Lighting Building on landscape : Black shadow spot and places too enlightened

Hello, I was working on a simple Landscape for practicing with mountains for the limits and simple grassy hills at the center. When the Lighting building comes, everything seems to be fine exept that at some places in the mountains I get black spots where I shouldn’t have, at the bottom of a mountain I have a black line, and at the bottom of another mountain I get some spots looking too enlightened.
I looked over the internet but couldn’t find any good solution. Can you help me ?

Note : Using the retopology brush seems to fix some black stain on the mountains