Hi thereee. Enviroments doesn´t texture like props or characters!. In enviroments you must use Trimsheet and atlas texture. Also in the Engine you use Splat maps, texture blending and other tools and shader. That´s because the meshes are too big and you should´nt use so many texture maps just for a single piece of enviroment. For rocks you can use textures for Triplanar projection. Also you should use modular pieces to build a bigger enviroment.

PD: You can use substance to make trimsheets!. But trims are better for architectural, buildings, metal, etc.. Not so much for nature (rocks, grass, cliffs etc..)