After 1718 the Scotch-Irish began colonizing in the Cumberland Valley and gradually pushed the frontiers toward W Pennsylvania. Their rugged independence and the peculiarities of their frontier problems made them rebellious against the established order. Throughout the province agriculture was the chief occupation, although industry was spurred by abundant water power and plentiful natural resources.

In the west settlement was hindered by a growing unrest among the Native Americans. Penn’s heirs lacked both the good sense and the ethical values that prompted Penn’s fair and considerate treatment. Resentful of encroachment on their lands and of the land purchase made by the Albany Congress (1754), the Native Americans allied themselves with the French, who were then fortifying positions in the Ohio valley. The frontier settlements were severely ravaged until, after several reverses, the French abandoned (1758) Fort Duquesne to British and American forces under Gen. John Forbes.

The power of the Native Americans was not completely broken until the suppression of the uprising of 1763. The inept defenses provided by the Quaker-controlled assembly during the crisis aroused bitter resentment and intensified efforts to overturn proprietary rule. The struggle between proprietary and antiproprietary parties was soon overshadowed, however, by the opposition to British imperial policies that culminated in the American Revolution.