When the first explorers came to what became Minnesota, they found three very different landscapes: great northern forests of white pine and other conifers, the flat fertile prairie of the Great Plains to the south and west, and in what is now south-central Minnesota the dense broadleaf forest settlers called the Big Woods. The Big Woods marked the western edge of the vast deciduous forest that stretched across the middle United States from the Atlantic coast to the Great Plains, and in Minnesota this deciduous belt ran from the northwest to the southeast, thickening in the middle to form this forest. Elm, basswood, sugar maple, and red oak covered more than 3,000 square miles there, rising into a canopy so high and dense that it nearly blotted out the sun in summer. Fire helped determine the forest's boundaries, as prairie fires started by lightning or by Indians for hunting kept broadleaf trees from spreading into the grasslands, while lakes, rivers, and rough terrain acted as natural firebreaks that kept those fires from spreading into the forest. Because fertile soil lay beneath it, much of the Big Woods was cleared for farming during the last half of the nineteenth century, and only a few remnants of the great forest survive today, the largest east of Northfield at Nerstrand Woods State Park.