78 IOWA STUDIES IN NATURAL HISTORY 



after the old age of the valleys has been reached. Another 

 principle in the normal erosional cycle is that divides are 

 not degraded much, before permanent divides have been 

 established, that is, before valleys have reached their width 

 and length limits, that is, before the valleys have ap- 

 proached or reached old age. Now the summit divides in 

 the Driftless Area are known to have been degraded by 

 amounts varying from 10 feet to 885 feet (see the fourth 

 column from the left in the above tables), and yet the val- 

 leys show few signs of old age, and few of the divides are 

 permanent. If the present surface was formed in a single 

 cycle of erosion, this cycle was not normal. 



(9) Assuming that but one cycle has been involved in 

 the erosion of the surface of the Driftless Area, recon- 

 structing the original surface by projecting the Niagara 

 and older formations over portions where they do not now 

 exist, and getting the altitudes of the lowest points reached 

 by streams beneath the present valley fills ; it is found that 

 the streams at La Crosse must have reduced their beds 

 from about 2200 feet A. T. to 600 feet in order to reach 

 grade; and the streams in the south portion of the Drift- 

 less Area in the vicinity of Dubuque, could have become 

 graded by cutting from approximately 1300 feet to 300 feet. 

 The Mississippi and its tributaries should have reached 

 grade at Dubuque after cutting through 1000 feet of rock 

 of varying hardness long before they brought their beds to 

 grade at La Crosse, where they had to cut through 1600 

 feet of the same rock, and the topography around Dubuque 

 should now be in a distinctly later stage of development 

 than that in the neighborhood of La Crosse. But the op- 

 posite is true. Due to the relative non-resistance of the 

 Cambrian sandstone on which the streams have their 

 courses at iwesent in the north part of the area, there is a 

 greater area of lowland there than farther south, and the 

 topography around La Crosse has an appearance of greater 

 age than the surface near Dubuque. 



(10) In advance of complete description and interpreta- 

 tion it should be made clear here that there are in many 

 places on the Dodgeville plain considerable areas of gravel 



