The probable location of the "Ancestral Rocky Mountains" was first described by Willis T. Lee in 1918. Inasmuch as he failed to delineate their areal extent, subsequent references and maps have differed in regard to the location of these mythical mountains. The writer has brought together all the evidence which bears on the problem. He has prepared isopach maps of the Cordilleran basin in order to show the probable source of sediments during the Mississippian, Pennsylvanian, and Permian periods. On these maps he shows the areas where clastics make up the total or a large part of the stratigraphic section for these systems, again showing the probable source of sediments or the close proximity of land.

The evidence tends to prove that the Ancestral Rockies probably began their history in Proterozoic time; that they were rejuvenated at the close of Ordovician time, and again at the close of Mississippian time. The Ancestral Rockies of Lee, however, began their history in middle Pennsylvanian time and probably constituted a range quite as high as the present mountains. The evidence shows that the following tectonic elements were the framework of the Ancestral Rockies: Uinta-Front Range element; Uncompahgre-Sangre de Cristo element; Defiance-Zuni element; Pedernal-Mesa de Maya element; and Amarillo-Wichita element. Each is described as to location, height, source of sediments, and geologic history.