GRADATIONS FROM CONTINENTAL TO MARINE CONDITIONS OF DEPO- 

 SITION IN CENTRAL MONTANA DURING THE EAGLE AND JUDITH 

 RIVER EPOCHS. 



By C. F. BowEN. 



INTRODUCTION. 



The large amount of geologically unexplored 

 territory in western Rosebud and Dawson 

 counties, Mont., led to an examination by the 

 %\Titer of a part of this territory for the purpose 

 of getting general information on its stratig- 

 raphy and structure. The region proved to be 

 one of considerable geologic interest. 



The work demonstrated (1) that pro- 

 nounced dome structure exists in a region 

 heretofore supposed to be tmderlain by nearly 

 flat-lpng beds; (2) that the Eagle sandstone, 

 which is a thick formation farther west, thins 

 eastward and disappears near the western 

 border of the area herein described; (3) that the 

 Judith River formation, previously known only 

 as of fresh- water and brackish-water origin, 

 grades into a marine formation in passing 

 eastward from the shore line of the sea along 

 which it accumulated; and (4) that east of the 

 meridian of 107'' 30' (approximately) the 

 deposition of marine sediments was continuous 

 throughout the Colorado and Montana epochs 

 of Upper Cretaceous time in this part of the 

 Great Plains region. 



The area herein considered comprises about 

 1,200 square miles in Rosebud and Dawson 

 counties and extends from Forsyth northward 

 for a distance of 40 miles, thence westward 

 to Musselshell River. Its geographic position 

 is shown on the key map (PI. IV). 



The area was mapped on a scale of 1 mile 

 to the inch. Horizontal control was based on 

 the land survey of the General Land Office 

 and was obtained by stadia and triangulation. 

 A stadia traverse along the Bearpaw-Lance 

 boundary was carried thi'oughout the field. 

 From this traverse triangulation points were 

 established which furnished control in other 

 parts of the field. Elevations were obtained by 

 means of vertical angles, the initial elevation 

 being a point on the Chicago, Milwaukee & 

 St. Paul Railway a few miles west of Forsyth. 

 131049°— 20 2 



The field sheets thus prepared and the land 

 plats obtained from the General Land Office 

 furnished the data from which the accompany- 

 ing base map (PI, IV) was prepared. This map 

 shows the principal streams and roads of the 

 area, as well as the geologic formations ex- 

 posed. The formation boundaries are repre- 

 sented by solid lines where they are accurately 

 determined and by broken lines where, because 

 of lack of exposures or for other reasons, they 

 could be located only approximately. 



The writer was assisted in the field by C. A. 

 Bonine, to whom much credit is due for the re- 

 sults obtained. 



SURFACE FEATURES. 



The area is a part of the Missouri Plateau of 

 the Great Plains, and its surface features are 

 those produced by the erosion of such a region 

 in a semiarid climate. Erosion has changed a 

 comparatively level surface into one dissected by 

 streams and surmounted by numerous buttes. 

 This sculpturing is most pronounced along 

 Musselshell River, where in a belt 1 to 3 miles 

 wide intricately dissected badlands have been 

 formed. In most parts of the area the sand- 

 stones of the Judith River and Lance forma- 

 tions form conspicuous ridges that rise abruptly 

 from the lower and more nearly level surfaces 

 occupied by the shale formations. The irreg- 

 ularities of the surface are due chiefly to dif- 

 ferences in erosion induced by the unequal 

 resistance of the formations. 



GEOLOGY. 



STRATIGRAPHY. 



GENERAL SECTION. 



The formations exposed in this area range 

 in age from Upper Cretaceous to Tertiary ( ?) 

 and include the Colorado shale (upper part), 

 Eagle sandstone, Claggett and Judith River 

 formations, Bearpaw shale, and Lance forma- 

 tion. With the exception of the Eagle sand- 



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