Any formation or product of nature (as opposed to structures or erections made by man) which may serve to define and fix one or more of the lines inclosing an estate or piece of property, such as a watercourse, a line of growing trees, a bluff or mountain chain, or Hie like. See Peuker v. Canter, 62 Kan. 303, 63 Pac. 617; Stapleford v. Brin- son, 24 N. C. 311 ; Eureka Mining, etc., Co. v. Way, 11 Nev. 171.
TLD Example: The Rio Grande is a river forming a natural boundary between Mexico and the United State.