257 



County, Wisconsin, lying between 42° 21' and 42° 35' north latitude 

 and between 87° 48' and 87° 49' west longitude. The western bound- 

 ary of the region under consideration, is the Glenwood ridge, which 

 w^as the upper limit of glacial Lake Chicago, a brief discussion of 

 which will presently follow. The region is entirely covered by the 

 Racine (Wisconsin) and the Waukegan (Illinois- Wisconsin) quad- 

 rangles of the United States Geological Survey. The latter is by far 

 the more detailed sheet and covers the greater part of the area. Parts 

 of these two sheets have been used directly in making up Plates 

 XXXVII and XXXVIII. Ditierences in elevation are very slight. 

 The highest elevation on the Beach region proper is but nine meters, 

 while virtually all of the area, with the exception of a few of the 

 ridges, is less than five meters above the level of Lake Michigan. 

 The Glenwood ridge, which forms the western boundary, is about 

 seventeen meters above the Lake Michigan level. 



Geologically the region consists of a sand and gravel beach super- 

 imposed upon glacial clay. In but one place, so far as was discov- 

 ered, is the clay exposed. The sand is arranged in long ridges not 

 quite parallel to the present shore-line. Between the ridges are 

 swales, only a few of which are able to drain directly into Lake Michi- 

 gan. Drainage is largely accomplished by seepage of the water 

 through the sand and finally into the lake. In the vicinity of Wau- 

 kegan, as indicated on the map (PI. XXXVII), are two bodies of 

 water located at practically lake level. These drain into the lake only 

 during periods of rather heavy rainfall and during the spring thaws. 



Physiographical History 



The western boundary (Glenwood ridge) of the region under 

 consideration was formed by Lake Chicago, the body of water that 

 occupied the southern end of the Lake Michigan basin during the re- 

 treat of the Late Wisconsin Glacier. This glacial lake had a south- 

 western outlet into the Illinois River. By erosion of the outlet the 

 lake level was reduced to 16.8 meters (55 ft.) above the present Lake 

 Michigan. The process known as "stopping" caused a rather sudden 

 transition from the Glenwood level to the Calumet level, which was 

 about 10.6 meters (35 ft.) above the present one. During this period 

 the ice-sheet retreated into the north until a low pass to the northeast 

 was uncovered, which caused a lowering of the lake below the pres- 

 ent level. A re-advance of the ice-sheet raised the water to approxi- 

 mately the 7.6 meter level which is known as the Tolleston stage. At 

 this time Lake Maumee, which occupied the upper Erie and lower 

 Huron basins, emptied into Lake Chicago through the Grand River, 



