(i) Running water or river is the most prominent agent of change in humid areas receiving heavy rainfall. As the rainfall occurs, water flows down the slope of the land and removes upper layers of the earth in the direction of water flow. Gradually small and narrow sills are formed which develop into gullies. The gullies further deepen, widen, lengthen, and unite to form a network of valleys. In the upper state of river, down cutting dominates during which irregularities are removed. In the middle stage, river cut their beds slower and lateral erosion becomes dominant. At a later stage, the valley sides are reduced to lower and lower slopes. Similarly, the divides between drainage basins are lowered until they are almost completely flattened. This type of plain forming because of stream erosion is called a peneplain. The work of a river can be divided into three stages- youth, mature and old. In dry areas, there is more down cutting because sides of the valley are not eroded due to lack of rainfall. In such areas landforms likes gorges and canyons are formed. The Valley of Kaveri River near Hogenekal, Dharmapuri district, Tamilnadu is one such example.
(ii) Limestones behave differently in humid and arid climates because there is change in climate. In humid climate, surface water percolates in the limestone rocks. After some time, the underground water flows horizontally through the bedding planes and joints. The downward and horizontal movement of water erodes rocks. The surface water and the groundwater through chemical process 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 water. However, in dry areas, much water does not percolate in limestone rocks and the work of underground water is insignificant in such area.
(i) Meandering courses are found over floodplains and delta plains indicate that a former floodplain with a widening stream had been uplifted, enabling the stream to cut downward. Incised meanders are occur when a river’s base level has fallen giving the river a large amount of vertical erosion power, allowing it to downcut. They give an indication on the status of original land surfaces over which streams have developed.
(ii) When sinkholes 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 areas have such rocks, which are permeable, thinly bedded and highly jointed and cracked.
(iv) Moraines and eskers are important linear depositional forms of a glaciated region. Lateral moraines form along the sides parallel to the glacial valleys. Medial moraines are formed at the confluence of two glaciers. Terminal moraines are found at the end of the valley glacier. When glaciers melt in summer, the water flows on the surface of the ice. Eskers are sinuous ridges formed by the depositional work of a glacier.
(i) (a) Youth stage
(ii) (d) Canyon
(iii) (a) Humid region
(iv) (d) An irregular surface with sharp pinnacles, grooves, and ridges
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