30 TWELFTH REPORT. 



From this moraine the ice border shrank within the limits of the 

 present hike and the lake became expanded eventually over the greater ' 

 part of the basin (Fig. 3). It is knoAvn as Lake Chicago, and its outlet 

 as the Chicago Outlet. As the outlet stood but little higher than the 

 present lake, the extent bevoud the present lake was slight. The highest 

 shore is 55 to 60 feet above lake level, so the bed of the outlet at first 

 was about 50 feet above the lake. There are three distinct beaches dififer- 

 ing about 20 feet in altitude, knov>n as the GleuAvood, the Calumet, and 

 the Toleston. The highest and oldest marks the level of the lake during 

 the cutting of a trench in the gTavel formations along the outlet. The 

 second or Calumet beach marks the. level of a rock sill over which the 

 lake outlet flowed for considerable time before cutting it awav. The 

 removal of the rock, it has been suggested by Chamberlin, was by a 

 stoping process, there being rapids at first at the down stream end of the 

 rock sill, which by wearing backward gradually narrowed the rock 

 barrier until it finally gave way and let the lake drop to the level of 

 the third or Toleston beach. The Toleston stage persisted until the 

 ice had melted out of the Michigan basin and connection was established 

 with Lake Algonquin in the Huron basin. It is probable, though not yet 

 fully established, that the Chicago Outlet served for a time as the line of 

 discharge for waters in both these basins and also of the Superior basin. 

 But with the opening of the eastern outlet for Lake Algonquin through 

 the recession of the ice in Ontario the lake seems to have been drawn 

 down below the level of the Chicago Outlet. Another rise of water took 

 place at the close of the lake stage known as the Nipissing, discussed 

 below, the water reaching an altitude about 16 feet above the present 

 lake Michigan. The outlet became blocked by sandbars at this stage 

 and the lowering of the lake to the present altitude has been a result 

 of cutting in the St. Clair outlet. The Chicago Outlet is now occupied 

 by the small Des Plaines River, which takes a straggling course across 

 its bed, and in its meanders scarceh^ touches the banks of its great 

 predecessor. 



HURON-ERIE BASIN. 



The portion of the Huron basin south of Saginaw Bay lies in a low- 

 land which is continuous with that of the Erie basin across the Cana- 

 dian Peninsula. This lowland is termed the Huron-Erie basin. There 

 were converging ice currents in the Huron and Erie basins which be- 

 came confluent and formed what may j)roperly be termed the Huron- 

 Erie ice lobe. 



The Huron-Erie basin has had perhaps a more complicated lake 

 histor}' than any other of the Great Lakes basins owing to the oppor- 

 tunity for discharge of water at various points on its border, ex- 

 j>osed one after another in the course of the recession of the ice. It 

 also aftects more closely the history of the southern portion of Michi- 

 gan. 



Lake Ala u nice. The first discharge Avas from several small lakes on 

 the southern border of the Erie basin into the drainage of the Ohio. 

 These eventually became connected by border drainage and led into a 

 lake known as Lake Maumee, at the southwest end of the basin, which 

 discharged past Fort Wayne, Indiana, to the Wabash River, along 



