What physical barrier follows the approximate boundary set by the proclamation of 1763?

The Appalachian Mountains.

The boundary of the Proclamation of 1763 was placed at the western edge of the English colonies which fell on the Appalachian Mountains.

The Proclamation of 1763 was issued along with the ending of the French-Indian (Seven Years') War. Great Britain obtained the land from the Mississippi to the Appalachian Mountains up into French Canada from France through the Treaty of Paris 1763. Great Britain established the Proclamation line to reserve the new lands for Native American tribes and French settlers.