Fremont's Map of Oregon and Upper California . . . is without question of the most important 19th Century maps of the American West. As noted by Carl Wheat, "in the history of the American West, the year 1848 is signalized by three events above all others, the discovery of gold at Sutter's Mill… the formal close of the Mexican War… which brought the cession of California and New Mexico… and the creation of the Territory of Oregon… All three events figure in the characteristic maps produced during the year, and particularly the cartographic monument of 1848, the magnificent 'Map of Oregon and Upper California'" (Wheat, 49).
Fremont's map was originally published to accompany Fremont's Geographical Memoir upon Upper California, in illustration of his map of Oregon and California, 1848. "The map and Geographical Memoir have a two-fold importance in history, first as contributions to geographical knowledge in the year 1848, and second as historic documents concerning Fremont's notable 3rd expedition… [The map] is a wonderfully graphic report on where the expedition of 1845-46 went and what it saw… As a contribution to cartographical knowledge, the case for the map was well put by Fremont himself: 'The map has been constructed expressly to exhibit the two countries of Oregon and the Alta California together. It is believed to be the most correct that has appeared of either of them; and it is certainly the only one that shows the structure and configuration of the interior of Upper California'" (Wheat). Schwartz calls it an "epochal" map and states that the map is the "most accurate general map of the Far West for its time" (Schwartz and Ehrenberg, The Mapping of America, 275, 278). "More than any other persons, John Charles Fremont and Charles Preuss dominate the cartography of the American West during the three years before the Gold Rush brought a human tide surging into that land which had so long lain beyond the ken of most Americans" (Wheat, Mapping of the Transmississippi West, 523).
The northern and southern borders are outlined in green, as is the border between Oregon and California and the eastern boundary of the Rocky Mountains, the South Pass and Sierra Madre. At the top of the map is an elevation profile of the route from the South Pass to San Francisco. Both the Plumas and American Rivers show areas marked "'El Dorado' or Gold Region," one of the earliest graphic announcements of the discovery of gold in California. According to Howes, this Preuss map was "not issued in all copies, if any, [of Fremont's Geographical Memoir] but [was] intended to accompany the pamphlet" (Howes F366). Graff lists a first edition copy of the Fremont work with first issue Preuss map laid-in as well as an identical copy of the map issued separately.
"This important and beautifully drawn map became the model for many of the later gold region maps. The California portion is based on Fremont's map of 1845, but the legend 'El Dorado or Gold Rush Region' has been added along the 'Rio d. l. Plumas' (Feather River), and the 'R. d. l. Americanos' (American River), which is shown flowing out of 'Lake Bonpland' (Tahoe) The map covers the territory from the boundary of the 'British Possessions' on the north to the Mexican border on the south, and from the Pacific Ocean on the west to Fort Laramie and the "Great Plains" on the east (Wheat)."[B]y far the most accurate map of the Far West up to the time of its publication The rapidly changing political character of the West is not neglected on the map. 1848 was a milestone year for establishing United States territories and boundaries, and the recognition of these most recent developments make the map an up-to-date document. 'Oregon Territory' established by Congress on August 114, 1848, is clearly delineated, as are the boundaries with Mexico laid out by the Treaty of Guadalupe Hidalgo. Missouri Territory is also indicated, as was, for the first time on a published map, the presence of the Mormons in the Great Basin" (Cohen).