 Indian de-population 1500-1800 a.d. reduced burnings, which led to the oak savannas succeeding into sugar maple - basswood and red oak "closed forest" savannas.Native vegetation based on soils information (note the bright green color) from the Natural Resources Conservation Service of the United States Department of Agriculture shows the historic extent of oak savannas in the Big Woods region (See accompanying pie chart, below).The soil of the Big Woods is thick glacial till of crushed limestone, deposited by the Des Moines lobe of the Wisconsin glaciation 10,000 years ago.