A RECONSTRUCTION OF WATER PLANES 463 



Ontario at about the same time. Lawson^ used it along the north 

 coast of Lake Superior in 1891; but between that time and 1905, tlie 

 hand level and aneroid were very generally used, instead; and 

 accordingly the correlation and identity of the beaches of the Lake 

 Michigan basin were imperfectly known. 



Absolute accuracy is of course impossible even with the Y -level. 

 On the one hand are original variations in height of the beaches and 

 benches, due to local conditions under which they were constructed 

 or cut; for which five feet has been allowed. On the other hand 

 a certain amount of error is involved in the process of leveling; first, 

 through the slight inaccuracy in the use of the instruments, and 

 second, through the use of Lake Michigan as the datum or starting- 

 point. It frequently happened, on days when levels had to be run, 

 that a strong on-shore wind was blowing and the waves were running 

 high, so that one could not tell within half a foot what the normal 

 level of the lake would be at that place. To be quite fair, then, we 

 must expect in these measurements occasional discordances due to a 

 combination of these errors of six feet or so. 



THE SHORE LINES OF LAKE CHICAGO 



The old shore lines fall into two distinct groups. There is an 

 earlier group, well registered around the south part of Lake Michigan, 

 but unknown in the northern part. These belong to the so-called 

 Lake Chicago, a lake which was confined to the Michigan basin, with 

 its outlet at the extreme southwest corner, into the Desplaines valley 

 at Chicago. A later group, represented by but a single shore line 

 in the southern part of the basin, below the beaches of Lake Chicago, 

 but rising to a considerable height northward, and splitting into a 

 large vertical series, records the complex history of Lake Algonquin 

 and the Nipissing Great Lakes. 



The shore lines of Lake Chicago, as distinguished at the south 

 end of the basin, and described by Leverett and Alden, mark three 

 distinct stages, to which the names Glenwood (or 60-foot), Calumet 

 (or 40-foot) and Toleston (or 20-foot) stages have been given. To 

 be more exact, the average altitudes of these three shore lines are 55, 



I A. C. Lawson, "Sketch of the Coastal Topography of the North Shore of Lake 

 Superior, with Special Reference to the Abandoned Strands of Lake Warren, " Minn, 

 ■Geol. Surv., 20th Ann. Rept. p. 231. 



