Part of a mountain that weathers fastest

The very top of a mountain weathers the fastest .

My reason being is mainly because it snows at the top because it's by the sky into the clouds and if it gets hot the top of the mountain will feel it first.

 Example : in Spring the top of a mountain will basically bloom flowers , saying that the top will be effected first.

I really hope that this helps you out a lot.

Digestion because it removes waste