What is it where there's a hill...?

Question

then a flat spot, and then a hill again. What forms this?

Answer 1

It's called a valley.

A valley is a low land between hills or mountains.

Answer 2

A rise in the land, which has a flat top and then goes down again, is generally referred to as a plateau.  If the land continues upward from the flat area it is known as a ridge.

Irregularities in land masses are caused by many things: glaciers, from a long time ago,

erosion from water and/or wind, volcanic eruptions and even man!