THE TEMPORARY MISSISSIPPI RIVER 431 



the abandoned channel is not as well developed as Goose Lake 

 channel. 



It is commonly recognized that during the maximum advance 

 of the Illinoian ice sheet the displaced Mississippi followed the 

 course outlined in the preceding pages. On the assumption that 

 a river occupied the valley of Iowa and Cedar rivers instead of 

 Lake Calvin, the absence of the fluvial deposits can readily be 

 accounted for by supposing that the stream has changed its course 

 at Columbus Junction. On the other hand, if the filling up of the 

 Iowa and Cedar River valleys is due to stream deposition, there 

 appears no logical reason why deposits similar to those found north 

 of Columbus Junction and in the Mud-Elkhorn-Mud Creek VaUey 

 should not be seen in the abandoned channel. There is no reason 

 to beUeve that the streams were no longer overloaded when the 

 waters used the channel, or that erosion has subsequently removed 

 materials which may have been deposited there. The absence 

 of such deposits demands explanation. A plausible and adequate 

 explanation is offered by the supposed former existence of Lake 

 Calvin. A lake acts as a filtering plant for a river, and accordingly 

 it is easy to account for the absence of the fluvial deposits in the 

 abandoned channel if Lake Calvin reaUy existed and this channel 

 served as its outlet. For comparison the streams emptying into and 

 draining the Great Lakes may be cited. It is well known that many 

 of the streams emptying into the Great Lakes are more or less 

 discolored and muddy by reason of their load of sediment. Also 

 it is true that such streams as the Niagara and the St. Lawrence 

 which flow outward from the lakes are relatively free from sediment, 

 and hence have little erosive power. Because of the settling of 

 sediments in the lake basin and perhaps also because the ground 

 in which the abandoned channel was excavated may have been 

 frozen as suggested by Chamberlin,^ it is not to be expected that 

 the outflowing stream would have much erosive power nor much 

 material to deposit. The absence of any considerable deposits 

 of sand and gravel within the channel is then quite natural. This 

 too miay explain why this abandoned channel is not so well developed 

 as the Mud-Elkhorn-Mud and Goose Lake channels. 



^ F. Leverett, "The Illinois Glacial Lobe," U.S. Geol. Survey Monograph XXXVIII 

 (1899), p. 93. 



