Question: Which mountain range, found in eastern North America, was seen as the western frontier prior to about 1800?
Context: Hemlock woolly adelgid ("Adelges tsugae"), or HWA, is member of the Sternorrhyncha suborder of the Order Hemiptera and native to East Asia.  It feeds by sucking sap from hemlock and spruce trees ("Tsuga" spp.; "Picea" spp.) .  In eastern North America, it is a destructive pest that gravely threatens the eastern hemlock ("Tsuga canadensis") and the Carolina hemlock ("Tsuga caroliniana").  Though the range of eastern hemlock extends north of the current range of the adelgid, it could spread to infect these northern areas as well.  Accidentally introduced to North America from Japan, HWA was first found in the eastern United States near Richmond, Virginia, in the early 1950s.  The pest has now been established in eighteen eastern states from Georgia to Massachusetts, causing widespread mortality of hemlock trees.  As of 2015, 90% of the geographic range of eastern hemlock in North America has been impacted by HWA. The Appalachian Mountains ( , French: "les Appalaches" ), often called the Appalachians, are a system of mountains in eastern North America.  The Appalachians first formed roughly 480 million years ago during the Ordovician Period.  It once reached elevations similar to those of the Alps and the Rocky Mountains before naturally occurring erosion.  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. 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.  The mountain range's name derives from its proximity to the sea coast, and it is often referred to as the Coast Range.  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.  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.  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. The Front Range is a mountain range of the Southern Rocky Mountains of North America located in the central portion of the U.S. State of Colorado, and southeastern portion of the U.S. State of Wyoming.  It is the first mountain range encountered moving west along the 40th parallel north across the Great Plains of North America.  The Front Range runs north-south between Casper, Wyoming and Pueblo, Colorado and rises nearly 10,000 feet above the Great Plains.  Longs Peak, Mount Evans, and Pikes Peak are its most prominent peaks, visible from the Interstate 25 corridor.  The area is a popular destination for mountain biking, hiking, climbing, and camping during the warmer months and for skiing and snowboarding during winter.  Millions of years ago the present-day Front Range was home to ancient mountain ranges, deserts, beaches, and even oceans. The Bashagard Mountains or the Bashagerd Mountains (also known as Bashagird or Bashakerd) is a mountain range that is located in southeast of Iran.  The mountain range runs in an arc almost in a northwest-southeast direction from eastern parts of Hormozgan Province along the border with Kerman Province and stretching into southern part of Sistan and Baluchestan Province.  With an elevation of 2185 metres, Mount Buniken (Kuh-e Buniken) is the highest point of the range located in the western section of the Bashagard Mountains, east of Sardasht in Bashagard County in Hormozgan Province, and in the vicinity of a place that marks the borders of Kerman, Hormozgan, and Sistan and Baluchestan provinces.  Made of ophiolite, an assemblage of mafic igneous rocks representing remnants of former oceanic crust, the mountain range was formed finally in the Miocene and the Pliocene during the Alpine orogeny.  The mountain range is located in a region with a hot semi-desert climate. The Baffin Mountains are a mountain range running along the northeastern coast of Baffin Island and Bylot Island, Nunavut are part of the Arctic Cordillera.  The ice-capped mountains are some of the highest peaks of eastern North America, reaching a height of 1525 - above sea level.  While they could be considered a single mountain range as they are separated by bodies of water to make Baffin Island, this is not true, as they are closely related to the other mountain ranges that make the much larger Arctic Cordillera mountain range. Amanita bisporigera is a deadly poisonous species of fungus in the family Amanitaceae.  It is commonly known as the eastern North American destroying angel or just as the destroying angel, although the fungus shares this latter name with three other lethal white "Amanita" species, "A. ocreata", "A. verna" and "A. virosa".  The fruit bodies are found on the ground in mixed coniferous and deciduous forests of eastern North America south to Mexico, but are rare in western North America; the fungus has also been found in pine plantations in Colombia.  The mushroom has a smooth white cap that can reach up to 10 cm across, and a stipe, up to 14 cm long by 1.8 cm thick, that has a delicate white skirt-like ring near the top.  The bulbous stipe base is covered with a membranous sac-like volva.  The white gills are free from attachment to the stalk and crowded closely together.  As the species name suggests, "A. bisporigera" typically bears two spores on the basidia, although this characteristic is not as immutable as was once thought. 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.  Because European settlement in the U.S. expanded westward after its founding, the meaning of the West has evolved over time.  Prior to about 1800, the crest of the Appalachian Mountains was seen as the western frontier. The Piedmont Mountains are outlying mountains, sometimes called “low mountains”, that typically occur in the western Piedmont near the Blue Ridge.  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.  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.  Most of these mountains, or hills, are what is left of ancient eroded mountains.  Some, like Stone Mountain in Georgia, are solitary rock domes called Monadnocks which become further exposed with erosion.  The Piedmont is part of the greater Appalachian Mountain Range and is also referred to as the Appalachian Plateau.  The French definition of piedmont in itself translates as foothill; however, a Piedmont Mountain may be that of greater significance or prominent elevation. The Precordillera terrane of western Argentina is a large mountain range located southeast of the main Andes mountain range.  The evolution of the Precordillera is noted for its unique formation history compared to the region nearby.  The Cambrian-Ordovian sedimentology in the Precordillera terrane has its source neither from old Andes nor nearby country rock, but shares similar characteristics with the Grenville orogeny of eastern North America.  This indicates a rift-drift history of the Precordillera in the early Paleozoic.  The Precordillera is a moving micro-continent which started from the southeast part of the ancient continent Laurentia (current location: North American plate).  The separation of the Precordillera (also named Cuyania) started around the early Cambrian.  The mass collided with Gondwana (the ancient supercontinent in the southern hemisphere) around Late Ordovician period.  Different models and thinking of rift-drift process and the time of occurrence have been proposed.  This page focuses on the evidence of drifting found in the stratigraphical record of the Precordillera, as well as exhibiting models of how the Precordillera drifted to Gondwana.
Answer: Appalachian Mountains