The Green River lobe of Tazewell (Shelbyville) ice extended west of the Mississippi River to the eastern border of the Goose Lake Channel in Clinton County, Iowa. It extended northwestward to within 1½ miles of the village of Goose Lake, across an area previously mapped as Iowan and Kansan drift. A small area of Illinoian drift is present between the Shelbyville drift and the Goose Lake Channel.
The Iowan drift is separated from the western border of the Goose Lake Channel and the Shelbyville drift by a small strip of Kansan drift on the north and a small area of Illinoian drift on the south.
When the Shelbyville ice reached the “big bend” of the Illinois River in Bureau County, Illinois, it blocked the eastward-flowing drainage of the old Mississippi River and formed the first stage of glacial Lake Milan which extended from the “big bend” to the rock-defended outlet near Andalusia, Illinois. Later when the Shelbyville ice had advanced westward to the west side of the present Rock River valley, glacial Lake Milan was restricted to the lower Rock River valley, and glacial Lake Cordova formed north and northwest of the ice dam. The outlet for glacial Lake Cordova was over a divide composed of Illinoian drift between Cordova, Illinois, and Le Claire, Iowa. Later when the Shelbyville ice crossed the present Mississippi River valley, glacial Lake Savanna was formed to the north. Its waters rose until they found an outlet through the Goose Lake Channel to the Wapsipinicon River, down the Wapsipinicon and through the new valley past Cordova and Le Claire. When the Shelbyville ice withdrew, the present course of the Mississippi River between Fulton, Illinois, and Keokuk, Iowa, had been established.
Two sets of loess-covered terrace remnants, deposited under different controlling conditions, are present between Geneseo and Lomax, Illinois. One set between Geneseo and Andalusia, Illinois, rises to a nearly uniform height and resulted from quiet-water deposition in glacial Lake Milan. Below Muscatine, Iowa, lie remnants of deposits made up largely of fine sands and silts, whose deposition was controlled by a constriction in the valley of the Mississippi River near Keokuk. When the great volume of water from the upper Mississippi drainage was added to that already flowing past Keokuk, constriction caused higher water levels upstream with areas of relatively quiet water in marginal portions of the valley and in the ponded tributaries.
The possible contemporaneity of the Shelbyville terrace on the lower Iowa with the high terrace of Lake Calvin upstream calls for a re-examination of the terraces along the Iowa River and raises the question of the possibility that the Shelbyville and the Iowan were contemporaneous.