a Geographic structure is a map feature that cannot be clearly separated by sim boundaries. Geographic structures can be found inside a single sim or they can cover multiple sims. It is found inside a single grid structure, like a Continent, a Subcontinent or a Microcontinent and it has a continuous shape.
Unlike a Subcontinent, a Geographic structure is not separated by sim boundaries. Its borders can be made of ground text features, parcel borders, buildings, shorelines or variations in altitude or declivity.
All Geographic structures are listed on List Of Geographic Structures.
There are many examples of Geographic structures that can be found throughout the grid, but mainly on continents:
A river crosses many sims, but it is a sharp stretch of water. There are, also, dried rivers.
Mountains can exist inside a single sim or they can form a complex mountain range (see Mountains Of Heterocera. Their borders are not made by sim boundaries, but by surrounding plains, plateaus or waterways.
An island is separated by water and can be found in one or more sims.
The East River Community is a good example, it stretches on many sims, but there are parcels almost in each sim that are not owned by the community.