verge on/upon

: brink, threshold

: something that borders, limits, or bounds: such as

: an outer margin of an object or structural part

: to be contiguous

: to be on the verge or border

: to move or tend toward the horizon : sink

: to move or extend in some direction or toward some condition

: to be in transition or change

: on

: on the surface : on it

: thereafter, thereon