate up the upstream side of the dune, collecting at the peak until the angle of repose is reached. At this point, the crest of granular material has grown too large and will be overcome by the force of the depositing fluid, falling down the downstream side of the dune. Repeated avalanches will eventually form the sedimentary structure known as cross-stratification, with the structure dipping in the direction of the paleocurrent.
In tidal areas, which have bidirectional flow, structures are formed with alternating layers of cross-beds dipping in opposite directions that reflect the alternating paleocurrent. These sedimentary structures are not common because they require the current to be equal in both directions, which rarely happens in nature. The time period represented by each cross-stratified layer is likely to be many years.
The pattern of the structure is said to resemble the backbone structure of a herring fish.
To log this EarthCache you have to e-mail me the answers to the following questions:
1. What is the approximate length of the exposed area?
2. Pick up a handful of the "beach sand". Describe the similarities and differences between this and the sand you will find on a beach today.
3. When was this geological formation formed according to the panel?
4. Describe any Herringbone structures that you can see. Pics will be welcomed (optional).
Uploading photos to the cache page is the best way to say thank you to the cache developer and to encourage others to visit the location but is only optional.
NOTES:
- There are no other facilities along the trail except for very nice picnic spots.
- The distance is about 40km from Barberton on a narrow and winding road up to the Bulembu border post to Swaziland.
- You can email me your answers in either Afrikaans or English.
Note that a handy guidebook specially compiled for this route is on offer at Barberton Tourism in town.
Enquiries: 013-712 2880.
Guided tours by Tony Ferrar on 072-376-2581 and Dave Mourant on 082-923-5366.
Thank you to the Barberton Chamber of Business, Barberton Tourism Biodiversity Corridor and all those involved to develop this geotrail.
