Samples of quartzite were collected from ridges that stand one to two meters above the surrounding loess- and till-mantled landscape and are 20 km beyond the youngest ice margin (the Wisconsinan, less than 12,000 years old). The rocks were analyzed for Be-10 and Al-26 by a laboratory that specializes in accelerator mass spectrometry. Four samples analyzed from the outcrops indicate that the striated and grooved surfaces have been uncovered for at least half a million years (575 ka ± 57 ka). Oxygen isotope data from deep-sea marine sediments imply extensive glaciation about, and prior to, 500,000 years ago. This activity could correspond with continental glacial ice that created the marks in the rocks. Extensive mapping of the glacial deposits in southwestern Minnesota indicates that younger glacial events, if they reached the study area, did not cover the Sioux Quartzite outcrops for significant periods of time. The 575 ka date compares favorably with other data on the till stratigraphy where the two uppermost pre-Wisconsinan tills in the area are known to be younger than 600,000 years old. These tills are thought to be associated with the glacial ice that created the striated surfaces. Minimum dates established by comparing the ratio of Be-10 to Al-26 suggest that most of the till deposited before the Wisconsinan glacial sediments was Middle Pleistocene or older in age.
For additional information on the history of glaciation in southwestern Minnesota and Beryllium-10 dating of surfaces on the Sioux Quartzite see: Contributions to the Quaternary geology of southwestern Minnesota . C.J. Patterson, ed. CONTENTS: Chap. 1, Surficial geology of southwestern Minnesota, by C.J. Patterson; chap. 2, Revised gravity investigation for potential ground-water resources in northwestern Rock County, Minnesota, by Val W. Chandler; chap. 3, Terrestrial gamma radiation in southwestern Minnesota and an assessment of its utility in surficial geologic mapping. Report of Investigation 47, Minnesota Geological Survey, 1997. 77 p.
Bierman, P.R., Marselia, K.A., Patterson, C., Davis, P.T., and Caffee, M., Mid-Pleistocene cosmogenic minimum-age limits for pre-Wisconsinan glacial surfaces in southwestern Minnesota and southern Baffin Island: a multiple nuclide approach, 1999, Geomorphology,v. 27, pp. 25-39.