Question: Which mountain range, found in eastern North America, was seen as the western frontier prior to about 1800?

<P0-S0> The Appalachian Mountains ( , French: "les Appalaches" ), often called the Appalachians, are a system of mountains in eastern North America. <P0-S1>  The Appalachians first formed roughly 480 million years ago during the Ordovician Period. <P0-S2>  It once reached elevations similar to those of the Alps and the Rocky Mountains before naturally occurring erosion. <P0-S3>  The Appalachian chain is a barrier to east-west travel, as it forms a series of alternating ridgelines and valleys oriented in opposition to most roads running east or west. <SEP> <P1-S0> The Piedmont Mountains are outlying mountains, sometimes called “low mountains”, that typically occur in the western Piedmont near the Blue Ridge. <P1-S1>  Most of the features within the Piedmont physiographic region of North America lie either on the eastern border where the plateau plunges onto the Coastal Plain at the Fall Line, in the broad valleys of the river systems, or on the western border where Piedmont Mountains likely occur. <P1-S2>  Occasionally, due to diverse rock formations, folds and outcroppings, these mountains can rise at various locations across the Piedmont like the Uwharrie Mountain Range in North Carolina or the Pine Mountain Range in Georgia. <P1-S3>  Most of these mountains, or hills, are what is left of ancient eroded mountains. <P1-S4>  Some, like Stone Mountain in Georgia, are solitary rock domes called Monadnocks which become further exposed with erosion. <P1-S5>  The Piedmont is part of the greater Appalachian Mountain Range and is also referred to as the Appalachian Plateau. <P1-S6>  The French definition of piedmont in itself translates as foothill; however, a Piedmont Mountain may be that of greater significance or prominent elevation. <SEP> <P2-S0> The Coast Mountains are a major mountain range in the Pacific Coast Ranges of western North America, extending from southwestern Yukon through the Alaska Panhandle and virtually all of the Coast of British Columbia south to the Fraser River. <P2-S1>  The mountain range's name derives from its proximity to the sea coast, and it is often referred to as the Coast Range. <P2-S2>  The range includes volcanic and non-volcanic mountains and the extensive ice fields of the Pacific and Boundary Ranges, and the northern end of the volcanic system known as the Cascade Volcanoes. <P2-S3>  The Coast Mountains are part of a larger mountain system called the Pacific Coast Ranges or the Pacific Mountain System, which includes the Cascade Range, the Insular Mountains, the Olympic Mountains, the Oregon Coast Range, the California Coast Ranges, the Saint Elias Mountains and the Chugach Mountains. <P2-S4>  The Coast Mountains are also part of the American Cordilleraa Spanish term for an extensive chain of mountain rangesthat consists of an almost continuous sequence of mountain ranges that form the western backbone of North America, Central America, South America and Antarctica. <SEP> <P3-S0> The Western United States, commonly referred to as the American West, the Far West, or simply the West, traditionally refers to the region comprising the westernmost states of the United States. <P3-S1>  Because European settlement in the U.S. expanded westward after its founding, the meaning of the West has evolved over time. <P3-S2>  Prior to about 1800, the crest of the Appalachian Mountains was seen as the western frontier.