Natural boundary means those points of the boundary of a region that marks the beginning of a boundary line. Usually it is formed naturally and it might be something like a river, mountain range, or an ocean. A person would generally require additional equipment or assistance, such as a boat or horses to cross a natural boundary. For example, ‘rio grande’ is the natural boundary between Texas and the country of Mexico. It is also known as a public boundary.
In Sherrod v. Battle, 154 N.C. 345 (N.C. 1911) and Hoge v. Lee, 184 N.C. 44 (N.C. 1922), the court held that “when the natural boundary is unique, or has properties peculiar to itself, these pointers or guides can have but little effect; in fact, the court believes, none. Where there is more than one natural object in the neighborhood answering the description--that is, having common qualities--then those pointers or guides may be reverted to ascertain where the object called for is, or which is the object designated. They do not then contradict or controvert natural boundary; they explain a latent ambiguity created by there being more than one object which answers the description”.