330 B. WILLIS — LEWIS AND LIVINGSTON RANGES, MONTANA 



of uplift and erosion. The characteristics of the gravels and the phys- 

 iographic record of the mountains may decide the relation on closer 

 study. 



Structure 



general statement 



The structural geology of the region comprises three dominant facts, 

 to which all other phenomena are incidental. These facts are, first, the 

 synclinal structure of the Front ranges; second, the superposition of 

 Algonkian strata on Cretaceous in consequence of an overthrust fault, 

 and, third, a normal fault which probably separates the mass of the Liv- 

 ingston range from the equivalent rocks beneath the Flathead valley. 



SYNCLINE OF THE FRONT RANGES 



General features. — The strata herein described as Algonkian, from the 

 Alt}m limestone at the base to the Kintla argillite at the top, are flexed 

 in a shallow basin. Throughout the eastern, the Lewis range, the strata 

 dip gently south westward. The amount of dip varies from 5 degrees or 

 less to 30 degrees. Throughout the western, the Livingston range, the 

 strata dip northeastward, usually at angles between 30 and 40 degrees/ 



Thus each crest is a limit of the syn- 

 cline, and the intervening valley fol- 

 lows the synclinal axis with a trend 

 of north 25 degrees west. The struct- 



FwvRvS.-Mounts Heavens and Stanton, looking Ure is of large proportions. Thebeds 



south jrom Trapper Peak. involved in the flexure are at least 



Showing the steepest northeastern dip ob- IQ^OO feet thick. The width of the 



S6FV6Cl 



syncline is 8 miles near the top of the 

 Siyeh formation, which forms the conspicuous elevated rims, and in the 

 Appekunny argillite, the lowest bed which appears on both sides, it may 

 be measured at 20 miles. The structure is exceedingly simple (figures 1, 

 plates 50 and 51, in panorama, and section 5, plate 53). In broad views 

 irregularities of dip are scarcely noticeable. 



Details of local folds. — The Grinnell red beds locally exhibit internal 

 folds a few yards in dimensions, representing movements within the soft 

 mass of argillites. In a peak known as mount Stanton, southwest of 

 mount Heavens, on the western margin of the syncline, gray argillites 

 of the Appekunny formation stand vertically, and are even overturned. 

 (See figure 3.) Again, east of the head of lower Logging Creek lake 

 and about 2,000 feet above it, cliffs of this argillite exhibit marked 

 cleavage, which traverses the bedding at an angle of 20 degrees. The 

 local strike of bedding is north 55 degrees west ; dip, 20 degrees south- 



