EROSIONAL HISTORY OF DRIFTLESS AREA 119 



plain not with the present flood plain, but with the rock 

 bottom of the valley beneath the present river level. The 

 streams rejuvenated by the uplift of the Lancaster plain 

 continued to cut downward until the rock bottoms of the 

 present valley were reached. The present flood plains of 

 the streams were established later, under diflferent condi- 

 tions. Although it is impossible to determine the exact 

 slope of the surface represented by the buried bock bottoms 

 of the valleys, it is shown in the last table that the depths 

 of the valleys cut during the post-Lancaster cycle and con- 

 sequently the amount of uplift which inaugurated that 

 cycle are notably uniform. This suggests that the pene- 

 plain which would have been developed if this cycle had 

 gone to a late stage would have been roughly parallel with 

 the Lancaster plain. This being the case the uplift of the 

 Lancaster plain is more likely to have been uniform than 

 accompanied by tilting. 



The rather low altitudes of the parallel Dodgeville and 

 Lancaster plains in Minnesota suggests the possibility that 

 the movement which interrupted the Lancaster cycle was 

 accompanied by warping in that state. On the other hand, 

 a slightly decreased slope in old age of each of the two cycles 

 or a slight change in the direction of slope of the two plains 

 would explain the slight discrepancy equally well. 



There seems to be no escape from the conclusion that 

 there has been still another period of diastrophism in the 

 Driftless Area, this time a subsidence rather than an eleva- 

 tion of the surface. The evidence of subsidence is found in 

 the fact that the Mississippi river and its main tributaries 

 are now at grade at levels on the average 180 feet above 

 levels to which they were formerly able to reduce their 

 beds. It is not believed that this fact is to be explained on 

 the supposition that the present grade is merely temporary 

 and controlled by some obstruction such as the rock ledge 

 at Rock Island or the rapids at Keokuk. These obstructions 

 are far from sufficient to explain the difference in grade 

 levels now and as they were, for the Mississippi river to- 

 day has a gradient of less than 6 inches per mile from La 

 Crosse to the Gulf, including the rapids. It is believed, 



