Avalley is formed by a river flowing, and fast flowing rivers tend to pick up more pebbles, sand and other materials that cause erosion. Given these facts, how does water cause a valley to form among the following options:
- Physical weathering
 - By the process of erosion
 - By falling from clouds
 - It used to be covered by water
 - fissures of hot, acidic water
 - Heavy snow accumulation
 - By forming puddles
 - Water expanding


 

  By the process of erosion       