The United States laid the groundwork for becoming a major power in part by developing territory that was not utterly dependent upon the sea, which, in the case of North America, was the Greater Mississippi Basin. Achieving such strategic depth was both an economic and a military imperative. With few exceptions, the early American population was based along the coast, and even the exceptions — such as Philadelphia — were easily reached via rivers. The United States was entirely dependent upon the English imperial system not just for finished goods and markets but also for the bulk of its non-agricultural raw materials, in particular coal and iron ore. Expanding inland allowed the Americans to substitute additional supplies from mines in the Appalachian Mountains. Collectively the Louisiana Purchase, the National Road and the Oregon Trail facilitated the largest and fastest cultural expansion in human history. From beginning to end, the entire process required fewer than 70 years. However, it should be noted that the last part of this process — the securing of the West Coast — was not essential to American security. The Columbia River Valley and California's Central Valley are not critical American territories. Any independent entities based in either could not generate a force capable of threatening the Greater Mississippi Basin. This hardly means that these territories are unattractive or a net loss to the United States — among other things, they grant the United States full access to the Pacific trading basin — only that control of them is not imperative to American security.