A surface is a living system with its own structure and cycles of production. It is a performative medium that conveys water and supports organisms like bacteria, fungi, plants, and animal life. It is the result of processes that take place under it such as the decomposition of rocks and their migration upwards from the depth of the ground. It is also the result of processes that take place over it like erosion caused by wind, water, and human activity. It responds to external systems like climactic patterns that evolve in their own composition. In its biological sense, the surface in landscape architecture is less a boundary and more a zone of connectivity. It is a place where vegetational, hydrological, and soil systems interact.
If we had this conversation twenty years ago there would be discussion of the formal properties of surface. We might be looking at the paintings of Malevich or Kandinsky, or the photography of Gursky.