Did you ever wonder why Table Mountain was flat? If there is one thing that distinguishes one mountain from another is its shape and the quality of the soil. You are looking at the eastern slopes of Table Mountain, Fernwood Peak is the eastern end of the flat ‘table’.
The geology of Table Mountain is simple, it is essentially three different rock types stacked on top of each other. The top layer, the grey rock of the cliffs overhanging Kirstenbosch, is sandstone, a sedimentary rock. This is the Peninsula Formation, a 600 m thick slab of hard and durable, coarse, pabbly quartz sandstone.
But all this is better explained and observed by other Earthcaches such as GC60RPN Table Mountain. Here we want to focus on why its shape and what we can observe from here: The Tree Canopy Walkway (aka the «Boomslang») in the Kirstenbosch National Botanical Garden.
Mudstone in the middle
At the level of the contour path (clearly indicated on the info board) are alternating layers of siltstones, mudstones and sandstones. This is the 70 m thick Graafwater Formation, also a sedimentary rock. It is not easy to see from here as it is hidden by the vegetation. These formations are part of the Table Mountain Group and are the bottom layers of the Cape Supergroup.
Granite bedrock
The lower slopes and bedrock are Cape granite, an igneous rock. It is almost all covered by soil and vegetation, but you can see the granite exposed in the waterfall on the contour path in Skeleton Gorge. Ganite underlines Kirstenbosch and the southern Peninsula, and Malmesbury shales underlie the City and Cape Flats. Malmesbury shale is a sedimentary rock that was formed 560 million years ago. The Cape Granite intruded into the shale 540 million years ago. These two rock types formed an ancient coastal plain and river delta. The Graafwater and Peninsula Formations were formed from the mud and sand washed off an ancient continent and deposited on the granite and shale between 510 and 440 million years ago.
Ancient mountain
Table mountain is made up of the bottom layers of Cape Supergroup, and is part of the Cape Fold Belt Mountains. This mountain chain was formed between 280 and 235 million years ago. When it was young, it would have been as mighty as the Rocky Mountains in North America. The mountains we see today are all that remain. Most of them have been eroded away, the sand washed into the sea and deposited off shore to form mountains of the future.
In order to log this EarthCache, please send me the answers to the following questions :
All answers can be found on the info board and the fun part is that this board is on the Tree Canopy Walkway (aka the «Boomslang»).
1. What is the elevation level of the contour path (mentioned at least 3 times on the info board)
2. On the contour path, what colours are the siltstones, mudstones and sandstones?
3. When it was young (before its erosion), how high would have been the peaks of the Cape Fold Belt Mountain chain?
4. Why is Table Mountain flat?
5. [optional] Take a picture of yourself on the Boomslang and attach it to your log.
Log in this cache «Found it» and send me your answers proposals or via my profile or via geocaching.com messaging (Message Center), and I will contact you in case of problems.
This cache lies in Kirstenbosch National Botanical Garden.
Entrance fees
Adults:R60
SA Students (with card): R30
Children (6 - 17 years): R15
Under 6 years: Free
BOTSOC members: Free
SA Senior Citizens (with ID): Free on Tuesdays except on public holidays
Operational Hours
Sep-Mar (Summer) Mon-Sun 08h00-19h00
Apr-Aug (Winter) Mon-Sun
08h00-18h00
Conservatory Mon-Sun
09h00 -17h00