This earthcache will take you to a view of a cave high above today’s sea level. There are several caves about and another fine example of one is on the walk to the waterfall - see Batman Returns GC1K62K - a cave that you can enter for approximately 100m.
An Earth cache is a special type of Virtual Cache that is meant to be educational. Therefore to log a find you must demonstrate that you have learnt something from the site and experience.
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Logging Tasks:
1) As an optional request please take a photo of you and your GPS with the west bank cave in the background and post it with your log.
2) Estimate the west bank cave height above current sea level.
3) In your own words describe how this cave was formed.
4) How do you think this gorge was formed?
5) At the listed coordinates (which should take you to the end of the swing bridge) take a look at the basement rocks and describe them.
The story of this cave
This cave was cut by wave action along a line of weakness in the rocks many thousands of years ago, when the sea levels were higher. Since then, collapse has built up a tumble of roof blocks in the entrance. Stone Age people used the cave for shelter. The most recent Later Stone Age people left behind proof of their presence in a shell midden that was uncovered in the excavation.
The shells and bones found in the excavation shows the range of shellfish that the Later Stone Age people collected, the species of the fish that they caught and the kind of animals that they hunted more than 2000 years ago. The midden must be at least 2000 years, since no evidence of pottery has been discovered in the excavation. Let’s look at how this cave was formed and interesting facts relating to different sea levels.
Geology of the area
The major part of the lower plateau consists of Cape sediments (Bokkeveld and Table Mountain sandstone) and granite bosses.
Characteristic of the Tsitsikamma area is the well-defined wave-cut platform, with an elevation of about 150 m formed during a period of high sea level in the Tertiary (70 - 20 million years ago) when the seashore lay along the Tsitsikamma Mountains. Rising of the land exposed the wave terrace and the rivers, which were rejuvenated, steadily cut deeply into the landscape behind the retreating sea. The many kloofs are evidence of a fairly rapid regression.
Changing global temperatures through time have affected sea levels.
On a timescale of million of years, global sea levels has risen as much as 300m above the present level and dropped 200 metres below. Astronomers and marine biologists have shown that warm-cold-warm cycles, about 100 000 years long, have been a feature of Earth’s history for at least the past 2-3 million years. During cold (glacial) cycles the water is bound in continental ice sheets and sea level drop about 130m below our present sea level, but, during warm (interglacial) times, like those of the past 6 000 years, the ice melts and the sea rises to levels within a few metres of that today. During the Cretaceous period, 140-80 million years ago, temperatures and sea levels were highest and lapped our coastal mountains. There was a great ice age during the Carbonisferous period and scratch marks made by an ice sheet are preserved in the underlying bedrock about 290 million years ago near Kimberley. Also refer to Geocaches GC1XWW1 in Durban in KwaZuluNatal and GC1E06B near Nieuwoudtville in the Western Cape.
Marine Geologists can determine sea temperatures in the distant past.
· Seagoing Geologists take cores penetrating up to 60m into the sea sediments
· They examine these cores, layer by layer, looking into the sand and the calcite shells of microscopic protozoa, called foraminfera (forams).
· They date the sediment layers using carbon dating and other radiometric methods.
· They can determine the sea temperature during which each layer was deposited by analysing the relative proportions of oxygen isotypes in foram shells using a mass spectrometer.
How ice sheets affect sea levels and oxygen isotopes
The waxing and waning of large ice sheets on land can affect the ratio of light and heavy isotopes of oxygen in seawater. Water with light oxygen evaporates more easily from the oceans and some will eventually accumulate in ice sheets. During the cold periods the ice sheets grow, taking water out of the oceans and lowering the sea levels and the seawater becomes enriched with heavy oxygen. The calcite shells of microscopic forams growing in cold periods also are enriched in oxygen. Forams thus contain a record of past oxygen isotope conditions in ocean and sea temperatures.
What causes regular, long-term sea levels to rise and fall?
Astronomical evidence explains the warm-cold-warm cycles of the past 2-3 million years.
About 100 years ago Milankovitch discovered a long term, gradual variation of the Earth’s orbit around the sun from nearly circular to highly elliptical over 100 000 years. When the orbit was nearly circular, summers were cooler, and continental ice sheets that had accumulated during winters did not melt and sea levels were low. In contrast, when the Earth’s orbit is elliptical, it approaches closer to the sun, summers are warmer, ice melts and sea levels rise.
Acknowledgements and references
Text and Research – John Rogers and Margo Branch
Earth story (Lamb) E
Ice Age of Norway N Coe C
Mining and Geology of SA
CSIR research report 418