THE LIFE OF GLACIAL LAKE CHICAGO 109 



Green Bay, Wisconsin, but is not now found within a distance of 80 miles from 

 the latter locality or Chicago. It is likewise not now living in the Great 

 Lakes drainage. The other naiad, Pleurobema coccineum magna! aaistr is, 

 is not now found in the Mississippi Valley drainage. The same may be said 

 of the gastropod, Amnicola letsoni. One species of bird (Mergus serrator) 

 lived at this time, as did also the mastodon, mammoth, and a species of deer. 

 Shallow water types of mollusks lived in the shallow water near the shore. 



6. The lake suffered a second low water period forming the Sag Stage, when 

 the lake fell to about 10 feet above the present level. The life of this stage is 

 the richest of all the episodes, including 75 species and varieties of mollusks, 

 3 species of fishes and two or more species of mammals. The increase in the 

 Pisidia as well as in the fresh water pulmonates is noteworthy, as is also the 

 absence of the heavy naiades. During this stage the outlet was by way of the 

 Trent Valley and the Chicago outlet was used only in a slight degree. 



7. The lake rose again (due to differential uplift) to the 600 foot level, 

 forming the Hammond Stage (Lake Algonquin outlet shifted to Chicago). 

 The life of this stage consisted of species of mollusks (39) typical of quiet 

 waters of large size and moderate depth, such as bays and ponds. In place of 

 the ponderous naiads there were a number of species of the smaller bivalves 

 (Sphaeriidae) and a large percentage of shallow water gastropods, principally 

 fresh water pulmonates, which became widely distributed. A few species of 

 the naiades remained in the more open parts of the lake. Crayfish were abun- 

 dant in the shallower parts of the bays. 



8. A third period of low water (or possibly a land surface) followed the 

 Hammond Stage, after which the water rose to a higher level forming the 



9. Englewood Stage, the level being about 12 feet above the present lake. 

 The Wilmette embayment became reduced to a bay about seven miles long, 

 from one-half to one and a half miles wide, and from one foot to twelve feet in 

 depth. The molluscan fauna consisted of a few shallow water bivalves and 

 gastropods. The naiades probably retreated to the beds of the rivers Chicago, 

 Desplaines, and Calumet, as well as to the lakes left in the southeastern portion 

 of the lake basin. Bars were formed which finally shut in the Chicago basin 

 north of the Sag outlet, causing a huge marsh to develop. 



10. When the water fell to the present level of Lake Michigan the aquatic 

 life retreated to the rivers, ponds, lakes, and small streams. Wilmette Bay 

 became a marsh, wet during the spring and dry during the fall. The region 

 of the Sag outlet was shut off by bars and also became a great marsh. 



11. The Blinoian ice sheet passed over the Chicago region and upon this 

 till an old soil was formed. Rock valleys also occur indicating preglacial 

 erosion. 



