Quixel has just released a high-quality collection of surfaces and assets including sand, soil, gravel, stones, boulders, rock formations, stumps, and branches.
All textures are provided in 4096×4096 resolution and include:
Physically Based Rendering (PBR): Yes
LOD: Yes
Collision: Yes
Texture size: Up to 4K
Number of Meshes: 31
Number of Materials and Material Instances: 10 Materials, 33 Instances
Number of Textures: 122
Documentation: No