http://www.edu.pe.ca/southernkings/valleyfl.htm This website is simple and describes the process
Gravity causes rock to fall away from steep mountains or cliffs and even by Landslides.
Erosion!
you see here, water from the rain and wind can erode and weather away a part of land, creating a valley
Look at the Grand Canyon. The Colorado River has eroded the rock (as it has risen due to other geological forces).