Digging Mountains

Hello,
I have a Question about Digging Mountains.
Sometimes I´m able to dig near Mountains, sometimes not. Why?
Why could I dig at the red point and not at the two blue points?
Is it a bug and would it be fixed, later?
The Picture is only a Example.

I have a Question about Digging Mountains.
Sometimes I´m able to dig near Mountains, sometimes not. Why?
Why could I dig at the red point and not at the two blue points?
Is it a bug and would it be fixed, later?
The Picture is only a Example.
