64 A NATURALIST IN THE GREAT LAKES REGION 



region it is underlaid by a deposit that has been regarded as 

 belonging to one of the earlier ages, probably the Illinoian. A 

 coarse conglomerate made up of glaciated pebbles, 5-10 per cent 

 of which are of granite or diabase, is seen underlying the Wis- 

 consin drift north of Lockport just beside the Chicago and 

 Joliet trolley line and at several points within the city limits of 

 Joliet, one opposite the brick switch house about a mile north of 

 the Rock Island Depot. This deposit is well cemented together 

 with carbonate of lime, transforming the original bed of gravel 

 into rock. Its surface is said to give some indication of having 

 been abraded by the later ice sheet. It seems more probable 

 that these deposits are part of the valley train described 

 below. 



The surface features of the Chicago area are chiefly due to the 

 deposits of the last of the great glacial periods the Wisconsin 

 and to postglacial changes in these. The front of this ice sheet 

 retreated by several steps, standing still long enough several 

 times to make a succession of terminal moraines as seen on the 

 accompanying maps (Figs. 42, 43). 



Each of the moraines marks a stage in the recession of the ice border, 

 when the rate of melting was temporarily checked and the edge of the ice 

 became nearly stationary. At such times the drift, which was being moved 

 forward to the melting border and deposited there, accumulated to great 

 thickness. When the ice border receded a relatively smooth lowland was 

 laid bare behind the belt of thick drift. This extended to the point where 

 another halt of the ice caused the making of another ridge of drift. 



Of these moraines the last formed -the one nearest Chicago, 

 the Valparaiso moraine is the highest and broadest. It marks, 

 evidently, a prolonged stand of the glacier's front. Its sur- 

 face varies from an almost smooth or gently undulating plain 

 (Minooka Ridge) to typical rounded hills and saucer-shaped 

 valleys with contained ponds or swamps. The last type is well 

 seen on Mount Forest Island, as one walks south from Willow 

 Springs, or in the hill territory about Palos Park. The hills and 

 valleys are not so abrupt or so high as they are in the kettle- 



