340 



NOTES UPON THE GEOLOGY OF SOME PORTIONS OF MINNESOTA. 



the Ohio Valley, point to a former continuity of the formation over the ancient Mississippi Valley-; while some 

 teeth of sharks, etc., found in the debris between the points indicated, help to sustain this view. 



The occurrence of supracarboniferous strata, containing the gypsum beds,* in the valley of the Des Moines 

 River in Iowa, and which may be of Permian or Triassic age, or of both, suggests their synchronism with the red 

 marls, marly sandstones, and limestones below the Cretaceous formation on the Minnesota Iliver. 



The great amount of erosion and denudation which has taken place in this region, can scarcely be appreciated 

 from the surface aspect of the country. We now know that these later formations were deposited in valleys of 

 erosion, many miles in width and hundreds of miles in length. These formations in turn have been eroded in the 

 most extensive manner, but the work of destruction has in a great degree been hidden, owing to the friable nature 

 of the deposits, which left no bold escarpments or abrupt valleys ; but the whole has been toned down, with rare 

 exceptions, to gentle undulations, and the surface covered by line debris. 



The evidence of this destruction of the Cretaceous strata, such as characterize the Minnesota Valley, is shown 

 by the vast accumulations of the prairie formation of Illinois, Iowa, and Minnesota, in many localities of which the 

 cretaceous sands can be recognized but little changed ; while the earthy ligniform coal of the same formation, 

 already mentioned, is thickly scattered throughout the deep accumulations. 



# See Goological Report of Iowa, vol. i, part i (Report of A. II. Worthen, for 1856), page 175. Also pages 142-3 of same volume. 



