The iconic sea stack is an example of Bako’s geological treasures.
IN 1957, Bako was gazetted as Sarawak’s first national park. Lying at the north-eastern end of the Muara Tebas peninsula, it covers an area of 2,727ha (27.27 sq km).
Most visitors to this national park go in search of its diverse flora and fauna and to visit its many secluded beaches. Many will remember the boat ride from the jetty at Kampung Bako, past the mangrove-fringed shorelines to land at Telok Assam alongside the giant sea stack.
My first visit to Bako was now 15 years ago and, until relatively recently, my memories were of the variety of vegetation to include the pitcher plants, the plank walkways alongside the mud of the mangrove swamps and the wildlife. Yes, I saw proboscis monkeys and a sleeping slow loris but most vividly embossed in my brain is the speed I ran, with three others, away from a chasing troop of long-tailed macaques eagerly snapping at our heels!
Geology
Most regrettably, on that initial visit, I did not appreciate the geology of the areas over which I trekked. On subsequent visits my mind focussed on the rock formations there, based upon geological maps and previous research.
Essentially, the park is composed of Plateau Sandstone, which is approximately 75 million years old, deposited on the ocean floor in Upper Cretaceous times. Close inspection of the sandstone, here and there, will reveal that it is interbedded with mudstone and pebbles in quite distinct layers in the rock strata. This implies that in Upper Cretaceous times, this area was a huge river delta, not unlike the River Ganges delta today. In river deltas, worldwide, rivers in spate and tidal movements allow slumping of river sediments to occur, which is, indeed, reflected in the bedding of the rock strata at Bako.
Interestingly, the Santubong and Matang mountain ranges consist of very similar material but these were the result of more violent folding of the delta deposits than Bako experienced and hence their higher altitudes.
All the areas mentioned were uplifted from the seabed by tectonic plate movements very many moons ago.
At a later stage in geological time, and it is uncertain exactly when, but probably 19 to 22 million years ago, in Mid-Miocene times, the Bako Plateau sandstones experienced igneous intrusions from magma beneath the Earth’s Crust.
These volcanic and plutonic intrusions exploited the vertical joints (cracks) and near horizontal bedding planes in the rocks to solidify as solid and more resistant rock than in sandstone country rock.
These igneous intrusions are composed of diorite and dacite, taking the form of dykes and sills.
Diorite is a coarse grained rock containing little quartz but, because of its iron content, weathers into a dark red clay soil. It has cut across the sandstone in the form of an almost seven-metre long dyke at Telok Limau and again appears in sill form at Sungai Serait. Diorite is a relatively rare rock and a favourite one of sculptors over the ages.
Dacite intrusions, of volcanic origins, are mainly found in the Serait Valley and on the north-eastern tip of the park again at Telok Limau.
These dykes are not dissimilar to those found at Bau and, in both dacite intrusions, the igneous rock contains minimal gold-bearing properties. Early in the last century, gold was panned from the bed of Sungai Serait and up until the Japanese Occupation of Sarawak, a small gold mine existed on a hill of weathered dacite in the Serait Valley.
Over millions of years, waves have excavated long caves in the cliffs of Bako.
Erosion and weathering at Bako
The Bako landscape as we see it today is the result of erosion, by the elements, of overlying less resistant layers of sedimentary rock exposing the thick, resistant sandstone beds. The latter are best seen in the cliff faces along the shoreline and on the higher plateau areas where the sandstone has weathered to produce sandy soils.
There, these thin podzolic soils are colonised by Kerangas or heathland species of vegetation, with low growing pitcher plants tolerant of such acidic conditions.
This sandstone is characterised by its north-westerly trending joints, which have determined lines of weakness for the courses of many streams.
Most of the coastal headland shapes at Bako have been determined by the trend lines of these joint-patterns.
The action of the sea has had its impact, too, as the South China Sea waves have pounded the cliffs for millions of years.
From Tanjung Pandan to Tanjung Sapi, long caves have been excavated, exploiting the weaknesses of the aligned joints in the sandstone. The overhanging cliff tops are characterised by a honeycomb pattern, the result of both weathering and sea spray.
If you visit Bako, do take the Lintang Trail, one of some 17 trails over various distances and times taken in completion.
This trail provides just over three hours’ walking over a distance of 5.25km. Along the higher parts of this trail you can observe small, bowl-like cavities which are found individually or in groups on the steeper scrambles.
On the plateau top, with its stunning views right across the sea to Santubong, circular depressions in the near horizontal sandstone abound. These hollows are about 2.5 to 15 centimetres deep and up to a metre across.
It is thought that these have been created by rainwater charged with acid from decaying lichens and algae, which colonise the bare rock surfaces.
This acidic water has dissolved the sandstone over the years. Rainwater has again played its part in exploiting the generic and structural weaknesses in the Bako Plateau Sandstones.
The beauty of Bako
In conversation with many of my friends in Kuching, they recall their childhood visits to Bako yet, surprisingly, have never returned.
Few children in Kuching have ever visited this amazing national park, only 37km and a 30-minute drive to Kampung Bako and from there a boat trip to land at Telok Assam beach. This famous national park is right on Kuching’s doorstep.
What a truly wonderful place for a family day out, to see its flora and fauna as well as its geology and its natural arches and sea stacks. Moreover the park fees are realistically priced, and as a senior citizen, and an ‘ang-moh’, I even get a reduction.
What a choice for a school’s class visit or for geography students’ field courses to study the natural world at its very best.
There is still much research to accomplish in fully understanding the geology of Bako. In that respect I hope I have whetted the appetite of more professional geologists than myself and equally Malaysian university students studying that subject or Earth Sciences. There is still much to be explored at Bako.