Abstract

The Monarch Valley, in Grand County, Colorado, on the western slope of the Front Range, was studied to determine evidence of glacial advances other than the two generally recognized on the eastern slope of the range. Greater extent of the Pleistocene ice on the western than on the eastern slope necessitated a careful review of criteria used in former studies. Five terminal moraines, of four ages, were found. Methods of correlating these moraines with polished rock surfaces and rock terraces were developed, and some trans-range correlations were found possible.

The four major ice advances, called “stages” in this paper, were named, in stratigraphic order, Monarch, Arapaho, River, and Stillwater. The ages tentatively assigned them are, respectively: Late Wisconsin, Middle Wisconsin, Early Wisconsin (Iowan), and not younger than Illinoian.

Some investigation of preglacial stratigraphy and history was found desirable in connection with this study. Climatic inferences were drawn from recessional history; the present condition of existing glaciers in the area was described; and the work necessary to secure a complete picture of Pleistocene glaciation in the Rocky Mountain region was mentioned.

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