Mainly material relating to Wheeler's investigations of Custer and the Battle of Little Big Horn, but also a few items regarding the Pacific railway route, newspaper articles about Wheeler, and reviews of his The Trail of Lewis and Clark.

Battle of Little Big Horn material includes correspondence with individuals interested in or involved in the affair (Custer's wife, the photographer D.F. Barry, F.F. Girard, Capt. E.S. Godfrey, Theodore W. Goldin, Capt. Charles King, C.A. Lounsberry, Valentine McGillycuddy, Richard A. Roberts, and T.A. Warburton). There are also battlefield maps drawn by Indian guides in 1892, a questionnaire completed by Bob Failed Horse, Medicine Bear, and Spotted Blackbird in 1902, and Wheeler's manuscript and proof copy for the 1893 issue of Wonderland.

Pacific railway route material includes typed transcripts of Senate and House proceedings regarding the issue, 1853-1859, and an undated typescript of an article by Wheeler, "Pacific Railway Congressional Debates of Antediluvian Days."

Photographs collected by Wheeler and of him comprise the Olin D. Wheeler Collection of Photographs of Yosemite Valley and the Route of the Lewis and Clark Expedition, which is part of the Edward E. Ayer Photograph Collection.