From Ohio History Central
Angiosperms are flowering plants that have protected seeds and are the dominant and familiar plants of today. They first appeared in the Cretaceous but Ohio's fossil record dates to the Pleistocene Ice Age, as rocks from the Mesozoic and most of the Cenozoic are absent from the state. Many sediments deposited in association with the glaciers have a rich record of vegetation living in the area at the time of deposition. These plants and trees in most cases are from species still living, although during colder glacial times many species do not live in Ohio today in abundance but do live in the boreal forests of Canada.
The most recognizable plant remains are logs and branches of spruce trees and other conifers. Some of them represent living forests that were destroyed as ice advanced and overrode the trees. Bogs and glacial kettle lakes may have many specimens of logs and branches entombed in the sediment along with seeds and a rich record of microscopic pollen. The larger logs and branches look like modern wood and will burn when dried out. Many people are surprised to learn that these "sticks" are more than ten thousand years old. These specimens have been used extensively for radiocarbon dates at many sites in the state.
Glacial ice had retreated northward and was gone from Ohio by about 14,000 years ago. The open spruce forests that characterized Ohio during glacial episodes persisted until about 10,000 years ago when the vegetation changed in a relatively short period of time from boreal forest to northern hardwood forests. This was in response to a warming climate. Perhaps not coincidentally, many of the large mammals, such as mastodon and mammoth, became extinct at this time.
- Cross, A. T., Gillespie, W. H., and Taggart, R. E., 1996. "Ohio Vegetation--the Last Million Years," in Fossils of Ohio, edited by R. M. Feldmann and Merrianne Hackathorn. Ohio Division of Geological Survey Bulletin 70, p. 480-505.