IOWA ACADEMY OF SCIENCES. 83 



This bed has been discussed at some length by Mr. Gordon 

 in the Geology of Iowa,* and three interpretations for its 

 origin are presented. 



First. — That it was formed by river action alone, /. e., as an 

 alluvial bar. 



Second. — That it is due to the cutting down of a till sheet, 

 the coarse material being left as a residue. 



Third. — That it is a bowldery moraine dropped at the edge 

 of the ice sheet at the Illinoian stage of glaciation. 



Of the three interpretations the second seems to Mr. 

 Gordon, as well as to the present w^riter, the most applicable. 

 Dr. Kellogg 's suggestion of a fall as the cause seems, at best, 

 to be poorly sustained. A similar bow^lder bed occurs near 

 Warsaw, 111. It there forms a capping for an eroded till sur- 

 face and bears clear evidence of removal of the fine material 

 by a stream, with the retention of the bowlders as a residue. 

 A bow^lder bed is also found along the face of the west bluff of 

 the rapids near Sandusky, about six miles above Keokuk, at a 

 level forty to sixty feet above the stream, that probably w^as 

 derived from the erosion of a sheet of till, though the exposure 

 is scarcely extensive enough to show clearly the relationship. 

 It seems referable to the period tf erosion that produced the 

 beds at Keokuk and Warsaw. 



The amount of erosion effected is so great that the begin- 

 ning of this new channel seems to date from near the close of 

 the Kansan stage of glaciation.' This becomes more evident 

 as we study into the later stages of the history of the river. 

 Even if the river had been forced into a channel farther east 

 than the lower rapids, it seems scarcely probable that it 

 remained long in that course. It apparently began its work of 

 opening the course across the rapids long before the Labrador 

 ice tield had reached the region. 



FILLING AT THE ILLINOIAN STAGE OF GLACIATION. 



Following this great erosion there came a partial filling of 

 the part of the valley immediately outside the limits of the 

 Illinoian drift sheet. It is well displayed below the rapids, 

 and some remnants are to be seen along the borders of the 

 rapids. This filling appears to have occurred at the Illinoian 

 stage of glaciation: Evidence of this relationship is to be 

 found in the connection, or close association, of this filling 



'Geology of Iowa, Vol. Ill, 1893, pp. 252 255. See also PI. XV. 



