Final answer:
The Appalachian Mountains were the first natural barrier to the expansion of the American frontier, reinforced by Britain's Proclamation Line of 1763 which sought to prevent westward settlement.
Step-by-step explanation:
The first natural barrier to the expansion of the frontier for American colonists was the Appalachian Mountains. These mountains served as a significant obstacle, discouraging and preventing the westward movement of settlers. The British Proclamation Line of 1763 officially restricted settlement beyond the Appalachians, intending to prevent conflict with Native American tribes and to avoid the costs associated with maintaining security on such a distant frontier. This led to significant dissatisfaction among colonists, including those like George Washington, who had interests in western lands for economic diversification and relief from the stagnated tobacco economy. The tension over land rights and the desire for westward expansion contributed to the growing rift between Britain and its North American colonies, setting the stage for the American Revolution.