The main geographic feature that formed the boundary of the thirteen British colonies in North America was the Appalachian Mountains. While the British had control of most of the land east of the Mississippi River as a result of defeating France in the French and Indian War, the colonists weren’t allowed to go west of the Appalachian Mountains when the British government passed the Proclamation of 1763. The British were concerned there would be conflicts with the Native Americans, who weren’t happy when Great Britain gained control over the land previously owned by the French. The British government stated that its main goal was to keep the colonists safe from potential attacks by the Native Americans, although many colonists viewed this action as a way for the British to control them. Thus, the geographic feature that formed the westward boundary of the thirteen British colonies was the Appalachian Mountains.