404 THE ILLINOIS GLACIAL LOBE. 



places wnere wells encounter a more pebbly material at the base of the 

 sand than near the surface, but it is not certain that these pebbles were an 

 outwash from the ice sheet. Indeed, it seems quite probable that they may 

 have been worked over and deposited by the waters of the lake. 



ASSOCIATED TILL PLAINS. 



The ridg'es just discussed occupy but a small portion of the area 

 embraced between the Valparaiso morainic system and the shore of Lake 

 Michigan. The greater part of the area is a plain, underlain by till deposits. 

 The plane tracts, as is shown later, were largely covered by Lake Chicago 

 and have received deposits of sand or gravel from the waters of that lake. 



ALTITUDE AND SLOPES. 



Ill the portion of the plain west and south from Chicago the altitude 

 at the inner border of the Valparaiso morainic system is 40 to 50 feet above 

 Lake Michigan. From this border there is a gradual descent toward the 

 lake, and the till scarcely rises above lake level along the present shore 

 within the limits of the city. At the time the upper or Glenwood beach 

 was formed lake water covered the entire plain west and south of the city 

 as far as the borders of the Valparaiso morainic system, the altitude of that 

 beach being 55 to 60 feet above the present lake level. 



Upon passing northward this till plain is divided into several narrow 

 plains which separate the till ridges, and these plains soon rise above the 

 level of the upper beach. The plain that lies between the west ridg*e and 

 the Valparaiso moraine slopes eastward at the rate of several feet per mile. 

 Its rise toward the north amounts to but 1£ to 3 feet per mile. At Oving- 

 ton Station, on the Omaha Division of the Chicago and Northwestern Rail- 

 way, the west border next the Valparaiso moraine is but 635 feet. It rises 

 to about 690 feet at the line of Cook and Lake counties, a distance of 

 18 miles, and to about 725 feet in northern Lake County, a distance of 24 

 miles farther. The plain between West and Middle ridges rises from 630 

 feet at Oak Glen to 680 feet at Deerfield, a distance of 6 miles. From 

 Deerfield north to the State line, a distance of 24 miles, it is shown by the 

 Chicago, Milwaukee and St! Paul Railway profile to stand at 670 to 685 



