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A sea cave, or littoral cave, is a cave that has been formed on the shoreline by wave action. The force of sea waves breaking against a rocky coastline or cliff is tremendous, and removes the rock by erosion. Even moderately large waves have an impact force of a few thousand tons per square metre! Waves can also carry sand and pebbles, adding more destructive power to the mix. In addition, air trapped within any voids in the rock is compressed and forced into cracks creating a splitting action. On top of these physical forces, the chemical weathering action of the salt water getting forced into each and every crack or void further weakens the rock, making it more susceptible to the physical erosive forces.In order for a sea cave to form, the rock on the shoreline must have a weak zone. This could be either a softer formation of rock or a fracture or bedding plane. Erosion from the wave action occurs more rapidly in the weak zone, and so it deepens and widens at a greater rate than the rest of the host rock until a sea cave has formed.
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Sea caves are found all over the world, and in all rock types. Areas known for large concentrations of sea caves are the Pacific Coast States of the USA, the Na Pali coast of Kauai, the Greek Isles, the British Isles and New Zealand. Matainaka Cave on New Zealand’s South Island is the world’s longest sea cave by length, measuring in at 1.54km while Riko Riko Cave on Poor Knight Island off the coast of New Zealand’s North Island is the world’s largest by volume, measuring over 200 000m3.
The St Blaize Cave at the listed co-ordinates is located in Mossel Bay, and is a fine example of a sea cave and a popular spot for whale, dolphin and seal watching. There are many more sea caves along this portion of the coastline, including the Pinnacle Point Caves a few kilometres south.
The St Blaize Cave is also an important archaeological site, first excavated in 1888 by George Leith and later by John Goodwin in 1929-1930. More can be read about the stone-age findings in the information boards within the cave. Parts of the cave were recently explored as part of the Mossel Bay Archaeology Project (MAP), which is the currently the largest scientific project of its kind in the world, studying finds at various sites in the Southern Cape. Interestingly, discoveries in the Pinnacle Point Caves (not open to the public) are suggesting that the humans that existed here about 165 000 years ago are the core population from which all humans today have stemmed.
References:
Press, F. and Siever, R. (1986) Earth
Moore, D.G. (1954) Origin and Development of Sea Caves
Bunnell, D. (2004) Littoral Caves
www.visitmosselbay.co.za
www.humanorigin.co.za
In order to log your find please email the CO with your own answers to the following questions. You are welcome to log your find before emailing the answers, but any logs unaccompanied by an email with suitable answers will be deleted:
- From your observations at the site, in what type of rock would you say the St Blaize Cave formed?
- Describe the appearance of the rock (noting the colour, grain size, presence of any layering and its orientation, jointing/fracturing, hardness etc).
- What are the two main physical forces that help to form a sea cave?
- Estimate the altitude of the cave, and then explain what this tells you about the current sea level relative to the sea level when the cave formed.
- The cave was home to stone-age dwellers a long time ago, but what species will you now find occupying the cave and surrounding area?
- It would be fantastic if you posted a photo of yourself at the cave together with your log, but this is optional.
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