For the rocks I went and took my own reference photos here in Texas. I also took my own photogrammetry of rock walls and took the resulting photogrammetry geo and converted them into booleans in Zbrush to help out in the sculpting process and believability in about 75% of my rocks. Even the ones that used my photogrammetry zbrush booleans there was a good amount of hand sculpting to get the shape and detail just right.
The rock booleans were basically created by taking the photogrammetry scan I took from trips thoughout Texas during early development, sculpting/refining/cleaning the scan and then inverting the geo to turn into a mesh to boolean around with. Then to create a new rock I'd start off by sculpting a base rock shape that I wanted that I though was needed to construct the level and then to get the medium and small shapes I went around with my scan booleans to create the finished rock. Throughout the process there were often areas that just needed to completely be sculpted by hand but it kind of gives you an overview as to my very organic process to creating these.
------------------------------------------------
Environment Art Lead:
https://www.artstation.com/davidhenchey
Art Directors:
https://www.artstation.com/pombero
& Chris Sutton

Go check out the rocks in-game:
https://www.artstation.com/artwork/496OEn