Limestone landforms

STUNNING SETTING: White limestone cliffs are visible from Camp 5 at Mulu National Park.

By Alan Rogers

WHILST recently driving across the Mendip Hills in Somerset, UK — a hill mass made of 250 million old Carboniferous limestone — I thought of the other limestone areas I have visited in the UK,Slovenia, Malta, Crete, and Madagascar, but far from least those in Malaysia,Vietnam and China.

What have these places in common? Karst scenery or landforms made in massive limestone — a limestone containing impurities laid down on the ocean beds up to 350 million years BP(Before Present Time) and subsequently uplifted through world plate movements to dry out on exposure to air.

As the limestone was deposited in the sea, it developed layers often with deposits of alluvium and

sand in between layers of calcium carbonate. Fossils can often be seen in the rocks.

These layers were separated by horizontal cracks along bedding planes and as the sediments were

gradually uplifted from the seabed, the limestone dried out and contracted creating vertical cracks called joints.The very word karst derives from the Slovenian language and was adopted in 1893 by a Serbian Professor Jovan Civjic. The landforms produced in karst areas are the direct result of solution action.

Rainwater (H2O) combines with atmospheric and soil carbon dioxide (CO2) to create a mild or aggressive carbonic acid (H2CO3) to dissolve limestone (CaCo3).

This is a reversible process as witnessed in stalagmites and stalactites in caves —dripstone features — made of calcite once the water and CO2 have evaporated. The rainwater percolates through the soil into the joints and along the bedding planes underground whilst dissolving the limestone en route.

It is in the tropical and subtropical humid climates where the greatest variety of karst and most striking

landforms are seen, especially at Gunung Mulu, Gunung Api, Niah, Bau, and Santubong in Sarawak;

Gomantong and Madai Caves near Sukau and Semporna respectively in Sabah; in the Kinta Valley,

Perak; at Bukit Batu in Selangor; the Langkawi Islands; South Central Java; Haiphong Bay in Vietnam; Papua New Guinea; and at Guilin alongside Lijang River, Guangxi, China. The Karst Geology Institute is located in China.

Remember the characteristic tower scenery depicted in classical Chinese paintings, rising on either

bank of the meandering Lijang River? These are the very products of humid tropical karst processes.

What have all these areas in common? Limestone bedrock albeit of different geological ages ranging from 350 to 150 million years BP and all containing impurities. They are massive limestones with the

mechanical strength to support cave roofs for thousands of years, all apart from the Niah Caves at Gunung Subis where the limestone is only 20 million years old.

Professors Yuan and Zhang at the Guilin Institute have identified three typesof karst scenery applicable

to all Southeast Asian areas:

1. Fengcong — composed of clustered residual hills joined together at the base with closed depressions into which water drains downwards. (Cockpit karst as it is termed in the Blue Mountains of Jamaica.)

2. Fengling — lesser relief with valleys dividing up the hills (peak forest plain).

3. Kufeng — low hills scattered over wide alluvial plains.

German geomorphologists recognised two specific forms of macrokarst in tropical areas — Turmkarst (Tower Karst) and Kegelkarst (Skittle shaped Karst).

Turmkarst (Fengcong) rise like islands from a plain and are steep sided residual limestone towers some 100 to 300 metres high.

It is currently thought that structural controls — the angle that the limestone was pushed upwards, may play a large part in the creation and preservation of tower karst.

Where natural forest is retained, as in New Guinea and at Santubong, the towers are covered with vegetation and vertical rock faces are hidden by plant growth with the roots of taller trees penetrating the joints. Forest litter accumulates in depressions, hollows and niches.

Chemical reactions double for every 10 degree Celsius increase in temperature, thus in high mean annual temperatures, high annual rainfall and high intensity of rainfall areas, as in Malaysia, there is rapid solution and removal of limestone. Rapid plant growth and decay and intense microbiological activity also make tropical water very aggressive. It has been recorded that soil CO2 values are greatest in the wet season.

With a worldwide rise in sea level, some 10,000 BP to the present, since the melting of the ice of the

Pleistocene glaciations, the tower karst of Haiphong Bay, Vietnam and at Langkawi have created an

archipelago with islands and inlets, in a shallow sea, fringed with mangroves and tidal flats.

At Guilin, as in the Kinta Valley, accumulations of alluvium have sealed off the permeable limestone below to allow rivers to flow in and around the tower karst. There the rivers undercut the base of the towers creating notches as indeed waves do where limestone cliffs occur on the coast.

It is interesting to observe caves at different heights in tower karst scenery, nowhere better exemplified

than at Coffin Cliff in Danum Valley, Sabah, where dead people were hauled up in coffin boats to their fihigh altitude caves.

How did these caves get tosuch a great height? The answer is relatively simple for as the continents collided and the limestone was gradually squeezed upwards those high level caves were created at land base level and then slowly uplifted.

The same principle applies to the Batu Caves at Bukit Batu, with today its Hindu temple, and to the Niah Caves where abandoned river gravel and deep accumulations of bat and swiftlet guano cover the floors.

Our Malaysian caves, whilst bearing witness to water penetration of the limestone along the joints and bedding planes, have some of the oldest and most compacted limestones in the world, similar to the French and Slovenian caves.

The rock in all three countries were uplifted from the sea at identical times during the Alpine orogeny

(orogenesis = mountain building) when the Indian sub continent slammed into Eurosia and Africa also hit Eurasia.

Yet the Gunung Api pinnacles at Mulu are harder than the Melinau limestone at Mulu Caves and thus

stand out as vertical sharp edged features due virtually to the vertical alignment of the bedding planes as the limestone was thrust upwards. As for the caves created initially at the base of the limestone, where rivers in flood, in monsoon conditions rushed over the impermeable rock below, Malaysia tops the world.

Locally, the Wind and Fairy Caves at Bau well exhibit the scalloping on their walls of rising waters of

past times with some interesting dripstone features and easily seen swiftlet nests.

Equally accessible caves are at Gunung Subis near Miri where in the 1950s Tom Harrison pioneered

anthropological research on humans living there in Pleistocene (Ice Age) times 40,000 years BP.

There the Great Cave is well worth visiting, but wear headlamps and avoid tripping over the guano

collectors’ tents. On a short flight from Miri, in good weather, look out for the rock pinnacles of Gunung Api and then explore the Sarawak Chamber at Good Luck Cave (Lubang Nasib Bagus),

a spectacular cave system 700 metres long x 400 metres wide and some 280 metres high.

This article is written in memory of my former tutor at Oxford University in the 1960s. Dr Marjorie Sweeting was a world expert on karst morphology and abseiled, at over 60 years of age, on the Royal Geographical Society Expedition to Sarawak (1970), down Gunung Mulu to discover new caves.

Had she been alive today, I just wonder what her findings would have been, for certainly she would have helped sort out the continuing debate on the origins of tropical karst landforms. Good tower karst

and cave hunting and further exploration.

For further revelations, go to Liz Price Caves and Limestone Hills of Malaysia (www.cavesofmalaysia.com), www.forestry.sarawak.gov.my, http://en.wikipedia.org/ 

wiki/Niah_Caves, www.karst.edu.cn, or Karst Topography of Sarawak by Wilford GE and Wall JRD

(1965), Journal of Tropical Geography Vol 21.