SOURCES OF THE CRETACEOUS DEPOSiTS. 101 



strata such as outcrop on the Sauk and Minnesota rivers. Concerning- the 

 eastern hmits of Cretaceous beds in this State he Avrites : 



A line drawn from the west end of Hunters Island, on the Canadian boundary 

 line, southward to ^Minneapolis, and thence southeastwardly through Rochester to 

 the Iowa State line, would, in general, separate that part of the State in which the 

 Cretaceous is not known to exist from that in which it does. It is not intended to 

 convey the idea that the whole State west of this line is spread over with the Creta- 

 ceous, because there are many places where the drift lies directly on the Silurian or 

 earlier rocks, but throughout this part of the State the Cretaceous exists at least in 

 patches, and perhaps once existed continuously.' 



SOURCES OF THE CRETACEOUS DEPOSITS. 



Deposits of Cretaceous clay are found in waterworn hollows of the 

 Lower Magnesian or Shakopee limestone forming the walls of the channel 

 or valley of the Minnesota River at numerous places in Blue Earth, Le 

 Sueur, and Nicollet counties. It is thus known that before the Cretaceous 

 period, when western Minnesota and the region of the Upper Missouri were 

 depressed and covered by the sea, a deep channel had been cut by some 

 river in the Lower Silurian and Cambrian strata of the Minnesota Valley ; 

 but the small width of this channel indicates that the stream then flowing 

 here, probably westward, was not larger than the present Minnesota River 

 This and many other streams of similar size, flowing into the Cretaceous 

 ocean as it spread to the east over the former land surface of Iowa, Minne- 

 sota, and Manitoba, contributed part of the detiitus which made its vast 

 mass of sediments, probably averaging a quarter of a mile in depth over 

 most of its area. These beds could be supplied only by extensive and 

 deep denudation upon the land areas both west and east of the Cretaceous 

 mediterranean sea. 



The very great disturbances of the region on the west in the elevation 

 of the Cordilleran Mountain ranges, since the Cretaceous period, make it 

 impossible to trace there the course of the larger tributaries to this sea. 

 On the east half of the continent the principal drainage system, carrying 

 its vast freight of detritus west to the Cretaceous ocean, is probably marked 



' Bulletins of the Minuesota Academy of Natural Scieuces, Vol. I, p. 348. Geology of Minne- 

 sota, Final IJeport, Vols. I and II. 



