A relative-age classification of Pleistocene moraines in the northern Sawatch Range, Colorado, has been developed using information- and graph-theoretic methods applied to moraine morphology and weathering characteristics.
At 89 stations on moraines in the study area, measurements were made of the six best age-dependent criteria; listed in order of importance, they are percentage of pitted granite, percentage of fresh granite, depth of pitting, width of moraine crest, surface-boulder frequency, and distal-moraine slope angle. Data were evaluated using computer programs CHARANAL (character analysis) and GRAPH (similarity clustering analysis) to produce an age classification of stations according to similarity with respect to the six criteria. Groups of moraines were then tentatively assigned Rocky Mountain geologic-climate names on the basis of weathering parameters and qualitative consideration of deposit morphology, downvalley position, and soil-profile development.
Two pre–Bull Lake(?) glaciations are recognized in the area and tentatively correlated with the Cedar Ridge and Sacagawea Ridge Glaciations described by Richmond (1960, 1962, 1965) in the Rocky Mountains. Early(?) and late Bull Lake(?) glaciers occupied Lake Creek Valley; however, evidence of Bull Lake Glaciation was not found in the valleys of East Brush Creek or Nolan Creek. Early Pinedale glaciers occupied all major valleys in the study area, and most were more extensive than late Bull Lake glaciers, possibly explaining the lack of evidence in some valleys for Bull Lake Glaciation. Most valleys in the northern Sawatch Range were again glaciated during middle and late Pinedale time.
The information- and graph-theoretic methods used in this study are useful tools for relative-age dating of Pleistocene deposits. Use of the methods (1) allows more objective age classification of glacial deposits to be made on the basis of qualitative data and measurements of several or many weathering characteristics, (2) may help to determine whether deposits represent different glaciations or phases of the same glaciation, and (3) may aid in correlation of glacial deposits between regions of similar climate and bedrock over great distances.