SEDIMENTARY ROCKS 



477 



east of the three buttes, and the deeper 

 coulees almost invariably cut into it, 

 with resulting picturesque canyons. 

 Around the buttes the Eagle is turned 

 up in encircling rims which inclose the 

 inner lowland of Colorado shale. 



The change from the calcareous 

 shales of the uppermost Colorado to the 

 massive buff sandstone is very abrupt 

 and bears witness to a rapid retreat of 

 the sea and the spreading of Eagle as a 

 retreatal sandstone. It is probably 

 marine at the base, with seaweed casts 

 and worm tracks, but near the top the 

 coal bears witness to its emergence from 

 the sea. On the southern edge of West 

 Butte this coal, by reason of proximity 

 to the intrusive rocks, has been meta- 

 morphosed into a high-grade semi- 

 anthracite. 



Above the Eagle, in the eastern part 

 of the region, are marine shales, of 

 which 259 feet were cut in Montana- 

 Canadian well number 4. It is at least 

 200 feet above the collar of this well 

 to the base of the Judith River beds, 

 which cap the ridge to the northeast; 

 so that, in all, about 500 feet of Clag- 

 gett is here represented. In well num- 

 ber 1, however, north of West Butte, 

 sandstone is practically continuous 

 from the base of the Eagle up. Be- 

 tween these . points, therefore, is the 

 ap])r()ximate shoreline of the Claggett 

 Sea, where the nuirine shales give way 

 to sands from the west. This shoreline 

 can he traced southward through sec- 

 tions 3, 4, 5, and 6. 

 This completes the list of formations which are represented in their 



entirety in the Sweet Grass Hills region. The sections there seen fix 



definitely the following important facts : 



