NO. lO DISCOVERIES OF CAMBRIAN BEDS RESSER 3 



Structural province is regarded as including the Southern and Middle 

 Rockies as recently defined by Fenneman. This includes the ranges 

 in central Colorado, Wyoming, and south-central Montana, but ex- 

 cludes the Wasatch and western Wyoming ranges. 



Although the Southern Rockies are in line of strike with the ranges 

 to the northwest, nevertheless they have a wholly different geologic 

 history and consequent structure, consisting essentially of a Cryp- 

 tozoic ^ core fringed wath belts of Paleozoic rocks. We may take the 

 Big Horn Range as a typical example of the Southern Rockies. This 

 range consists of an elongate dome of peneplaned Cryptozoic rocks, 

 the edges of which are surrounded by a band of early Upper Cambrian 

 overlain by younger strata. It is certain that the overlapping edges of 

 the Upper Cambrian strata have been stripped back along the pene- 

 planed surface on which they rest, but they appear never to have 

 covered altogether the higher, central portions of the dome. Owing to 

 the positive nature of these domes, coupled with their stability through- 

 out long periods of time, it is not surprising that real geosynclines are 

 apparently absent from the Southern Rockies, and that, in consequence, 

 all sediments from Cambrian to Recent times are basin deposits laid 

 down in the same manner as the Tertiary beds of the present Big 

 Horn Basin. 



The northern boundary of the Southern Rockies is naturally an 

 irregular line. Along the main strike the province terminates with the 

 Beartooth Mountains, northeast of the Yellowstone National Park. 

 However, the Gallatin and other similar Montana ranges to the west 

 of the Beartooth mass should be excluded, even though in their evident 

 stability and peneplaned cores they retain characteristics of the 

 Southern Rockies. Their Cambrian, or initial Paleozoic, strata clearly 

 belong to the northern subdivision, so that they represent the southern 

 shore line of that province. Eastward of the Beartooth mass the 

 northern boundary of the Southern Rockies extends far northward to 

 include the Little Rockies and Big Snowy Mountains of central 

 Montana, and to the east to embrace the Black Hills in South Dakota. 



Applying the same criteria to the delimitation of the western 

 boundary of the Southern Rocky Mountains, it is necessary to exclude 

 the Wasatch, Teton, and intervening ranges from this province and 

 include them with the Great Basin, even though the Tetons and pos- 

 sibly the Salt River ranges partake somewhat of the structural nature 

 of the Southern Rockies. Cambrian and other Paleozoic strata in 



^ Term recently used by Schuchert and Dunbar, Textbook of Geology, 3rd ed. 

 John Wiley & Sons, New York, 1933. 



