84 IOWA ACADEMY OF SCIENCES. 



with the opening of a temporary course for the Mississippi 

 across southeastern Iowa, which occurred at the time the 

 Mississippi valley above the rapids was covered by the Labra- 

 dor ice field. 



The drainage line referred to leaves the present Mississippi 

 at the mouth of the Maquoketa, passes southward along that 

 valley (reversed) to Goose Lake channel, and thence to the 

 Wapsipinicon valley, coming to that valley a few miles above 

 its present mouth. It follows up the Wapsipinicon a few 

 miles to the mouth of Mud creek, a southern tributary, which, 

 together with a small tributary of Cedar river also called Mud 

 creek, furnishes the line of continuation for the old valley to 

 the Cedar river near the great bend at Moscow. The val- 

 ley continues southwest to the Iowa river along the course 

 now followed by the Cedar river in its lower twenty -five miles. 

 It then passes southward from Columbus Junction to Winfield 

 and thence westward to Skunk river at Coppock, opening in 

 its westward course two lines, one of which is now utilized by 

 Crooked creek. From Coppock the old drainage line follows 

 the course of Skunk river southward to Rome, and Cedar 

 creek (reversed) to Salem. It there turns southeastward, 

 being known as "Grand valley " in northern Lee county, and 

 joins the Mississippi about six miles west of Ft. Madison, 

 nearly opposite the head of the rapids. Its continuation was 

 evidently across the rapids into the broad valley below 

 Keokuk. 



The altitude of the bottom of this old valley, near the head 

 of the rapids, is fully 100 feet above the present stream, but 

 connects well with the surface of the valley filling in and 

 below the rapids. It is nearly 100 feet lower than at the 

 point where it leaves the Iowa valley, seventy -five miles to 

 the north. The portion above the point where the Iowa valley is 

 crossed has been so modified since the Illinoian stage of glacia- 

 tion that very little is known concerning its condition at the 

 close of that glacial stage, but the portion south from the Iowa 

 valley has been only slightly modified. 



Very little material was deposited on the bed of the 

 temporary channel of the Mississippi in the seventy-five miles 

 from the Iowa valley to the head of the rapids, but a great 

 filling occurred in the broad valley below the rapids, and 

 some filling along the rapids, especially at their lower end. 

 The valley, which, at the foot of the rapids, had been cut down 



