Terrain and character turn black after moving for a while,Terrain and Model turn black after movement in the scene

I am quite new to unity and have searched on the forums for similar scenarios but to no avail.

When I play the scene the game and character loads normally and everything is fine, however once i begin to move forward after a small amount of time some black flickering appears on the terrain such as in the image below.

As I then continue to move forward the terrain blackens more and appears to create a big black circle around the character for a large distance.

Any help would be greatly appreciated