Development of Landscapes

Erosion, transportation and depositional effects of the geographical agents are the dominant factor to shape up the earth surface and responsible for its irregularities.

Land scape is study base on three viewpoints.

• Structure – present topographic lines are depends on their structure. Examples are origin of the rocks, its components, nature of elements.

• Processes – It intend the processes that work upon these structure. These are determined by climate, elevation and nature of the rock.

• Stage – Finally, the landscapes are influenced by its stages of development.

Regions changes are result of impacts of erosion factors. Erosion factor caused changes through its ‘cycle of erosion’ or ‘geomorphic cycle’. Earth land forms are take changes through ‘youth’ to ‘maturity’ to ‘old age’. These are regarded as stages of development.

The erosion cycle in a temperate humid region

In temperate humid region moderate rainfall is the dominant factor. The effects of running water and chemical weathering are responsible for land surface modification.

In younger region there are few streams with poorly branched. Most of the area untouched by river valleys. Under these conditions, poorly drainage water accumulated in low areas, and forming lakes and swamps. These are not intermittent. Lakes are continually filling up with sediments by inflowing streams and out flowing streams cut their outlet. Vegetation grows with in them and adding organic materials with sediments to filling the basins. Lakes length disappears. Tributaries occur from head stream and they form branched and re branched.

Erosion which has contribution on this process depends on rock structure. If these rocks are horizontal, tributaries cut the intervening lands into ridges. While rocks are tiled, streams are flow through weaken rock layer. Chemical weathering dissolved the rock minerals. Therefore rock those near the surface not stand for long at high angles. Valley walls become crumbling or wash downward into stream. Steep slopes and sharp angles are starts to disappear and form mellow curves on surface land.

These stages know as maturity. At this stage extent dissection and relief occur. This relief increases the soil creep. Soil erosion starts outside of the valley and the same time side surface almost destroyed.

Land becomes lower and old stage comes on. Region becomes flat through the effect or impact of erosion and weathering. These landforms are called as peneplain. At this stage it reaches final stage.

The cycle in a dry climate

Arid and semi-arid regions elevated and flat surface will undergo with specific changes same as temperate humid region. Stream flows are irregularity because of irregularity of rainfall and lack of vegetation which has retarding effect upon runoff. Many streams not endure. 

In dry region weathering process is slow and mechanical type. This type of weathering has little impact on rocks which lie beneath the earth surface. If stream banks are steep, channels deepen at relatively rapid rate. Valleys become canyon, when elevation reaches enough height. Small streams and deep valleys taken canyon shapes. 

Small depressions or basin exists in arid region same as in humid region. In earlier stages of development, the water of intermittent streams evaporates or undergo into surface before it reach the bottom of basins. But growth and development stream systems carries the water into lower parts of the small depressions and form playa lakes. In this way the floor of the valleys raised. Best example is Death Valley. 

If basin filling up has continued, stream come from higher elevated become higher and find the way to outlet and in lower stage flow into basin. This integration drainage system intended maturity stage. 

In most places weathered materials obscured the bedrock structure and soften the outlines of topography. Horizontal layer carved into terraces. Hard layer forms higher steep, softer slopes. 

Soften rocks at considerable altitude above the main streams converted by sheet, gully and rain erosion into an intensely dissected landscapes. It known as badlands. There is no vegetation cover. Heavy rainfall washes away the weathered material before the plant life rooted. For example South Dakota & Northern Nebraska 

                                                           South Dakota badlands

At final stage dry region reduce as peneplain. In this condition wind erosion become as dominant erosive agent factor. In very arid region wind erosion reduce the land below the sea level. 

The cycle in mountainous regions

The stages of erosion cycle in mountainous region different from other level regions. Youngest shapes determined by their fundamental structure of the mountain region rather than effects of erosion. Streams are flow toward down and cut the local canyons and leave masses untouched. If glacier present, cirques excavated cavities will appear in their summits and sides. But original form of the mountain not changes. For example Southern Rockies & Bighorns. 

 

Maturity stage knows as maximum dissection by weathering, running water and some time by glaciers. Only little form of original landscape remains. Canyons and their tributaries cause inroads upon elevated or uplifted masses. If the region covered by glaciers cirques has start to work and leave steep peaks and jagged crests between them. 

Finally mountain gives place for weathering and erosion. The ruggedness and grandeur which take form in maturity stage slowly changes soften contours. Mountain stream also become tributaries because of their silt. At last mountain is brought as low.

Example: Norfolk and western railway

The cycle in a Limestone region  

In this region dominant factor is solvent action of ground water. The early stages of development, the region is similar as those under the influence of surface region. Stream valleys will visible soon and their development starts as usual way. When they cut down the layer that lie under the limestone, the water begin to find the way to outlets through underground channels.  At this point limestone starts to disappear from the surface. 

But eventually very little of the original surface is remain. In stage of maturity sinks become large. Its leads to the formation of solution valleys. These differ from stream valleys by their irregularity. Surface stream starts to disappear and subterranean drainage system has to develop. 

The stage of old age reaches its basic level. Here it intends the lowest level of solubility. This may be the level of water table or insoluble rock which lies beneath the limestone. Either, the surface becomes more flat and less rugged. Scattered monadnocks start to rise. Surface streams reappear and stream erosion takes a place to reduce the land to sea level. 

 

 The cycle under glacial conditions. 

The progress of alpine glaciers has already described in erosion cycle of mountain region. Some erosion effected by valleys deepen and widening. But these are extent of cirques. 

In southern parts of the continental land contain the evident of unfinished glacier erosion process. This caused by melting of the ice. The glacier motion is slow, ponderous and irresistible progress. In glacier land scape contains lack of sharp outlines, the angles, peaks and spurs. Landscapes appear as reluctance. 

 The shore-line cycle

It is constantly changing due to the effects of land, air and marine processes. On many coastlines dominant factor is wave action. Recently sunken coastal areas will present irregularity forms. The stage of young age, river mouth becomes as estuaries and hills have been converted into peninsulas or islands. Waves will continually attacks the islands and main land forcefully. 

Wave actions widen the weaken area and forms cave at a headland. Cliffs and wave cut terraces appear. If cave formed at headland it will be widened and deepened until the sea cuts through to form a natural arch. If waves continually erode the foot of arch its roof becomes weakens to support and it will collapse. It leaves a part of the cliff isolated as a stack. 

At the late youth shore line shows as clear changes of stage. Bars closed the mouth of bays and they extend them self or form a line from one headland to another headland. Bays are filled by sands washed from the bars or from the main land. 

A wave attacks the remaining head lands and sand – choked bays. Sea-cliff take places in borders the coast and not similar as the original shore line that remains. This may be considered as maturity stage. Older stage intends sunken coasts. 

References: 

(1) Howard E Brown, Victor E monnett, J. Willis Stovall (1958); Introduction to Geology; Ginn    and Company; USA
(2) Michael Allaby, Dr. Robert R. Coenraads, Dr. Stephen Hutchinson, Karen McGhee, Dr. John O'Byrne, Dr. Ken Rubin (2008): The Encyclopedia of Earth, Sydney.

(3) David Waugh (2003); The New Wider World; Nelson Thornes; Uk.
(4) D.M. preece, H.R.B. Wood (1946); Foundations of Geography; University tutorial press; London