Black Dots on LandScape

So i imported a Heightmap and made my landscape , then applied the material into it with 4 layers . but after building lightning these wierd black dots appear on the landscape and they are literally everywhere .I tested both of the layer blending options < height and alpha > didnt work . how can i get rid of this artifact ? thanks.

  1. build the light with production quality
  2. increase the lightresolution of the landscape (in the properties)

Thanks Fighter . that solved the problem . i tried the value of 4 . and it got rid of the most of the dots.