20 



SHORTER CONTRIBUTIONS TO GENERAL GEOLOOY, li)l'.). 



of this uplift the sea hecame shallow and 

 probably withdrew from local areas, and the 

 accelerated streams brought down great quan- 

 tities of sand, wliich were spread over the shal- 

 low sea floor and now constitute the Eagle 

 sandstone. As the uplift progressed the sea 

 withdrew from its former western shore to a 

 point, in the latitude represented by the dia- 

 grammatic section on Plate IV, a few miles 

 east of section A, for from that place westward 

 the upper part of the Eagle sandstone contains 

 thin beds of carbonaceous shale and lignite. 

 The distance seaward to which this uplift was 

 effective in producing a change of sedimenta- 

 tion is 2 or 3 miles east of section B, Plate IV, 

 or about the present site of Musselshell River in 

 the western part of the area here discussed. 

 At that place the Eagle sandstone feathers out 

 between the Colorado and Claggett formations 

 and thence eastward the deposition of sliale 

 was continuous through the Colorado, Eagle, 

 and Claggett epochs. 



Another submergence in the Claggett epoch 

 restored the sea over at least a part of the 

 area from which it had been withdrawn, and 

 the deposition of shale was extended west- 

 ward, covering the Eagle sandstone in this 

 part of Montana. This submergence did not 

 extend as far northwest, however, as that of the 

 Colorado epoch, because, as showni by Stebin- 

 ger,^ the entire interval between the Colorado 

 and Bearpaw epochs in the vicinity of the front 

 range of the Rocky Mountains was character- 

 ized by the deposition of sediments that were 

 mainly of continental origin. 



After 500 to 700 feet of marine shale had 

 been laid dowai another emergence set in which 

 marked the beginning of the Judith River epoch. 

 This emergence was more widespread than 

 that of the preceding Eagle epoch, and during 

 its progress the sea was withdrawn from the 

 area herein considered to about the longitude 

 of 107° 30' — that is, to some position between 

 the points indicated by D and E in the dia- 

 grammatic section on Plate IV. The clay and 

 sand not transported by the streams from the 

 adjacent higlilands were in part spread out as 

 continental deposits on the emerged sea floor 

 and in part carried out to the sea and depos- 

 ited beneath its surface. Westward from 

 point B the resulting sandstones and shales of 



1 Stebinger, Eugene, The Montana group of northwestern Montana: 

 U. S. Geol. Survey Prof. I'aper 90, pp. 62-63, 19U. 



the Judith River formation contain the re- 

 mains of land and fresh-water vertebrates and 

 invertebrates, leaves, stems, trunks of trees, 

 and beds of lignite; whereas east of point E 

 these deposits contain the remains of marine 

 organisms. 



How far eastward tliis Judith River uplift 

 was effective is not known. Sections A to G in 

 Plate IV show that the formation gradually 

 thins from west to east, and apparently it 

 either disappears or is represented by not 

 more than 1.5 feet of sandstone at point II, in 

 the longitude of Glendive. The well drilled by 

 the Northern Pacific Railway Co. in sec. 20, 

 T. 14 N., R. 55 E., about 10 miles southwest of 

 Glendive, was begun near the top of the Pierre 

 shale and penetrated a bed of sandstone 15 

 feet thick at a depth of 745 feet and another 

 bed 6 feet thick at a depth of 900 feet. The 

 remainder of the weU log is given as shale down 

 to a depth of 2,710 feet, or near the base of the 

 Colorado. It is evident, therefore, that m the 

 longitude of Glendive conditions of sedimen- 

 tation were practically uniform throughout 

 Colorado and Montana time and that the oscil- 

 lations which gave rise to the Eagle and Judith 

 River deposits to the west did not extend that 

 far east. 



A renewed submergence brought the Judith 

 River epoch to a close and restored the sea 

 over the entne known area of the Judith River 

 formation. During this, the last widespread 

 submergence, 1,000 feet or more of marine 

 shale was laid down, constituting the Bearj^aw 

 formation. This submergence was followed by 

 a widespread emergence that resulted in the 

 withdrawal of the sea from this part of the 

 continent, which has since remained above sea 

 level except in small areas. This uplift was 

 probably of epeirogenic character, because the 

 next succeeding deposits seem to have suffered 

 the same amount of orogenic deformation as 

 the underlying Bearpaw shale. 



On the lowlands produced during this uplift 

 the continental sediments and associated coal 

 beds of the Lance and Fort Union were laid 

 down apparently without interruption and 

 imder only such changes in conditions as re- 

 sulted in the alternate deposition of sand and 

 mud and the more or less extensive accumula- 

 tion of vegetable growth. This period of depo- 

 sition was brought to a close b}- a widespread 

 orogenic disturbance, accompanied by folding 



