538 D. F. HEWETT 



INTRODUCTION 



It is the principal purpose of the present article to set forth 

 observations in the McCullock Peak region that bear upon the 

 extent and age of the Heart Mountain overthrust. It will be 

 apparent, however, that such a profound structural feature must 

 consistently fit into the complex Tertiary history of sedimentation 

 and orogenic movements in the region, so that it seems advisable 

 to set forth here some of the problems of the region and some of 

 the data that must be adjusted to a correct and comprehensive 

 interpretation of that history. As the writer has devoted parts 

 of the field seasons of 1911, 1912, 1913, 1916, and 1919 to detailed 

 investigations in three fifteen-minute quadrangles that He between 

 Cody and ThermopoHs, southeast of the region in which the over- 

 thrust has been observed, some conclusions must be stated without 

 giving much of the evidence on which they are based. 



SURFACE FEATURES 



The Bighorn Basin (4, 5, 14) is an elHptical area of low relief 

 with few conspicuous hills or mountains, surrounded on the east, 

 south, and west by mountain ranges. For purposes of physio- 

 graphic, stratigraphic, and structural description, it may be con- 

 sidered as made up of two parts, a central part 75 miles long by 

 45 miles wide, which largely coincides with the area of Wasatch and 

 younger beds, and a border belt 10 to 20 miles wide, which lies 

 between the central part and the mountain ranges that surround 

 the basin. These ranges include the Bighorn Mountains on the 

 east, the Bridger and Owl Creek ranges on the south, and the 

 Absaroka Range and Bear-tooth Plateau on the west. Figure i 

 shows some of the features and geology of an area in the north- 

 western part of Bighorn Basin, which extend from the eastern edge 

 of the Absaroka Range, across the border belt to the center of the 

 basin. Figure 2 shows two sections across this region. 



The central part of the basin contains large areas of flat uplands 

 that He seVeral hundred feet -above the nearby valleys. There are 

 also extensive areas of bad lands where erosion has cut back into 

 the uplands. The central part contains three conspicuous elevated 

 areas that rise above the uplands, and that range from 1,200 to 



