118 IOWA STUDIES IN NATURAL HISTORY 



average depth of the valleys cut during this cycle is ap- 

 proximately the amount of the uplift which closed the Lan- 

 caster cycle. The following table gives details of the depths 

 of valleys cut during this cycle. 



TABLE SHOWING DEPTHS OF VALLEYS WHICH MEASURE THE AMOUNT OF 

 THE UPLIFT WHICH INTERRUPTED THE LANCASTER CYCLE 



Considering the fact that not all the wells, the records of 

 which were used for the altitudes of the bedrock beneath 

 the surfaces of the fills, are in the middles of the valleys 

 and the probability that not all the streams had reached 

 grade when the filling began, the depths of these valleys 

 are quite remarkably uniform. This average depth, ap- 

 proximately 600 feet, seems to be a fair estimate of the 

 amount of uplift. If the streams were not at grade when 

 degradation ceased and aggradation began the amount of 

 uplift may be considered to have been more than this. Of 

 the two uplifts which have occurred since the land surface 

 of the Driftless Area was established the second one was 

 three times as great as the first. 



From the table showing the relations between the two 

 upland plains and the bottoms of the present Mississippi 

 Valley, (p. 117), the inference might be drawn that the up- 

 lift which interrupted the Lancaster cycle was accompanied 

 by tilting, for the Lancaster plain and .the present Missis- 

 sippi flood plain are not parallel. However, the nature of 

 this uplift is to be obtained by comparison of the Lancaster 



