PAPERS OX GEOGRAPHY AND GEOLOGY 193 



from the simple gently dipping to slightly flexed struc- 

 ture which is generally considered to be typical of north- 

 ern Illinois. Axes of sharp narrow anticlines with ac- 

 companying broad shallow synclines cross the area in a 

 westerly to northwesterly direction. A characteristic 

 feature of these anticlines is the narrowness of the folded 

 zone and the steepness of the dips in this narrow zone. 

 Gently dipping limestones will suddenly exhibit dips of 

 20 c or more. In one instance the dip changes sharply 

 from a few degrees to more than -40°. In another instance 

 the St. Peter sandstone unexpectedly appears with 

 steep dips, in the midst of gentle north-dipping Platte- 

 ville limestone. Between these folds the strata are com- 

 monly only very slightly tilted or horizontal. These few 

 examples indicate the character of the deformation in the 

 Oregon quadrangle. 



