GRASSES OF IOWA. 365 



trasted especially with the valley of the Mississippi proper they seem to 

 indicate a long period of undisputed action of the present streams. The 

 lower portions of the streams which come down to join the Mississippi 

 show usually considerable rounding of the cliffs, and quite extensive 

 slopes are developed. Along all but the streams which in time of flood 

 impinge against their banks, talus has accumulated, and long, grassy slopes 

 hide the cliffs. Traveling along the Mississippi north from Dubuque 

 one sees on the other hand a constant succession of high cliffs fronting the 

 river. If he climb these cliffs he will find usually long, grassy slopes lead- 

 ing away from the river, and down to some insignificant stream which has 

 often cut almost to the level of the main valley. This contrast of the 

 two slopes is well shown inTrempalo mountain, and may be seen at many 

 other points along the stream. The Mississippi is known to have filled 

 in its valley very considerably at some time during the ice period,* and it 

 is now bordered by a well developed gravel terrace, due doubtless to the 

 Hooding of the river during the Wisconsin period. The tributaries of 

 the Mississippi have corresponding terraces, containing, however, local 

 material, and due to the ponding of the side streams at the time the larger 

 river was flooded. Manifestly the present Mississippi channel has been 

 recently widened, and the smaller, neighboring valleys show what must 

 have been its character before the larger river was turned into its present 

 course. That the valley has long been the channel of the main stream or 

 the region is evident from the dendritic arrangement of the tributaries, 

 but that this main stream was, previous to the ice period, a relatively 

 small stream there is every reason for belief. 



It seems impossible, for the present, to fix definitely the period during 

 which the valleys of the region were cut. The attempt has been made to 

 correlate the plane at the top of the Galena with the Tertiary cycle of 

 erosion, and that touching the tops of mounds with the Cretaceous. f 

 There is, however, but little evidence to sustain this view, and there are 

 a few facts which seem to be against it, so that the matter may well be 

 considered very uncertain. 



THE OLDER DRIFT. 



Fringing the driftless area on the southwest, but much better dis- 

 played in the southern part of the state, is a topography notably different 

 from that just described. It is in the region of the older drift and is well 

 shown in all the area south of a line drawn across the state through Iowa 

 City and Des Moines. In this region rock exposures are relatively few. 



♦Leyerett ; Mon. S. Geol. Surv. 37. 

 +Hershey ; Amer. Geol. vol. 20, pp. 24S-2-63. 



