Iowa is on the borderland between prairie and forest. The topography of the state has been largely determined by the different glacial drift sheets which cover the greater part of the state. The streams too have been influenced by the different glacial drifts. Two mighty streams mark the boundary of the state, the Mississippi, finest and best stream of the United States Keokuk in southeastern Iowa to New Albin in northeastern Iowa there are numerous limestone outcrops of various ages. Although the topography is fairly uniform over much of this state, there are marked and abrupt changes brought about by different geological formations. For instance, in northeastern Iowa the Oneota limestone and the St. Peter and St. Croix sandstone; along the Des Moines from Keokuk to Webster county the carboniferous sandstone at such points as Cordova, Red Rock, and Pine Creek, Muscatine county. These formations have had more to do with the prevention of the outward extensions of certain trees, shrubs and herbaceous plants than topography. Let us use a few illustrations of the distribution of some trees.
Pammel, L. H.
"The Native White Pine of Iowa,"
Ames Forester: Vol. 9
, Article 8.
Available at: http://lib.dr.iastate.edu/amesforester/vol9/iss1/8