THE LAKE-BORDER MORAINIC SYSTEM. 333 



have been a complete obliteration of so strong- a belt as is present in districts 

 to the north. In the case of Middle and East ridges, as shown below, there 

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



Middle Ridge has its southern terminus as a well-defined ridge at 

 the point where it meets the upper or Glenwood beach of Lake Chicago 

 There is, however, near the southwestern limit of the city of Chicago, a till 

 ridge known as "Blue Island," and connected with it a bowlder belt, each 

 of which may be correlatives of this morainic ridge. This ridge leads north 

 to south for a distance of about 6 miles along the line of Calumet and 

 Worth townships, and 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 or 90 feet above the lake. Its width, including 

 the slopes, is only about 1 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 and gravelly beaches at an alti- 

 tude 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 and not to a rock nucleus, 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. 



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 part of the city where dwellings are numerous, the bowlders 

 still remain in sufficient numbers to be a noticeable feature. In the thinly 

 settled part of the city from South Lynne southward to Blue Island they 

 remain in about their natural abundance. The belt occupied by the bowl- 

 ders is a mile or more in width. There appear to have been several hundred 

 surface bowlders to the square mile along this line, while on bordering dis- 

 tricts there are estimated to have been scarcely 100 to the square mile. 

 From the north end of this bowldery tract to the south end of Middle 

 Ridge the interval is but a few miles, and is mainly covered by deposits 

 of lake sand and gravel which would obscure any bowlder connection 



