GEOLOGICAL AND NATURAL HISTORY SURVEY. 23 



GENERAL PHYSIOGRAPHIC FEATURES OF THE 

 CHICAGO AREA. 



The principal physiographic features of the Chicago Area 

 were produced by the ice sheet, the rock surface being generally 

 covered to a great depth by the drift. The variations in altitude 

 vvhich the region presents are produced by inequalities of drift 

 accumulations rather than by the underlying rock, the elevation 

 of the rock surface being but little, if any, higher in the hig-hest 

 parts of the area than it is in the lowest. 



The most conspicuous physiographic features are the lake 

 plain, the lake outlet, and the Valparaiso moraine. Less conspic- 

 uous features are found in the Lake Border Morainic System of 

 northern Cook and eastern Lake Counties. These are each dis- 

 cussed in some detail farther on, but a brief outline of their features 

 may be of service at this point. 



The lake plain has its most extensive development in Chi- 

 cago and its immediate borders as far north as Winnetka. It 

 extends back some twelve to fifteen miles from the lake, with a 

 general southward and westAvard rise, the elf^vation averaging 

 about ten feet at the border of Lake Michigan and about fifty 

 feet at the upper beach. This plain is continued into Indiana, but 

 is greatly concealed by dunes. North from V ^ukegan there is 

 a narrow plain bordering the lake, whose ek ation is but twelve 

 to thirtv feet above the present lake level. 



The portion of the sliore of Lake Michigar" between Win- 

 netka and Waukegan is occupied by one of the ridges of the Lake 

 Border morainic system, and the erosion of this ridge by the lake 

 gives the high banks which are present in that section. Between 

 this ridge and the Desplaines River there are a series of till ridges 

 separated by narrow plains. The ridges rise to a height of lOO to 



Explanation of Glacial Map of Chicago and Vicinity. — On 

 this map an attempt is made to give a bird's-eye view of the area 

 by means of broken profiles. The moraines appear with undulat- 

 ing contour, the plains with straight lines, the beaches with abrupt, 

 step-like descent of the plain. Striae are indicated by arrows and 

 represent bearings accurately. Some confusion of dunes with 

 moraines may arise if it is not borne in mind that the belt of ridges 

 bordering Lake Michigan in Lake County, Indiana, consists of 

 sand dunes. The dunes are found for eight to twelve miles south 

 from the lake in that county. The bars in Cook County, IlHnois, 

 also are to be distinguished from the moraines. They are much 

 lower and narrower ridges, as shown in the discussion. 



