Map showing spatial and temporal relations of mountain and continental glaciations on the Northern Plains, primarily in northern Montana and northwestern North Dakota

Abstract

This report is an overview of limits of glaciations and glacial history in, and east and southeast of, Glacier National Park, Mont., and on the Northern Plains farther east in Montana and northwestern North Dakota (lat 47–49 N., long 102–114 W.). Glacial limits east of long 102 W., in the United States and also in adjacent Canada, are shown on published maps of the U.S. Geological Survey Quaternary Geologic Atlas of the United States (I–1420) (for example, Fullerton and others, 1995, 2000). The glacial-advance limits shown on this map are from data compiled for the Lethbridge, Regina, Yellowstone, and Big Horn Mountains 4 × 6 quadrangles. Limits of Laurentide glaciations shown on the map supersede those mapped by Colton and others (196l, 1963) and Soller (1993, 1994). An abbreviated version of this report is presented in Fullerton and others (2004). This pamphlet is an expanded explanation of the map and supplemental illustrations. Reference to the map is required to follow this discussion. However, the reader can visualize spatial and temporal relations of the glacial limits and patterns of regional ice flow simply by study of the map and figures 1 and 2. The map and figures 1 and 2 depict ice-flow patterns and selected glacial limits in a very large geographic region. Comprehensive discussion of data from this region is beyond the scope of this report. In this pamphlet, we discuss selected glacial limits and selected stratigraphic, geomorphic, and sedimentologic data in a spatial and temporal context. In general, discussion of specific areas under a temporal subheading (for example, “Illinoian Glaciation”) proceeds from west to east. This pamphlet is a summary of published and unpublished information from field studies in Montana and northwestern North Dakota by D.S. Fullerton and R.B. Colton. Discussions of stratigraphic, spatial, and temporal relations of tills in Montana and North Dakota are primarily based on field observations, secondarily on regional synthesis. Discussion of these complex relations is intended for a scientific audience. Descriptions of physical characteristics of tills, discussion of stratigraphic, spatial, and temporal relations of specific till units, and discussion of the history of glaciation are provided for the reader who is interested in the glacial deposits and history in one or more specific geographic or physiographic areas and who also has some prior knowledge of Quaternary deposits and history in the region. We summarize distinguishing field characteristics of Laurentide tills on the plains in Montana in the appendix. Depiction and discussion of selected glacial limits in adjacent Canada is necessary to summarize the history of glaciation in Montana and North Dakota. Discussion of geologic relations in Canada is restricted to topics relevant to interpretation of Quaternary stratigraphy, chronology, and history in Montana and North Dakota. The term “Laurentide glacier” was applied by Chamberlin (1895) to a continental ice sheet east of the Rocky Mountains in North America. Flint (1943) referred to the same body of ice as the “Laurentide ice sheet.” Fulton and Prest (1987) and Dyke and others (1989) proposed that the name “Laurentide ice sheet” be applied only to the ice sheet of Wisconsin age. Dyke and others (1989, p. 184) suggested that ice sheets of Illinoian and pre-Illinoian ages be referred to as “Laurentide ice,” but not “Laurentide ice sheets.” “Laurentide ice sheet” here refers to any Quaternary continental ice sheet east of the Rocky Mountains in the United States and Canada. “Laurentide till” refers to till deposited by a Laurentide ice sheet. A Laurentide ice sheet here is distinguished from a Cordilleran ice sheet (Chamberlin and Salisbury, 1906) in the Cordilleran region in parts of Washington, Idaho, and Montana in the United States and in adjacent Canada. Clague (1989) indicated that Cordilleran ice sheets formed several times during the Pleistocene. He did not restrict the term “Cordilleran ice sheet” to an ice sheet of Wisconsin age. Numerical ages cited in reference to the astronomically tuned marine oxygen isotope time scale are from Martinson and others (1987), Bassinot and others (1994), Berger and others (1994), and Chen and others (1995). Most of the conventional radiocarbon (C) ages cited also are given as calibrated ages (CAL ages). The CAL ages (Bard and others, 1990a,b) cited were calculated by using the simple linear equation for calibration in Bard and others (1998):