THE LOMBARD OVERTHRUST AND RELATED FEATURES 271 



The principal structural features are shown on the map (Fig. 1) 

 and in the structure sections (Fig. 2). The dynamic features 

 consist of folds and faults. 



FOLDS 



There is no indication that any marked deformation took place 

 in this region during the Paleozoic and Mesozoic eras. At the 

 close of the Cretaceous period, probably, the great series of sedi- 

 ments which had been accumulating began to be deformed. In 

 this region they were compressed into a series of closed folds with 

 a general northeast-southwest trend. These folds are usually 

 overturned to the southeast and pitch to the southwest. Two of 

 these folds were named by Dr. Peale 1 the Horsehoe anticline and 

 the Cottonwood isocline, both situated north of Logan, Montana. 



East of Lombard, in the vicinity of Crane Station, there is a 

 northward-pitching anticline. In the long ridge west of the Mis- 

 souri River there is an elongate domal structure (Fig. 3) the 

 western side of which is interrupted by a normal fault and obscured 

 by an extensive overthrust. The southern part of this elongate 

 dome is overturned to the east and pitches steeply to the south 

 (Fig. 1; Fig. 2, section D-D; and Fig. 4). 



FAULTS 



The Lombard overthrust. — -The most important feature of the 

 structural geology of the region is an extensive overthrust fault 

 which has its southern end in the ridge north of Three Forks, and 

 extends a distance of at least 13 miles along the ridge to the north- 

 ern border of the map. The writer proposes the name "Lombard 

 overthrust" for this feature, because it is well exposed in the 

 canyon of the Missouri River near Lombard. Here the fault plane 

 dips about 40 to the west. This fault has brought strata of the 

 Belt Series over strata of Cretaceous age in the north, near Lom- 

 bard, and has brought the upper member of the Cambrian into 

 contact with the Carboniferous Madison limestone in the southern 

 end of the ridge (Figs. 2 and 5). The maximum displacement on 

 the fault plane near Lombard cannot be very closely estimated, 



1 A. C. Peale, Bull. U.S. Geol. Survey, No. no, 1893. 



