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tions of the Ordovician, the Silurian and the Devonian periods 
with their alternating limestones, sandstones and shales which 
bespeak changing relations of sea and land, or possibly arid 
climate, as is thought by some to be represented by the St. Peter 
sandstone. But what has given to this region its rugged charm 
is the erosion which has been ceaselessly at work for ages carv- 
ing deep valleys into the once level plains, sculpturing the mas- 
Fig. 7. Mississippi river south of Lansing - , Allamakee county. 
sive rocks into bold cliffs and battlemeiited towers, slowly, unob- 
trusively, irresistibly wearing away loose sand or solid ledge 
until the present picturesque topography has been developed. 
This region lies in what is known as the Driftless Area, an 
area which has not been invaded by any of the great glaciers 
which covered the state, unless perhaps it was the first, the Ne- 
braskan. Hence not only has the work of the erosive agents 
been uninterrupted but the region has not been subjected to the 
