terrane map

Application

The dating technique was recently applied to erosional surfaces on the 1.75 billion year old Sioux Quartzite in southwestern Minnesota. Composed of almost 100 percent quartz, the Sioux is very resistant to erosion and an ideal material for accumulating Be-10 and Al-26 because of the high content of silicon and oxygen in the quartz. Covered at various times over the past several million years (the Quaternary Period), by ice and sediments from glaciers, surfaces of the Sioux are today

terrane map

on ridges and stream valleys near Pipestone Minnesota. The outcrops of quartzite are marked by grooves, striations, and chattermarks created when glacial ice scraped across the rocks. The marks and surfaces are wind polished and cross-cut by shallow wind-eroded (ventifaction) grooves. This has been suggested as evidence that the outcrops were exposed during the Wisconsinan glaciation (and possibly earlier), when sustained, high-velocity winds blew in the direction indicated by the name tags in the picture. The goal of the dating project was to determine when the surface containing the striations and other marks was exposed. That age should post-date the time the surface was last glaciated. Glacial deposits mapped as stratigraphically above sediments associated with the surface would therefore be younger.


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