The Giant Boulders of the Iowan Drift and a Consideration of Their Origin

Samples of the large granitic boulders of the Iowan drift were analyzed and features of the boulder distribution were examined. The analysis gave evidence of a central body of single granite stock which included more than 80 per cent of all large boulders examined within the Iowan drift area. Characteristics of this main granite stock were then compared to the bedrock areas which could be considered source regions of the boulders. All evidence pointed toward a source region in the large granite body found in central and west central Minnesota. Weathering and structural features of the boulders suggested that they may have been formed by secular weathering prior to their glacial transport