240 G. F. WRIGHT ABANDONED BEACHES ABOUT LAKE MICHIGAN 



to a 40-foot level. Others resort to the supposition that there was an 

 elevation of the land north of the outlet sufficient to allow the accumula- 

 tion of peat, and that afterward there was a depression to the Calumet 

 level. 



The only direct evidence supposed to indicate such a temporary land 

 elevation is drawn from soundings in various narrow lakes tributary to 

 Lake Michigan on the east side, which in several cases descend at least 

 50 feet below the present level of the lake. But it would seem altogether 

 probable that these are preglacial channels or depressions preserved, by 

 the presence of ice, from being filled with glacial material. 



Supposed Changes of Land Levels 



A recent survey of the whole region, under the guidance of Mr. Charles 

 B. Shedd, suggests a theory which avoids the obvious difficulties con- 

 nected with the other theoi'ies and is supported by all the facts and causes 

 known to ])e in operation. 



The 60-foot water level, during which the first beach was formed, is 

 accounted for ])y the natural supposition that the drainage outlet from 

 Summit to Joliet was filled with glacial deposits, compelling the water to 

 rise to tliat level. Naturally the clearing out of this channel would take 

 place by a process that is called "stoping" — that is, by a wearing back 

 from the Joliet end by a recession somewhat similar to that which takes 

 place in waterfalls. Otherwise there would have been a succession of 

 numerous small beaches. Their absence shows that the water was kept 

 up to the 60 -foot level until the removal of the obstruction through its 

 whole extent, constituting the Glenwood period. 



Very interesting Glenwood dunes are to be seen on the west side of 

 Blue Island, already referred to as a narrow mass of morainic character, 

 some 5 or 6 miles in length, that stood above the 60-foot level of the 

 lake. These were evidently formed out of the shallow water deposit to 

 the west during the Glenwood stage, as is shown by their long slope to 

 the west and their abrupt slope to the east; they are of finer material, 

 more oxidized than most of the other dunes, and now covered with grass 

 and large trees. 



Supposed earliee Opening of the Sag Outlet 



Mr. Shedd has suggested that naturally the Sag outlet was opened 

 before the Des Plaines Eiver outlet to the north of Mount Forest Island, 

 which he thinks was probably closed during the Glenwood stage by the 

 prolongation of the moraine from Galewood down to Mount Forest 

 Island, probably maintaining the Sag outlet to the 60-foot level during 



