PEOPLE TAKE THINGS for granted. When you visit a nature park you listen to birds chirping, look at the branches swaying, leaves falling, tree standing tall, insects crawling. The last thing you go out of your way to look at when you are out in nature are the rocks.
Why would you look at rocks anyway? One rock looks like another; unless you are a geologist by profession.
Former geologist, Hans Hazebroek led a group of 18 people – myself included – on a trek through the Santubong National Park on Aug 2 telling us there were more to the rocks than meet the eye.
It was a trip called ‘Geology Walk’, one of the lead-up activities to the Santubong Nature Festival (SNF) jointly organised by Malaysian Nature Society Kuching Branch (MNSKB) and Permai Rainforest Resort and supported by Kuching City North Commission and Sarawak Museum Department.
Before we started our 45-minute trek to the waterfall in the national park, Hazebroek explained the geological basics of Santubong.
Hazebroek giving a briefing before starting the trek.
Mount Santubong is made up of sandstone. Sandstone is composed of sand-sized grains of mineral, rock or organic material.
Besides sandstone, Hazebroek also explained to us about intrusive igneous rocks. It is formed when magma flows from the cracks within the crust of our Earth which already cooled and solidified, surrounded by pre-existing rock.
Speaking of crust, do you know how Mount Santubong was formed? Our Earth’s crust is divided into continental crust and oceanic crust.
When these two crusts collide and slide under each other, the continental crust gets pushed upwards, which explains how Gunung Santubong is presumed to have risen up from the ocean.
Overall, here are some geological features you should take note if you find yourself in Santubong National Park:
The Rocks on the Streambed
Have you ever noticed how round the rocks in a streambed are? No corners, no sharp edges. Hazebroek had an explanation for it, “First we have the mountain being pushed up. As it is being pushed up, since sandstone is brittle, it will cause a crack in the sandstone. Water flows into the crack leading the crack to get bigger.”
Example of sandstone can be found in streambeds in Santubong.
“Eventually, part of the sandstone breaks off and rolls down because the mountain is quite steep.”
He explained that when the water started pushing them, these rocks start bouncing and knocking each other which explains why the rocks are smoother.
Hazebroek also asked us to imagine the power of water as it could push some of the big boulders that some bigger than us.
The Rocks Lying Around
We also noticed some boulders lying around the forest near the trail. Some were even taller than an average adult human’s height.
An example of a boulder found in the middle of Santubong forest, most probably weighing several tonnes.
According to Hazebroek, “The last ice age ended about 10 thousand years ago. There were several ice ages; in between these ice ages are warm periods called interglacial.”
During these interglacial periods, he explained that the tropical areas including Borneo had severe raining weather. Due to the heavy raining, there were big avalanches causing a lot of rocks to fall from the steep mountain.
Another example of big rock near the trekking trail of Santubong National Park.
Voilà! That is how those huge rocks get in the middle of Santubong forest.
The Waterfall
Our final stop during our trek before heading back was the canopy bridge built over a waterfall.
We took turns in groups of five going up to the canopy bridge to listen to Hazebroek explaining about the geological features beneath us.
Hazebroek (second right) explaining on the geological features to some of the trekkers while on the canopy bridge.
He introduced us to the term ‘cross-beds’ which we could see at the waterfall. They are sedimentary structures of big rocks with lines running across formed by downstream migration of bedforms such as ripples or dunes in flowing water.
Cross-bedded sandstone at the waterfall where you could some lines although most of the surfaces are covered with mosses.
From the canopy bridge, we could also see three bedding planes at the waterfall; surfaces separating layers of sedimentary rock.
So what does this geological structure tell us? Hazebroek said the cross-beds were where sands were deposited due to flowing water so thousand years ago this area could have been a riverbed or shallow seabed.
Hazebroek also shared how old these rocks were; based on pollens found inside these sandstones dating back to upper Cretaceous to early Eocene period time which is about 70 to 60 million years ago.
“But the intrusive rocks here are much younger, about 35 to 5 million years ago,” he stated.
So the next time you visit Santubong National Park, don’t take the rocks for granted. They have been there longer than you can imagine.
We are happy trekkers!
For further information on MNSKB and their upcoming SNF, contact them at mnskuching@gmail.com. You can also visit http://mnskuching@blogspot.com or www.facebook.com/mnskb.
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