W.M. Davis has been credited for standardizing the various stages in the evolution of landforms under normal climatic conditions where rivers flow upon the ground’s surface. He, along with Passarge, provided a cycle of erosion in arid conditions. But these cycles cannot be used to describe the forms that evolve in karst terrain.
It was only in 1918 when Cvijic provided an ordered description of successive changes that took place in the progress of the karst cycle. Karst topography develops mainly in limestone and dolomite regions because these rocks respond to weathering differently from other rocks. This is mainly because of the solubility of calcium carbonate in natural waters. In karst terrain, water circulates almost entirely underground.
The basic assumption for the occurrence of all stages of evolution is a mass of pure limestone, which is soluble and is formed of strata inclined at a great angle but without disruptions from crushing or faulting. Another important criterion is the thickness of the limestone mass; it should not have a thick protective covering of vegetation and must have a developed system of streams.
Cvijic also opined that there are three hydrographic zones in a well-developed karst landscape. These are:
- The zone immediately underneath the surface comprises channels and reservoirs that transmit water in times of storm but are generally dry.
- The inter-mediated zone lies between dry and wet, where caverns and channels are generally flooded for a considerable time but not permanently.
- The lowest zone lies immediately above the underlying impermeable strata; this zone has permanent streams and reservoirs which are always full of water.
He further states that there are four stages in the evolution of landforms in a karst terrain: Youth, Maturity, Late Maturity and Old Age.
Youthful Stage
The youth begins with surface drainage on an initial limestone surface or one that has been laid bare and is marked by a progressive expansion of underground drainage. This stage is marked by imperfect underground drainage that leaves most of the rain on the ground surface, although the cracks and crevices of the rock are filled with water.
Wherever the limestone is exposed to the rain, the ground is now covered by a network of furrows eaten out of the rock by the dissolving water. These have been named differently, such as Karren or Rascals. Cvijiv uses the term lapies for such landforms.
When water penetrates through lines of weakness, such as faults, and bedding planes of joints, these deep and narrow chasms are eroded and form what Cvijic calls bogaz.
Slowly these furrows and chasms are deepened, underground channels are created, and the surface streams disappear in the ground, leaving their valleys dry- either wholly or in part. Such valleys are called blind valleys.
There is the beginning of normal valley cutting in elongated depressions of tectonic origins i.e. poljes begin to appear along with scattered dolines – a funnel-shaped depression. Permanent lakes also exist. No great caverns are yet formed, as only one hydrographic zone has been established.
By late youth, two zones may be distinguished – an upper zone which is flooded intermittently and a lower zone which is entirely saturated.
Therefore, this stage is marked by a progressive loss of surface drainage. Rivers flowing above the ground start disappearing. Examples of such topography may be found in Tennessee, Kentucky and Virginia in the United States.
Mature Stage
In the mature stage, there is maximum development of underground drainage as the subterranean system is adequately developed to carry off all the surface water. As now all the surface water is carried immediately underground, no lakes exist unless the depressions are so deep that they dip below the upper level of the saturated zone. So the presence of intermittent lakes is common.
A large number of caves and caverns characterize this stage, developing a full network of caverns and channels permeating the entire mass of limestone. Surface drainage is limited to short sinking creeks, which end in swallow holes and blind valleys.
This stage is characterized by the development of thousands of swallow holes and dolines. This stage represents the maximum development of karst topography with all its characteristic landforms. Such topography is found to have developed in the Dinaric coastal strip of the Adriatic Sea.
Late Maturity Stage
Late maturity in the Karst cycle begins with the decay and decline of karst topography as now the limestone is gradually stripped off the underlying impermeable strata. As this happens, the drainage can no longer flow underground, and surface streams reappear.
Initially, they are not above ground for long distances, occurring most frequently where the limestone mass has been eroded as well as undermined by the extension of subterranean caverns.
Various features of the karst topography expand and coalesce to form uvalas. Poljes with their hums are to be seen on the surface. All over the karst region, cavern after cavern falls in and gorges are created everywhere. The polje flow is again covered in part by an intermittent lake and streams flowing in gorges exist side by side with dolines and uvalas.
Old Stage
With the beginning of the old stage, there is a return to surface drainage. Now, the caverns collapse, leaving open, flat-floored valleys. Solution activity has removed most of the limestone formations. The process of un-roofing of caverns and regressive erosion takes place at both the edges of the plateau and along the sides of the gorges, where the streams cut back and capture the dolines and uvalas.
As a result, the karst windows, natural tunnels and bridges and other solution features disappear. Only isolated knolls remain as remnants of the former limestone surface which are mere shells honeycombed with caverns.
A normal system of surface streams is now in possession of the land which was formerly dominated by limestone mass. These landforms may be found in Natural Bridge County, Virginia and in the Causse of central France.
These stages can be best understood through the following diagrams:
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