To achieve the look we were aiming for we researched all the tech currently available. We made very high resolution ground textures and added distant normalmaps to avoid repetition. We used a lot of procedural foliage and scattered hundred of thousands of rocks and pebbles; we also implemented atmospheric scattering, high quality soft shadows, god rays and so on.
This was fine, but we still needed iconic large assets that add scale to the environment. And what can be bigger then mountains or cliffs? The common technique is to sculpt or displace them using special tools, but these models lacked scale and shape. So we tried out 3d scanning and this was the key. We made hundreds of photos from nearby rocks and even created a small in-house photo studio for smaller assets.
Then we processed these images with special 3D scanning software. The resulting files looked marvellous but were gigabytes in size with a gazillion polygons, so we had to hand tune them to something we can use in a real time game. These final models gave us silhouettes and details, nearly impossible to sculpt from scratch. On top we added highres textures for details and normal maps.