14 BULLETIISr 16 9, UNITED STATES NATIONAL MUSEUM 



topographic feature. Sweetgrass County does not include some of 

 the most important localities, and does include a vast area, much of 

 it south of the Yellowstone, far outside that here treated. Melville 

 is an unimportant settlement local to one small part of the field. This 

 whole region, a great synclinal area with Fort Union rocks filling it, 

 may be called the Crazy Mountain region, for these mountains occupy 

 its approximate center and are the most prominent topographic 

 feature in this part of the State, and the major structure has already 

 been called the Crazy Mountain Syncline. The smaller area desig- 

 nated and discussed above is, in general, the eastern half of the 

 Crazy Mountain region, and is that where the Crazy Mountain Fort 

 Union is best and most clearly developed and where it has yielded 

 mammals. This area will herein be called the Crazy Mountain Field. 



GEOLOGY 



STRATIGRAPHY 

 General Stratigraphic Column 



Rocks exposed in the area between the Musselshell and Yellowstone 

 Rivers and east of the Crazy Mountains are from Lower Cretaceous to 

 Recent in age. Aside from terrace gravels, moraine and outwash 

 deposits, valley fill, and other unconsolidated young deposits, not to be 

 considered here, the sedimentary rocks are Cretaceous and Paleocene, 

 as far as definitely established. There are numerous igneous intru- 

 sions, all younger than the Paleocene sediments, and Kkewise omitted 

 from this discussion. The whole stratigraphic column, including 

 some rocks not exposed in the area of the map (pi. 1) but all within a 

 few miles of these and undoubtedly underlying this area, may be 

 summed up in a general way as shown in the column on page 15. 



From exposures outside this area, especially in the Big Snowy and 

 Belt uplifts, it appears that the sedimentary rocks here exposed are 

 probably underlain by many thousands of feet of earlier Mesozoic, 

 Paleozoic, and pre-Cambrian sediments, probably one of the thickest 

 piles of sediments in the world. 



The oldest surface beds of this region are exposed only in the centers 

 of domes north of the mapped area, for instance in sec. 34, T. 7 N., R. 

 16 E. Dr. Barnum Brown has recently obtained some interesting 

 dinosaurs from this locality. There is some question whether these 

 beds are closer to the Kootenai or to the approximately equivalent 

 Cloverley, but here they have generally been called Kootenai. 



The series here called "Undifferentiated Colorado" is probably 

 susceptible to definite subdivision and correlation, but this has not 

 yet been clearly accomplished and is outside the scope of the present 

 study. The lower part was given the local name "American Fork" by 

 Douglass (1909), who called the upper, marine, beds "Fort Benton", 



