SOME GEOLOGIC ASPECTS OF CONSERVATION 
135 
destructive planing action of the great ice-sheet. So it is that 
the unique and beautiful forms resulting from the erosive work 
of air and water have been preserved under the most favorable 
circumstances. In the country immediately to the west, on the 
other hand, such erosion remnants have been swept away by 
the repeated advance of the ice, the river valleys have been filled 
and the resulting topography is a level or gently undulating 
prairie. 
Fig. 8. Columnar cliffs along Oneota river, Winneshiek county. 
One of the striking topographic features of northeastern Iowa, 
one which becomes apparent with a glance qt the topographic 
maps cf the region and is equally evident to the traveler, is the 
relatively straight course and smooth, parallel walls of the great 
gorge of the Mississippi, which is in marked contrast with the 
intrenched meanders and extremely irregular slopes of the tribu- 
tary valleys. It is as if some gigantic plow had been forced 
down the main valley, cutting off all jutting headlands and leav- 
