56 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V, A. 1912 



Cretaceous. In other words, the eldest sedimentary beds visible in the Rocky 

 mountains at the Forty-ninth Parallel here overlie one of the youngest forma- 

 tions of the region. The relation is plainly one of overthrusting, which will be 

 discussed in the section devoted to the structure of the Clarke range. 



ALTYN FORMATION. 



General Description. — Tbe Altyn formation, immediately overlying the 

 Waterton dolomite, was so named by Willis, who described it from a typical 

 section near the village of Altyn, Montana, fifteen miles south of the Boundary 

 line. 



The Altyn is not exposed within that part of the Clarke range which is 

 covered by the Commission map. The writer studied the formation chiefly in 

 a fine section on Oil creek and thus on the Atlantic side of the Great Divide. 

 The exposures are there excellent for the greater thickness of the formation. 

 In this section the eroded edges of 3,000 feet of Altyn strata can be seen on 

 the long ridge running southwestward from the bend in Oil creek two miles 

 below Oil City (Figure 6). At least 500 feet of additional, basal beds are 

 exposed along the lower course of the creek and it is these which have been 

 referred to as conformably overlying the Waterton dolomite. 



Calcium and magnesium carbonate are the dominant constituents of the 

 formation. With these are mixed grains of quartz and feldspar in highly varia- 

 ble proportion. The rock types thus include arenaceous magnesian limestones, 

 dolomitic sandstones, dolomitic grits, and pure dolomites, named in the order of 

 relative importance. The character of the bedding and the colours of the rocks 

 were often found to vary in sympathy with the rock composition. On this 

 threefold basis the thick formation as exposed along Oil creek, has been sub- 

 divided, though only approximately, as follows: 



Columnar section of the Altyn formation, slxoiving thiclcnesses. 



Top, conformable base of the Appekunny formation. 

 -Medium-bedded, light gray, sandy, magnesian limestone, weathering gen- 

 erally pale buff or, more rarely, strong brownish buff; a few inter- 

 beds of magnesian limestone. 



Thin-bedded, light gri«y and greenish gray magnesian limestone, weather- 

 ing buff; subordinate interbeds of sandy limestone. 



Massive, homogeneous, light gray, sandy limestone; weathering yellowish 

 whit©; in some horizons bearing chert.y nodules and large, irre- 

 gularly concentric silicious concretions. 



Thin-bedded, buff-weathering magnesian limestone. 



Massive, highly arenaceous or gritty, gray magnesian limestone, weather- 

 ing white or very pale buff. 



Thin-bedded, relatively friable, gray or greenish gray magnesian lime- 

 stone, weathering buff or yellowish white. 



Light gray, thick-bedded, sandy and gritty magnesian limestone, weather- 

 ing pale buff; occasional thin intercalations of thin-bedded mag- 

 nesian limestones bearing cherty nodules and silicious concentric- 

 concretions. 



Base, conformable top of Waterton formation. 



a 300 feet, 



b 950 " 



c 550 " 



d 50 " 



e 750 " 



/ 650 " 



9 250 " 



3,500+ feet. 



