Sun (light) comes through the landscape


I am new with unreal engine and at the moment i a just fooling around to get a grip on it (i do have a idea for a game but for now i try to understand the engine) .

At the moment i have a problem with the “sun” coming through the landscape.
I tried fixing this with a plane (see photo) but then the shadow doesn’t follow the mountain correctly (come out straight).

Is there a way to cut the plane, so that is follow’s the mounting ? Or do i have to do something else.

Thank in advantages and greeting from The Netherlands.

PS. sorry for any grammer mistakes