Landforms And Their Evolution

Downward cutting is dominated in the youth stage of landform development.

A canyon is a deep valley characterised by steep step-like side slopes.

Limestone is a sedimentary rock that forms of CaCo₃ so in this process these regions are prone to chemical weathering specially action of water that forms a chemical reaction and the region gets eroded.

Lapies refers to grooves and ridges formed on a rock surface by solution of limestone.

A cirque is a deep, long and wide trough or basin with very deep concave high walls at its head as well as in sides.

(i) Incised meanders are formed during the youth stage of a river. With the passage of time, as the mountains and plateaus rise the meandering course starts vertical erosion and incised meanders are formed.

Meanders in plains of alluvium indicate formation during mature and old stage of the rivers.

(ii) The surface run off simply goes down shallow and sink holes and flow as underground streams and re-emerge at a distance downstream through a cave opening. 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.

(iii) Underground flow of water is more common than surface run-off in limestone areas because limestone is rich in calcium carbonate, the surface water as well as groundwater through the chemical process of solution and precipitation deposition, develop varieties of landform. These two processes of solution and precipitation are active in limestone occurring either exclusively or interbedded with rocks.

(iv) Glacier valleys show up many linear depositional forms. They are as follows:-

• Moraines- They are long ridges of glacial till. Terminal moraines are long ridges deposited at the end of the glaciers. Lateral moraines form along the sides parallel to the glacier valleys. Deposits varying greatly in thickness and in surface topography are called ground moraines.

• Eskers- It is a long winding ridge of a stratified sand or gravel found in glacier regions.

• Outwash Plains- The plains at the foot of the glacial mountains or beyond the limits of continental ice sheets.

• Drumlins- form beneath heavily loaded ice through fissures n the glacier.

(v) Wind is the primary agent for erosional features in deserts. It involves the lifting and removal of dust and smaller particles from the surface of the rocks. The wind creates a number of erosional and depositional features in the deserts.

Along with wind, running water or sheet erosion are also common in deserts.

(i) In humid areas which receive heavy rainfalls, running water is considered as the most geomorphic agents in bringing about the degradation of the land surface. There are two components of running water:- One is overflow on general surface as streams and river in valleys. Most of the erosional landforms made by running water are associated with vigorous and youthful rivers flowing over the steep gradients. With time, stream channels over stream gradients turn gentler due to continued erosion and as a consequence, lose their velocity facilitating active deposition. There may be depositional forms associated with streams flowing over steep slopes. When the stream beds turn gentler due to continued erosion, downward cutting becomes less dominant and lateral erosion of banks increases as a consequence hills and valleys are reduced to plains.

In arid regions, the rain is scare but when it comes heavily, the rocks devoid of any vegetation get eroded much faster and the rains help in removing the weathered materials easily. So running water is by far the most dominating geomorphic agent.

(ii) Limestones are permeable, thinly bedded and highly jointed and cracked and is a sedimentary rock that forms of CaCo₃ . After vertically going down to some depth, the water under the ground flows horizontally through bedding planes, joints or the materials themselves. The downward and horizontal movement of water erodes rocks. The surface water and the groundwater through the chemical processes of solution and precipitation deposition develop a variety of landforms. Limestone does not suffer much solution, erosion and chemical reaction in dry climate due to lack of moisture or water.

(iii) Masses of ice moving as sheets over the land or as linear flows down the mountains in broad-trough like valleys called glaciers. They are slow in moving as they are heavy. Erosion by glaciers is tremendous because of the friction caused by the sheet of ice. The material plucked from the land by glaciers get dragged along the floors or sides of the valleys and cause great erosion through abrasion and plucking. Glaciers can also cause significant damage to even un- weathered rocks and reduce mountains into low hills, plateaus and plains.

Several related landforms together make a landscape (large tracts of earth’s surface).

The two important aspects of the evolution of landforms are:-

• Climatic conditions

• Vertical and horizontal movement of landmasses.

Most of the geomorphic processes are imperceptible. The processes which are gradual and can be seen are melting of glaciers, volcanic eruption.

No, complete reduction of relief of a high land mass is not possible. It may be reduced to a great extent but total reduction is not possible. For example Hills and valleys are reduced to plains by the geomorphic agents but total reduction is not possible.

Natural levees are formed on the banks of the rivers. They are low, linear and parallel ridges of coarse deposits along the banks of the rivers, quite often cut into individual mounds.

Point bars are found on the concave side of the meanders of large rivers and are sediments deposited in a linear fashion by flowing waters along the bank.

The primary forces that influence currents are:-

• Heating by solar energy

• Wind

• Gravity

• Coriolis force

• Presence of moon.

The erosional features carved out by wind action and action of sheet floods are as follows:-

• Pediments and pediplains

• Playas

• Deflation Hollows and Caves

• Mushroom, Table and Pedestal Rocks