This is an SDE feature dataset showing drift thickness in Illinois. Data are originally from ISGS Circular 490 by Piskin and Bergstrom (1975), Plate 1. Contours (lines) are coded with thickness, and polygons (areas between contours) are coded with range of thickness. Nominal scale is 1:500,000. Unconsolidated deposits, mainly glacial drift, overlie the bedrock surface in most of Illinois. Glacial drift ranges from less than a few feet to approximately 600 feet in thickness. The thickest drift occurs in major preglacial valleys cut into the bedrock and filled with glacial sediments. Regionally thick drift occurs in the N.E. portions of the state. The thinnest drift, less than 25 feet thick and intersected by numerous bedrock outcrops, occurs widely in southern and western portions of the State. Drift, as shown on this map, may include any of the following: unconsolidated deposits, glacial drift, Wisconsinan deposits, Illinoian deposits, till, glaciofluvial deposits, glaciolacustrine deposit, wind-blown deposits (loess).