What I see is part of the terrain.
It looks like you ar using a fractal procedural terrain.
I have seen the 'lines' very often in terrains with 'lack' of detail.
Try some settings in the 'Terrain'-Group, Heightfieldshader, Tab 'Fractal Detail'.
The values 'Fractal Amount' and 'Fractal Variation' sound interesting.
Volker