Till samples were collected from the Valparaiso moraine around the southern end of Lake Michigan, and along a line paralleling glacial movement from Chicago south. The samples were analyzed, and. the pebbles classified according to lithology. In the Valparaiso moraine the mechanical composition was influenced by a loading of the ice with surficial material, and by drainage at the ice margin. The pebbles changed abruptly from dominantly limestone to shale and siltstone about 40 miles west of the corresponding bed rock contact. The westward movement of these pebbles was attributed to radial ice currents from the lake basin, over an area mantled with earlier drift. The samples in the line of glacial movement grouped themselves into three types on the basis of their mechanical composition. The results of the study are summarized in terms of the differentiation of tills by their textural composition. As an example of the method, the thickness of the Bloomington above earlier Wisconsin drift was determined in t...