Due to the river action, cylindrical holes are drilled vertically in the river bed with varying depth and diametre.

(i) A valley is made deeper by a stream of water (or) a river as it flows from the high land.
(ii) The valleys are either 'U' shaped or 'V' shaped.
(iii) The glacier debris get deposited in the low lying areas. So glaciers do not form deep narrow valleys.

(i) Guptadham caves - Western Bihar
(ii) Pandav caves Pachmuri Madhya Pradesh
(iii) Kutumsar - Bastar district in Chattisgarh
(iv) Borra caves of Visakhapatnam - Andhra Pradesh

(i) Meanders in due course of time become almost a complete circle with narrow necks
(ii) This is turn gets abandoned and forms a lake.This is called an Ox-bow lake

(i) The rain water mixes with CO₂ and enters into limestone region it dissolves and destroys much of the lime stone.
(ii) Subterranean drainage may limit surface water with few to no rivers.
(iii) The ability of water to penetrate the rock lowers. Joints and bedding planes make the rock permeable.
So surface drainage is rarely found

The snow line is at the sea level in Polar regions because higher the latitude lower the Snow line from sea level

Chemical weathering takes place through the processes of oxidation, carbonation, solution, and hydration. The agents of Chemical weathering are Oxygen, Carbon-dioxide, Hydrogen and water.

(i) The river channels of the upper course has more energy gradiant then the channels of lower course.
(ii) As the river flows in the lower course it corrodes sideways and makes it wider.

(i) As the desert are with little or no rainfall, wind is the only gradational agent.
(ii) Mechanical weathering is the dominant activity as there are limited plants.

Weathering is the disintegration and decomposition of materials of the earth's crust by their exposure to atmosphere.
There are three types of weathering,
(a) Physical weathering,
(b) Chemical weathering
(c) Biological weathering
Physical weathering: It is the breakdown of rocks without changing their chemical composition, through the action of physical forces. The constant freezing and thawing of rocks during the night and day leads to the expansion and contraction of rocks. Cracks are formed and disintegration occurs eventually. Exfoliation, block disintegration, granular disintegration etc., are the different types of weathering.
Chemical weathering: Disintegration and decomposition of rocks due to chemical reactions is called Chemical Weathering. This is predominantly high in the hot and humid regions such as the equatorial, tropical and sub tropical zones. Chemical weathering takes place through the processes of oxidation, carbonation, solution, and hydration. The agents of Chemical weathering are Oxygen, Carbon-dioxide and Hydrogen.
Biological weathering: Biological weathering occurs due to the penetration and expansion of plant roots, earthworms, burrowing animals (rabbits, rats) and some human activities

a)

b)

c)