Can I access/edit how the terrain basemap is generated?

Hi,

Editing the shader used for the Unity terrain is no problem (it is included with the built-in shader package), however the basemap seems to be generated differently - so any edits to the main shader is very visible as the basemap fades in/out. Does anyone know where the basemap is generated and if it is possible to edit the generation process? Is it a shader or built in code?

Thanks in advance!