Drop and sink

The drop function (and the per-biome parameters) control how terrain drops to the ground below (usually as gravel). with the drop function itself providing a scaling of the required depth (so if drop function is a constant -1, then terrain will ‘always’ drop when below the required depth).

The shrink function provides a little bit of noise onto how the height of the dropped terrain is shrunk down towards 0 (so if you had a constant plateau of thickness 10 which all dropped to the ground, the actual height of the columns of terrain resulting from the drop would vary based on the shrink noise).

The drop/shrink stuff is especcialy effective in opening up cave systems both to the surface, and within the cave systems themselves. Eg if two caverns are carved out such that there is only a thin layer of rock between them, the dropping of terrain will allow them to be opened onto eachother (with the associated pile of ground/gravel below the opening).