Johnson W.C, Willey K.L, Macpherson G.L.  2007.  Carbon isotope variation in modern soils of the tallgrass prairie: Analogues for the interpretation of isotopic records derived from paleosols. Quaternary International. 162:3-20.

Johnson W.C, Willey K.L, Mason J.A, May D.W.  2007.  Stratigraphy and environmental reconstruction at the middle Wisconsinan Gilman Canyon Formation type locality, Buzzard's Roost, southwestern Nebraska, USA. Quaternary Research. 67(3):474-486.