An attempt has been made to convert the mineral and rock-fragment components of a till into the original bedrock materials from which they were derived. The till used was a composite one (designated the "typical" till) that represented the averaged fraction-analysis data for 11 highly similar till samples from Marion County, central Indiana. For this study it was assumed that: 1) the glacier flow unit that reached Marion County was 20 mi. wide and coincided with a series of interconnected bedrock lowlands lying between northeastern Indiana and Lake Naococane, Quebec, 2) the flow unit eroded bedrock (or older drift) continuously throughout its journey from Lake Naococane, and 3) the different shales traversed by the ice were eroded equally. In terms of volume-percent composition, the typical Wisconsin till in Marion County probably consists of about 33% shale, 29% metamorphic and igneous rock, 18% limestone, 16% dolomite, 2% sandstone and siltstone, and 2% miscellaneous rocks. Approximately 90% of the typical till may consist of bedrock from outcrops more than 100 mi. upstream from the site of till deposition. An approximate 1 to 1 relationship has been found between the weight percent of a given rock type in the typical till and the percent of outcrop area of that rock type upstream. Implications of this relationship are that: 1) the ice streams that developed the Marion County tills eroded materials according to a weight rule, 2) the tills were not deposited by lodgement processes, and 3) the ice streams picked up and transported bedrock for distances in excess of 1,200 mi.