The park's three natural borders lend it distinct regional differences in plant and animal life. (somak.com)
Decorate With Linens For A Period Look - Prussian blue throw with natural border over classical hem stitch bed linen (theswedishfurniture.com)
I covered the geopolitical motifs that drive Russia (the need to protect itself from invasion from West / Southwest / South due to lack of defensible natural borders) in another Politics.SE Q&A. (politics.stackexchange.com)