Erosional Landforms – Landform and their Evolution
Pools, Sinkholes, Lapies and Limestone Pavements
Pool, also known as a sink, a sinking, a plunge or a bottomless pit. A pool is a depression in the ground caused by a natural process called dissolution in which water gradually erodes the surface beneath it creating a cave but never reaching all the way down to create an actual hole or shaft.
These two types of landscape feature are very common when travelling around limestone/karst areas and add interest to the karst landscape.
If the bottom of a sinkhole forms the roof of a void or cave underground, it might collapse leaving a large hole opening into a cave or a void below (collapse sinks). The term doline is sometimes used to refer the collapse sinks
When sink holes and dolines join together because of slumping of materials along their margins or due to roof collapse of caves, long, narrow to wide trenches called valley sinks or Uvalas form.
Caves
Caves are very important in the context of speleology since they are part of the subsurface landscape. A huge part of our planet’s biodiversity is found in caves.
Most of these cave dwelling species have developed various adaptations to the underground environment, such as good vision in low-light conditions or anhydrobiosis.
Caves normally have an opening through which cave streams are discharged. Caves having openings at both the ends are called tunnels.
Depositional Landforms
In limestone caves the rocks are subject to solution by underground waters after surface waters have dried. The caves thus illustrate a type of origin which is common also elsewhere, namely, a roomy space confined by walls of rock dissolved out from the surrounding intact rock.
Landforms showing this type of origin include caves and sinkholes as well as solutions caves and gours. The first two kinds furnish illustrations for this chapter, the others for the chapter on caverns.
Many depositional forms develop within the limestone caves. The chief chemical in limestone is calcium carbonate which is easily soluble in carbonated water (carbon dioxide absorbed rainwater).
This calcium carbonate is deposited when the water carrying it in solution evaporates or loses its carbon dioxide as it trickles over rough rock surfaces.
Stalactites, Stalagmites and Pillars
Stalactites hang as icicles of different diameters. Normally they are broad at their bases and taper towards the free ends showing up in a variety of forms.
Stalagmites rise up from the floor of the caves. In fact, stalagmites form due to dripping water from the surface or through the thin pipe, of the stalactite, immediately below it
Stalagmites may take the shape of a column, a disc, with either a smooth, rounded bulging end or a miniature crater like depression.
Glaciers
Glaciers usually form in mountainous areas where more snow falls than melts, and thus more ice accumulates than thaws and flows away.
The mass of ice and the movement it contains feeds the glacial river. Glaciers often form long scratches or troughs on the underlying bedrock.
These troughs give streamlined glaciers a banded appearance over geologic time, which can travel hundreds to thousands of kilometers like a giant conveyor belt moving rock, ice and sediment downslope.
The movement of glaciers is slow. Glaciers move basically because of the force of gravity. Erosion by glaciers is tremendous because of friction caused by sheer weight of the ice.
The material plucked from the land by glaciers (usually large-sized angular blocks and fragments) get dragged along the floors or sides of the valleys and cause great damage through abrasion and plucking.