The Dyfi valley provides the natural frontier between north and south Wales.

The river itself served as natural frontier.

No natural frontier marks the border, and the ethnic composition, language, value systems, and traditions of the two peoples are nearly identical.

Biologists call the canopy the richest but least-known natural frontier on earth.

Carolingians also flouted "natural" frontiers when they divided their lands.

"The natural northern frontier of the new Aryan province of Industan."

I knew the Main to be a broad river south of Taglios, a natural frontier and barrier.

Gender as social group seemed the natural next frontier.

The Elbe is the natural frontier of the Peaceloving Camp.

Together, both rivers marked the natural frontier from the Andes to the Atlantic Ocean.