The correspondence series includes approximately 1100 letters written between 1892-1939. The majority of the collection are letters between Frank Leverett and Frank Bursley Taylor; they discuss their field work, Monograph 53, other publications and various related problems. There is also other correspondence with other geologists, including T.C. Chamberlin, Grove K. Gilbert, J.W. Goldthwait, H.L. Fairchild, et alia. There is extensive correspondence with the U.S. Geological Survey, the Geological Survey of Canada, and the Michigan Geological Survey. The primary subject of this series is the surficial glacial geology of the midwestern U.S. and Canada. Leverett & Taylor's work was essential for understanding how the Great Lakes were formed as the Pleistocene glaciers advanced and retreated from the midwestern states. The letters describe the 30 year process of gathering data, mapping the data and constructing the picture of glacial processes during the last Ice Age.