Forests of elm, sugar maple, basswood, and oak once covered more than 2,000 square miles of south-central Minnesota, extending in a band 40 miles wide from Mankato to Monticello. It was not until 100 years later, through careful study of the notes of public land surveyors from the 1840s and 1850s and examination of fossilized pollen grains left in bog and lake sediments, that scientists worked out the origin of the Big Woods in detail. The development of the Big Woods turned out to be a relatively recent event that coincided with climate cooling over North America about 300 to 400 years ago. Before this cooling, fire maintained the brushlands, prairies, and oak savannas that covered southern and western Minnesota. When the climate cooled, wildfires diminished in the area that would become the Big Woods.
Forests spread outward from small, isolated groves into the brushlands and prairies.
Fires remained frequent enough on the flatter and often drier lands surrounding the Big Woods region that brushlands, prairies, and savannas persisted in these areas. Bounded by rivers and rolling, lake-dotted terrain, the Big Woods region burned much less frequently. Dense, tall forests of elm, sugar maple, basswood, and oak developed during the next few hundred years. The Big Woods was not only a curiosity to new inhabitants, but also a source of livelihood in the 1800s. Settlers who located their farms on nearby prairie land traveled to the Big Woods to obtain lumber to build houses and barns and firewood to heat them.