(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.