US Geological Survey
Date of this Version
1907
Citation
The Journal of Geology, Vol. 15, No. 6 (Sep. - Oct., 1907), pp. 526-549
Abstract
INTRODUCTION
Investigations of the United States Geological Survey during the summer of I906, covering the larger part of the Laramie exposures on the Laramie Plains, examined by the King and Hayden surveys, have revealed many new and important facts bearing on the Laramie problem.
By detailed areal surveys it was found: (1) that the lignitiferous series, which in the Laramie Plains lies between the Montana below and the Fort Union above, and has a maximum thickness of about 12,500 feet, is divided about the middle by an unconformity; (2) that this unconformity is in the same stratigraphic plane and continuous with the unconformity which in the vicinity of Carbon and to the southeast separates all the Laramie beds studied by the Hayden and King parties from the underlying Cretaceous; (3) that the beds above the unconformity rest, often with great divergence of dip, on all the underlying beds down to and including the Dakota; (4) that the basal conglomerate, locally well developed at the horizon of the unconformity, while composed largely of materials derived from the underlying Cretaceous rocks, notably the Benton, contains pebbles and bowlders from the pre-Cambrian crystallines now exposed in the hearts of the surrounding ranges. This unconformity thus involves the total thickness of the Cretaceous portion of the beds below the unconformity, and probably the whole sedimentary series of this region, or over 20,000 feet of strata.
The Laramie Plains section in brief is as follows:
It may be said that in the areas examined by the Hayden and King parties, all of which were immediately along and south of the Union Pacific Railroad, the lower division is usually absent, the upper resting generally on one or another of the members of the Montana, but at one point extending over on to the Dakota. The large development of the division beneath the unconformity and above the Montana, corresponding to the "Lower Laramie" of literature, lies entirely north of the area they studied, and its existence was not known until the work of the past season.
The three principal usages of the term "Laramie" today are shown in the recent textbook of geology by Chamberlin and Salisbury. Here "Laramie series" is applied to all the beds between the Montana and Fort Union. Upper Laramie and Lower Laramie or "Laramie proper" are applied respectively to the beds above and below the great unconformity, first shown by Cross to exist in the Denver region, then found by Weed in western Montana, in 1905 found in southern Uinta County, Wyoming, by the writer, and traced northward by Dr. Schultz in 1906 to a point just south of Yellowstone National Park, and again found in 1906, most strikingly developed, in Carbon County, Wyoming, in the western part of the Laramie Plains. The Lower Laramie has also been called "true Laramie," and sometimes simply Laramie. The complexness of the situation is excellently illustrated by the use of "Laramie" by Stanton and Knowlton in seven different combinations on a single page of their report, namely "Laramie series," "true Laramie," "so-called Laramie," "original Laramie."