340 C. A. FISHER GIAKT SPRINGS AT GREAT FALLS^ 3M0KTANA 



Kettleton,* Weed/ Calhoun,^ and others. The investigation of jSTettleton 

 was of a hydrographic nature, while that of Weed was mainly for the pur- 

 pose of obtaining information concerning the geology of the Great Palls 

 coal deposits. The recent work of Calhonn deals with the general glacia- 

 tion of northern Montana, including the Great Falls district. An 

 interesting feature of this work is the study and tracing out of the 

 modifications in the course of Missouri river caused by giaciation of the 

 region. 



Geologic Eelations 



Great Falls is situated in a plains region about 45 miles northeast of 

 the Eocky Mountain Front range. The geologic structure in this portion 

 of the plains is relatively simple, the surface rocks consisting mainly of 

 sandstone and sandy shale with an occasional limestone bed, all of Lower 

 Cretaceous age. They lie nearly horizontal, the prevailing dips being at 

 a low angle to the north, away from the mountains. The rocks upon 

 close examination are found to be slightly warped into a series of shallow 

 synclines and low anticlines. This structural feature is scarcely percept- 

 ible to the casual observer, being only revealed by a careful examination 

 of the beds exposed along the sides of some of the larger valleys. To the 

 south of Great Falls, in the vicinit}'' of Stockett and throughout the foot- 

 hill zone, the structure becomes more comj^lex, and local folds and minor 

 faults are not unconnnon. 



Physiographically the region is one of high broad plateaus sloping grad- 

 ually to the north, away from the mountains. These plateaus are trav- 

 ersed by numerous deep and relatively narrow valleys. The region has 

 been more or less glaciated, and the Cretaceous rocks comprising the 

 surface are in places covered by glacial material deposited by the Iveewatin 

 ice-sheet, which covered a portion of this area in Wisconsin time." There 

 is also considerable local giaciation along the base of the Eocky mountains 

 to the west. The continental glacial deposits are usually not conspicuous, 

 especial^ on the summits of the plateaus, iDut in the valleys they are very 

 noticeable. The sequence of events in the glacial history of this region 

 as first worked out by Calhoun is about as follows : The Keewatin ice- 



* E. S. Nettleton : Artesian and underflow investigation. Senate Executive Document 

 no. 41, part 2, pp. 74-78, 1892. 



^ W. H. Weed : Two Montana coal fields. Bulletin of the Geological Society of Amer- 

 ica, vol. 3, 1892, pp. 301-330. 



' P. H. Calhoun : The Montana lobe of the Keewatin ice-sheet, tl. S. Geological Sur- 

 vey, Professional Paper no. 50, 1906. 



' F. H. Calhoun : Op. cit. 



