EXPANSION, FRAGMENTATION, AND REUNIFICATION 133
Preuss drew these maps at the request of David
Atchison, Missouri’s second senator and as ardent
an expansionist as Benton. Atchison hoped that
detailed maps of the Oregon Trail would facilitate not
just migration but also the construction of strategic
military forts along the way. To compile the maps,
Preuss drew from Fremont’s journal as well as his own
field experience on these expeditions. Shown here is
the fourth sheet of the series, which covers the area
of Wyoming just northeast of Casper through South
Pass to Jackson.
Preuss oriented the maps to show the geography
that lay immediately ahead on the trail. Each sheet
gave practical field information about the presence
of Native Americans as well as climate, wildlife,
fuel, and game. Information about temperature and
rainfall was especially welcome in the 1840s, for
detailed knowledge of climate in this region—long
stereotyped as the “Great American Desert”—would
not be widely available for another decade. For
all these reasons, Preuss’ maps were among the
most important and accurate profiles of western
geography of the time. The Mormons also used the
maps, though ironically their goal was not Oregon
or California, but instead the blank spaces south of
the trail in what was then northern Mexico. There,
they hoped to live and worship together free from the
persecution that plagued them in the United States.
By the time the maps were published, the nation
was at war with Mexico and negotiating with Britain
over control of the Oregon Territory. Ultimately,
both of these engagements enlarged the nation’s
reach to the Pacific Ocean. Once these territories
were secured, maps such as Preuss’ supported a
mass migration to the Far West. From 1840 to 1860,
300,000 Americans made the overland trail to Utah,
Oregon, and California. In this context his maps
of the Oregon Trail are not just passive records of
geographical information but active instruments of
national expansion. If 1846 was indeed the “year of
decision” for the United States in the West, as Bernard
DeVoto has argued, Preuss and Fremont guided these
decisions. Their detailed maps enabled Americans to
see, understand, and take control of the West.