While this might appear to be the result of a landscape retexture gone bad, where two areas of land don't meet in a way that allows the textures to blend, or are of adjacent cells and are assigned by different mods, it is not actually landscape textures causing the problem: it's vertex shading. One cell quadrant has the artificial landscape shadows (vertex shading) and the opposite side does not.
In the latter case of textures in adjacent cells and assigned by different mods, please see the 'Checklist Item #15 & 16' entries in the wiki "Fallout NV Mod Conflict Troubleshooting" guide regarding the need to run both TES4LL and FNVLODGen; or xLODGen - Terrain LOD beta for FNV, FO3, FO4, FO4VR, TES5, SSE, TES5VR, ENDERAL. Also recommend you read the 'LOD/VWD Texture Packs' section of the wiki "FNV General Mod Use Advice" article first.
Otherwise you would have to use a landscape editor to fix the vertices. The procedure is outlined in the TES Texture Guide.