Evidence of Climate Change over the Last 10,000 Years from the Sediments of Lakes in the Upper Mississippi Basin http://pubs.usgs.gov/fs/fs-0059-99/
Walter E. Dean, United States Geological Survey
This USGS fact sheet discusses paleoecological records derived from sediments in the upper midwestern US (e.g. Minnesota). The paleoecological record was derived from varved (seasonally-laminated) sediment cores, geochemical data, and pollen.