Rochester Falls
Rochester Falls is the widest waterfall in Mauritius. The water flows over its famous feature, the basalt columns, before falling about 10m into a basin below.
You will find the falls surrounded by beautiful, quiet nature and lush plantations of sugar cane and bananas near the village of Souillac. Simply follow the handwritten signs from the main road through the sugar cane fields on narrow roads. Taxi drivers will also take you there.
Note that the roads may be impassable and flooded after prolonged and heavy rains. Parking for about 5 cars is available near the falls.
Another option is the hiking path starting on the main road between Souillac and Gris-Gris. It is an easy walk, suitable for families.
It is possible to swim in the basin and even jump into it from the top of the waterfall. This is often dangerous due to strong currents, low water levels, or hidden obstacles in the water, so only do it at your own risk. Always ask a local where to jump and whether the water is deep enough at the time.
Please watch your step and keep an eye on children that accompany you, there are no guard rails at the top of the falls and the river current is very strong at times.
Please be aware that there have been reports of tourists being ripped off by locals who charged unreasonable amounts for "watching the car", jumping for pictures, guiding around the falls, etc. You are a geocacher and you do not need a guide here. Whatever you might pay, it will not be enough.
Don't encourage the touts by accepting services. DO NOT PAY ANYTHING. You will only make the problem worse for other visitors.
The columnar basalt at Rochester Falls was uncovered by the Savanne River washing away all the soil, leaving an unobscured view of the dark colored, columnar jointed volcanics. What we see here today is evidence of a large volcanic eruption. The area around Souillac is quite young, the most recent eruptions have happened in the last 100.000 years, while the island of Mauritius rose up from the depth of the Indian Ocean 8-10 million years ago. Masses of hot viscous basaltic lava have covered the area and formed this geological phenomenon, which can be observed here.
The environment into which hot liquid lava is thrust, very much defines its later shape after cooling. Basalt can therefore be found in many different forms, such as in a pillow shape often found under water. It is among the most common types of rock on earth, covering much of the ocean floor, naturally dominates on volcanic islands and even covers vast landscapes on the mainland in the form of continental flood basalts.
When a thick lava flow cools down, joints and fractures form in the rock from contraction in the lava during the cooling process. As a lava flow cools from the outside edges toward its center, molten lava slowly solidifies into rock in the same direction. While a lava flow can shrink vertically without fracturing, it can't easily accommodate shrinking in the horizontal direction unless cracks form. As more rock forms toward the center of the flow, an early crack will grow toward the center of the flow's molten core. An extensive developing fracture network results in the formation of columns. These structures are often hexagonal in cross-section, but polygons with three to twelve or more sides can be observed elsewhere. The diameter of the columns depends on the speed of cooling. Very rapid cooling may result in rather small (<1 cm diameter) and fragile columns, while slower cooling is more likely to produce large columns. Basaltic lava flows immediately begin to cool from the top, bottom, and sides toward the center, where the most heat remains. While the bottom of the flow cools slowly because it is insulated by the ground below, the top cools more rapidly because it is exposed to the atmosphere's cooler air, wind, and rain, as well as standing or running water.
In some basaltic lava flow exposures, the joints form near-perfect hexagonal columns. This hexagonal symmetry is best seen in a top-down view, looking at a cross section of the column geometry. You may observe this right here, by standing at the top of the waterfall. Mathematical descriptions of the cooling process predict, that cooling flows should form perfect hexagonal columns with a series of 60- and 120-degree angles. These angles are the most effective at relieving the complex thermal stresses acting on a cooling body of rock. As it turns out, however, few things in nature do exactly what they should, and the result is that we see columns of different shapes—not always perfect hexagons. They look the way they do because the cracks occurred where they needed to be in order to relieve the internal thermal stresses. Even though many of the columns are not hexagonal, they have an overall hexagonal symmetry.
The difference in cooling rate often forms two distinctive types of joint patterns. Joints are fractures in rocks, and columnar joints are a specific type of joint pattern. The slower cooling bottom section typically forms thick, regular-sized columns called "colonnades" by geologists.
The faster cooling top section often forms thin and less regular columns. The resulting "rough and jagged" column pattern is called "entablature". It may be difficult to recognize entablature sections as columnar basalt because its highly fractured appearance lacks a dominant orientation of the columns.
At this location, we clearly find colonnades, very evenly shaped columns from bottom to top. This may be the result of very even cooling, or erosion of the top entablure layer.
Among the world's best-known examples of these "columnar basalts" are the Giant's Causeway in Ireland and the Devil's Postpile in California, USA. Columnar basalts have even been found on the surface of Mars, evidence of earlier volcanic activity.
At Rochester Falls the basalt has been transformed into steps with differing length columns: The work of thousands of years of erosion by the river water, as well as changes in temperature, breaking off the columns and smoothing their surface by abrasion.
Logging the earth cache:
Send me a message with all the answers to the questions posted below. You may log the cache straight away, but I will delete all logs with incomplete or incorrect answers. Do not reveal any answers in your log.
You are encouraged to attach pictures of yourself, the waterfalls or your adventure to your log, but this is not a logging requirement.
Please complete these tasks and questions to log the earth cache:
- Measure the average diameter of the columns. If you do not have a ruler or measuring tape with you, please approximate the diameter by placing your GPS (or any other object) on a column and multiply with the length of your GPS later.
- How many sides do most of the columns at Rochester Falls have? What would you call this geometric form?
- What do you think how the basalt columns were formed?
- Would you think the lava here has cooled slowly or rapidly? Explain what evidence found here supports your conclusion.
- Why do you think the lava can shrink vertically without fracturing, but not horizontally?
- Name another place on earth (or another planet), where columnar basalt has been found, however do not use the places listed above.
Feel free to answer in your own language.
Congratulations to the first finders:
- placc
- geosackboy
- Hägar von Buron & die Hausemer