Day 5 Bako coastal features

Remember the coastal cliffs and patterns on the sandstone? These are formed by a mixture of physical and chemical weathering.

A quick recap
Physical weathering is breakdown of rocks into smaller pieces by force. Content of the rock remains the same.
Chemical weathering involves breakdown when the content of rock is changed. This is mainly done when acidic rainwater dissolves certain content of the rock.

More about sandstone
Sandstone is a sedimentary rock, meaning it was formed when layers and layers of sand and other particles get cemented together over millions of years. The bonds between these particles are very strong.

However, when some of the particles get dissolved upon reaction with acidic water, holes are formed in the rock and the sand is loosened. That's why when you touched and rubbed the rock surface, you removed alot of sand.

The dark brown rings and patterns are caused by leeching of iron oxides. Leeching means iron oxides are washed down from the cliff-top soil (and vegetation). When they exit from the rock, these patterns are formed.

Coastal features

You guys are sitting in a notch. When the water level was higher, the waves "dug" into the rocks, causing erosion. That's why there's a horizontal cut in the rock.

If the waves do enough damage, a sea cave can even be formed!

Headlands and bays

The bay (beach) usually has cliffs at both sides. That was the case in Bako too.

This is how bays and cliffs form. The photo shows a top-view of the coast. The pink areas are the softer materials (sand) while the grey areas are more resistant (sandstone, granite, rocks). With wave erosion, the pink areas (softer) are eroded faster than the grey (harder). Therefore, the grey area sticks out while pink areas form at crescent bay.

Formation of a sea stack

This was the sea stack we saw on the last day. Watch the video below to understand how it was formed.

Source: http://www.rgs.edu.sg/events/geotrip/stack.html

Here's an animation of how stacks and arches form from a cliff. Basically, the tops of arches collapse to form stacks.

Top-down weathering

The top of the two rocks are still slightly connected to each other. This is in fact a narrow arch. The trees on top of the rock play a part in weathering too. Their roots force their way into the rock joints, breaking the rock into smaller pieces.

If you can recall, this weathering process was also visible at the entrance of fairy cave!

Take-away message
Waves have alot of erosive power and can shape coastal features. Other than the waves attacking the cliff bottom, tree roots exert force from the cliff top and break the rock into smaller pieces.

The videos should also have shown you how features like caves and notches are formed.

Good to read

A guide to Bako National Park - Very good book with many photos. Helps you understand many of the things we saw... solution pans, mangroves.
http://vistaweb.nlb.gov.sg/cgi-bin/cw_cgi?fullRecord+27045+3002+12852567+1+11