valleys
Its a hill
A low hill at the base of a mountain is typically called a foothill. It serves as a transition between the flatlands and the more rugged terrain of the mountain. Foothills are often characterized by gentle slopes and can be rich in vegetation.
Valley
valley
A foothill.
The low lands between hills or mountains is called a valley.
A synonym for mountain could possibly be a hill.
valley's
A bridge.
How high it is. Too low, and it's considered a hill instead.
There is no universally agreed definition of mountain; in the UK anything over 600m is considered a mountain, but in the USA some landscapes as low as 30m are called "mountains", also in the Gambia there is a "mountain" of 53m height. The short answer, with such an arbitary difference between hill and mountain, its impossible to say.
The main forces that can change a rugged mountain into a low hill are erosion and weathering. Erosion, including processes like water and wind erosion, breaks down the rocks and soil on the mountain, gradually wearing it down over time. Weathering, which involves the breakdown of rocks and minerals on the mountain's surface, also contributes to the transformation of a mountain into a hill.