The river formed a natural boundary between the two countries.

【form】这条河成了两国的自然边界。

SO THE RIVER FOYLE IS THE NATURAL BOUNDARY, OR THE BORDER, UNTIL IT COMES TO THE URBAN AREA.

所以福伊尔河是自然界线，或者边界， 直到它到达市区。

The other he ordered straight westward with orders to halt and go into permanent camp just beyond the great river which marks the natural boundary of the country that the big Bwana rightfully considers almost his own.

另一个他命令直接向西前进，命令停下来进入大河对岸的永久营地，这条大河标志着这个国家的自然边界，大布瓦纳理所当然地认为这几乎是他自己的。