Glasford crater - impact structure is, according to the Earth Impact Database, about 4km in diameter, completely buried, and less than 430 million years old.
The sedimentary target rock in the region of west-central Illinois, in which the crater is preserved, was north of the line of glacial advance, meaning that not only is the crater buried beneath 350 meters (McHone et al., 1987) of overlying sediment, any residual surface contour is not only eroded, but has been scrubbed away by thick ice sheets.
<note: The location circled in the map above is incorrect. The crater is located a little to the north of this site. I will correct in the next round of updates.>
Evidence of Impact Origin
The impact origin of each location listed on this website has been supported by unambiguous diagnostic evidence of hypervelocity impact that has been reported in a scientific (usually peer reviewed) context. Without such evidence, a geological structure is not a confirmed impact crater. This section, which is included for each crater on this website, is not an exhaustive list of such published evidence, but is meant to demonstrate that appropriate work has been done for each listing.
Other significant evidence: breccia, possibly more. I have not read Buschbach and Ryan, 1963.
Bibliography and References:
(If links to articles don't work, don't give up. Try pasting the link shown into a search engine or searching for the article authors, title, or other reference information. If your research leads you to additional scientific references related to this crater, please help improve this resource by sending a note with the new citation(s) to: firstname.lastname@example.org )
Buschbach, T. C., Ryan, R. 1963. Ordovician explosion structure at Glasford, Illinois. Bulletin of the American Association of Petroleum Geologists, v. 47, pp. 2015-2022.
Dietz, R. S., McHone, J. F. 1991. Astroblemes recently confirmed with shatter cones (abstract). Meteoritics, v. 26, p. 332.
McHone, J. F., Sargent, M. L. and Nelson, W. J. 1986. Shatter cones in Illinois: Evidence for meteoritic impacts at Glasford and Des Plaines (abstract). Meteoritics, v. 21, p. 446.
Snyder, F. G., Gerdemann, P. E. 1965. Explosive igneous activity along an Illinois-Missouri-Kansas axis. American Journal of Science, v. 263, pp. 465-493.