pubs.er.usgs.gov/publication/70136641
I took this amazing "Geologic map of the United States (exclusive of Alaska and Hawaii)" by Philip Burke King, Helen M. Beikman, and Gertrude J. Edmonston, published 1974, showing the age of rocks (Cenozoic, Mesozoic, Pre-Cambrian!) in the US, found its projection coordinates, and reprojected it to equirectangular so anyone can clip out interesting tidbits from it.
https://github.com/fasiha/steppe-map for the cartographic goodness.