D% for size and type of terrain, type then size, two rolls?

Biomes: grassland, forest, wetlands, desert, ocean.

Grassland can border ocean, desert, forest or wetlands, plus wetlands.

Ocean borders grassland or ocean 

Forest borders grassland, wetlands or forest.

Wetlands border grassland, forest, or westland.

Desert borders grassland or desert.

Add mountains to forest and desert?