The correct answer is - Watershed.
The drainage basis are places in which all the water in an area is accumulated and ends up in the same place. The drainage systems are divided by highlands, which can vary from small hills to very high mountains, depending on the topography of the place.
The line on the highland that is separating the drainage systems is called watershed. The watershed is simply a line stretching on the highest parts of a highland, thus having a slope on each side.
It's a form of weathering. If water gets into a crack in a rock and then freezes, it will expand, causing the crack to get bigger, which can happen continuously.
I hope this is the type of answer you wanted. :)
