Chapter 8: Limestone and Chalk Landforms (Karst Topography)
This chapter explores landscapes formed on Limestone or Chalk. These rocks are unique because they are chemically reactive and highly porous. The term “Karst” comes from the Karst district of Yugoslavia, where these features are most distinct.
1. The Nature of Limestone
To understand these landforms, you must understand two key properties of limestone:
- Solubility: Limestone is made of Calcium Carbonate. While pure water doesn’t affect it, rainwater absorbs carbon dioxide to become a weak carbonic acid. This acid dissolves the rock—a process called Carbonation.
- Jointed Structure: Limestone is a sedimentary rock with many vertical joints and horizontal bedding planes. Water seeps into these cracks, enlarging them over time.
- Absence of Surface Drainage: Because water sinks into the joints immediately, Karst regions have almost no permanent rivers on the surface. Most of the “work” happens underground.
2. Surface Landforms
- Grikes and Clint: As water dissolves the limestone surface, it widens the joints. The deep grooves are called Grikes, and the flat, isolated rectangular blocks between them are called Clints. Together, they form a Limestone Pavement.
- Swallow Holes (Sinkholes): Small depressions on the surface where a stream “disappears” underground.
- Dolines and Uvalas: Several swallow holes may merge to form a larger depression called a Doline. When many dolines join together, they form a massive, long trench called an Uvala.
- Polje: A very large, flat-bottomed depression, often covering several square miles, caused by the collapse of underground caverns or tectonic sinking.
3. Underground Landforms
The most spectacular features of Karst topography are found inside Caves and Caverns:
- Stalactites: Sharp, icicle-like spikes of calcium carbonate hanging down from the roof of a cave. (Memory trick: Stalactite holds “tight” to the ceiling).
- Stalagmites: Short, fat columns of calcium carbonate growing up from the cave floor. They are formed by the dripping water from above. (Memory trick: Stalagmite “might” reach the ceiling).
- Pillars: Formed when a stalactite and a stalagmite eventually meet and join together.
4. Chalk Landscapes
Chalk is a very pure form of limestone. It is extremely porous, so it rarely has surface streams.
- Downs: Rounded, grass-covered hills (e.g., the North and South Downs in England).
- Coombes: Dry, deep-sided valleys found on the slopes of chalk hills. They were likely formed during the Ice Age when the ground was frozen and water had to flow on the surface.
Quick Revision Questions & Answers
Q1: Why is there a lack of surface drainage in Karst regions?
- A: Because limestone is highly jointed and porous. Rainwater quickly sinks into the cracks and flows through underground channels instead of staying on the surface.
Q2: What is the chemical process responsible for limestone weathering?
- A: Carbonation. Rainwater +
𝐶𝑂2 = Weak Carbonic Acid. This acid reacts with Calcium Carbonate to form soluble Calcium Bicarbonate, which is washed away.
Q3: Distinguish between a Grike and a Clint.
- A: A Grike is the deep groove (crack) created by chemical weathering, while a Clint is the flat-topped ridge or block left between the grikes.
Q4: How does a “Blind Valley” form?
- A: It forms when a surface stream flows until it reaches a swallow hole and disappears underground, leaving the rest of the valley dry and “ending” abruptly.
Q5: Are stalactites and stalagmites erosional or depositional features?
- A: They are depositional features. They are formed when mineral-rich water evaporates, leaving behind solid calcium carbonate deposits.
