What is Natural Boundary?

Legal Definition
Any formation or product of nature (as opposed to structures or erecdons 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 the like. See Peuker v. Canter, 62 Kan. 363, 63 Pac. 617; Stapleford v. Brinson, 24 N. C. 311; Eureka Mining, etc., Co., v. Way, 11 Nev. 171.
-- Black's Law Dictionary
Legal Definition
A boundary formed by natural objects.
-- Ballentine's Law Dictionary