136 IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
the valley walls steep and rugged. And this is just what has 
happened. Great floods of water from the melting Wisconsin 
glacier, laden with rock, sand and silt, poured down the valley, 
scoured both floor and walls and then filled the valley to the level 
of the highest terraces of the present day. The lateral valleys, 
however, and the back slopes of the main valley, which were not 
subjected to this scouring, have retained their older, normal 
erosion forms. 
There are_ many spots of beauty in this scenic wonderland. 
Along Oneota river are the great columnar cliffs of Plymouth 
Rock, the vertical scarps at Bluffton, the Ice cave and Mill 
Spring at Decorah, Elephant Bluff, the Owl’s Head, Mount 
Hope and other hills of circumdenudation. The most unique of 
all these is the Ice Cave. This is a great gap left in the rock by 
the slipping out of a block of stone along the cliff face. The 
limestones of the region are honeycombed with fissures and into 
these the cold air of winter is drawn, to be forced out during the 
warm days of spring and summer. Coming into contact with 
the moisture-laden warm air of the cave this colder air causes a 
precipitation of the moisture along the inner wall of the cave and 
forms during the early summer months a boating of ice which 
sometimes becomes ten to twelve inches thick. Mill Spring is 
Fig. 9. Waterfall at Devil’s Den, Allamakee county. 
