28 CHICAGO ACADEMY OF SCIENCES. 



Island and Pacific, and the Omaha division of the Chicago and 

 Northwestern. These Hnes cross low points in the crest, twenty- 

 five to fifty feet lower than the general altitude of neighboring 

 portions. The highest parts of the crest in Illinois, as noted above, 

 are in northwestern Cook and southwestern Lake Counties, and 

 stand about 900 feet above tide. 



Surface Contours. — The Valparaiso niorainic system pre- 

 sents notable variations in surface contour, even in the short sec- 

 tion that lies within the State of Illinois. At the north, in Lake 

 County, there are numerous basins occupying an area ranging 

 from a few acres up to several square miles each, and containing 

 lakes or marshes. They are scattered over the slopes of the mo- 

 raine, indenting its surface to depths of twenty-five to fifty feet 

 or more. Basins become rare upon passing southward into Cook 

 County and the moraine is comparatively free from them througli- 

 out the remainder of its course in Illinois. The few which occur 

 are very small and shallow. These basins, witji their lakelets, add 

 greatly to the beauty of the scenery of Lake County, furnishing 

 attractive sites for summer residences and for camping grounds. 

 Among the lakes there are knolls and irregular ridges, with gentle 

 slope, ranging in height from ten feet or less up to about fifty 

 feet. The moraine has also a well-defined crest line in a ridge 

 twenty to forty feet higher than the border tracts and a mile or 

 less in width, which leads southward, with a somewhat winding- 

 course, throug'Ji the western part of Lake County, forming the 

 water parting between P'ox and Des Plaines Rivers: 



Passing south into Cook County the crest line continues as 

 definite as in Lake County. To the east of the crest line there 

 are only gentle swells seldom more than fifteen feet in height and 

 a rapid descent to the plain bordering the Des Plaines. To the 

 west of the crest line there is an irregular network of ridges and 

 knolls, inclosing winding sloughs. The height of these knolls and 

 ridges ranges from ten or fifteen feet up to fully fifty feet. Con- 

 tinuing south across Du Page Coimty the ridges tend to a paral- 

 lelism with the crest and the sloughs become less conspicuous. 

 This topography also dharacterizes the portion of the moraine in 

 Will County. In the latter county the tendency to a parallel ar- 

 rangement of the ridges and of chains of knolls is very marked. 

 Throughout the course in Du Page and Will Counties the inner 

 slope has a milder expression than the crest or outer slope, though 

 the contrast is scarcely so striking as in Cook County. 



The outlying belt in Will County carries low knolls twenty 



