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The large amount of geologically unexplored plats obtained from the General Land Office territory in western Rosebud and Dawson furnished the data from which the accompanycounties, Mont., led t.o an examination by the ing base map (Pl. IV) was prepared. This map writer of a part of this territory for the purpose shows the principal streams and roads of the. of getting general information on its stratigarea, as well as the geologic formations exraphy and structure. The region proved to be posed. The formation boundaries are repreone of considerable geologic interest. sented by solid lines where they are accurately The work demonstrated (1) that prodetermined and by broken lines where, because nounced dome structure exists in a region of lack of exposures or for other reasons, they heretofore supposed to be underlain by nearly could be located only approximately. flat-lying beds; (2) that the Eagle sandstone, The writer was assisted in the field by C. A. whieh is a thick formation farther west, thins Bonine, to whom mueh credit is due for the reeastward and disappears near the western suits obtained. border of the area herein described; (3) that the SURFACE FEATURES. Judith River formation, previously known only as of fresh-water and brackish-water origin, The area is a part of the Missouri Plateau of grades into a marine formation in passing the Great Plains, and its surface features are eastward fron1 the shore line of the sea along those produced by the erosion of such a region whieh it accumulated; and (4) that east of the · in a semiarid climate. Erosion has changed a meridian of lOT., 30' (approximately) the comparatively level surface into one dissected by deposition of marine sediments was continuous streams and surmounted by numerous buttes. throughout the Colorado and Montana epochs This sculpturing is most. pronouneed along of Upper Cretaceous time in this part of the Musselshell River, where in a belt 1 to 3 miles Great Plains region. wide intricately dissected badlands have been The area herein considered comprises about formed. In most parts of the area the sand1,200 square miles in Rosebud and Dawson stones of the Judith River and Lance formacounties and extends from Forsyth northward tions form conspicuous ridges that rise abruptly for a distance of 40 miles, thence westward from the lower and more nearly level surfaces to Musselshell River. Its geographic position occupied by the shale formations. The irregis shown on the key map (Pl. IV). ularities of the surface are due chiefly to difThe area was mapped on a scale of 1 mile ferenees in erosion induced by the unequal to the inch. Horizontal control was based on resistance of the formations. the land survey of the General Land Office and was obtained by stadia and triangulation. A stadia traverse along the Bearpaw-Lance boundary was earried throughout the field. From this traverse triangulation points were established whieh 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 GEOLOGY.

Gradations from continental to marine conditions of deposition in central Montana during the Eagle and Judith River epochs