46 CHICAGO ACADEMY OF SCIENCES. 



have terminated in water held between its front and the Val- 

 paraiso moraine in the brief period required for the cutting down of 

 the outlet to the level of the upper beach. But in that case, while 

 wave action may have removed weak morainic features, it seems 

 scarcely probable that there could have been a complete oblitera- 

 tion of so strong a belt as is present in districts to the north. In 

 the case of the Middle and East Ridges, as shown below, there 

 may have been some erosion by the lake outlet and lake waves. 



The Blue Island Ridge. — The Middle Ridge has its southern 

 terminus as a well-defined ridge at the point where it meets the 

 Upper beach. There is, however, near the southwestern limit of 

 the city of Chicago a till ridge, and connected with it a bowlder 

 belt, which may be correlatives of this morainic ridge. 



The till ridge referred to is known as "Blue Island." It leads 

 north to south for a distance of about six miles, along the line of 

 Calumet and Worth Townships. Blue Island village is situated 

 at its southern end. At that end the ridge stands about 60 feet 

 above Lake Michigan, but rises northward to an altitude 85 to 90 

 feet above the lake. Its width, including the slopes, is only about 

 one mile. The northern portion is gently undulating and is 

 strewn with bowlders, but the remainder of the ridge is smooth 

 and comparatively free from surface bowlders. Around this ridge 

 there are shore marks in the form of eroded banks or terraces 

 and gravelly beaches at an altitude 55 to 60 feet above the present 

 lake level. "On its west border sand from the old lake shore is 

 drifted into dunes that extend nearly to the top of the ridge. Blue 

 Island ridge owes its elevation to a thickening of the drift deposits, 

 for the rock surface is as low beneath it as on border plains. A 

 well at Morgan Park, near the crest, reaches a level 70 feet below 

 the base of the ridge before entering limestone. 



Bowlder Train. — A train of bowlders is traceable north from 

 the north end of Blue Island through the western part of the city of 

 Chicago to the vicinity of the Chicago River in Jefferson Township. 

 Although portions of the line fall within a thickly settled portion 

 of the city, the bowlders still remain in sufficient numbers to be a 

 noticeable feature. In the thinly settled district from South Lynne 

 southward to Blue Island they remain in about their natural 

 abundance. The belt occupied by the bowlders is a mile or more 

 in width. There appear to have been several hundred bowlders 

 to the square mile along this fine, while on bordering districts 

 there are estimated to have been less than 100 to the square mile. 

 From the north end of this bowl-dery tract to the south end of the 



