CKETACEOTTS SYSTEM. 287 



surface deposits. The first is, the great thickness of the latter, 

 both the Bluff and Drift deposits ; and secondly, all the rocks 

 of Cretaceous age are so soft and friable, that many of the 

 exposures which may have once been made by the erosion of 

 the valleys of the present streams, since the Glacial epoch, 

 soon become covered again by their own debris, which resulted 

 from their disintegration. The ease with which these rocks 

 were disintegrated and ground by the glaciers, thus augment- 

 ing the amount of drift material, may account, to some extent, 

 for the great thickness of the Drift deposit in that part of the 

 State. 



For these reasons it is very difficult to draw a line upon the 

 map of Iowa which shall indicate the exact boundaries of the 

 region occupied by them, but the following description may 

 approximately indicate the outlines of the area : 



Draw a line from the northeast corner to the southwest 

 corner of Kossuth county; thence to the southeast corner of 

 Guthrie county; thence to the southeast corner of Cass 

 county; thence to the middle of the south boundary of 

 Montgomery county; thence to the middle of the north 

 boundary of Pottawattamie county; thence to the middle of 

 the south boundary of Woodbury county; thence to Sergeant's 

 Bluffs ; thence up the banks of the Missouri and Big Sioux 

 rivers to the northwest corner of the State; thence eastward 

 along the State boundary to the place of beginning. 



While there are many reasons to believe that the Cretaceous 

 rocks occupy all, or nearly all, of the area thus enclosed, it is 

 true that over a large part of it no exposure of rock in place 

 of any kind is to be seen. Consequently, the boundaries 

 designated are not to be understood as precise. This is 

 particularly the case in the northern two-thirds of the area, 

 with the exception of that part of it bordering the Missouri 

 and Big Sioux rivers. The reasons for inferring the existence 

 of Cretaceous rocks beneath the Drift Deposit where they do 

 dot actually appear, may be stated as follows : 



Traces of rocks of this age are found in southern Minnesota 

 even farther to the eastward than any point indicated within 



