5 C. D. WALCOTT — ALGONKIAN FORMATIONS OP MONTANA 



DEARBORN RIVER SECTION 



Location of the section. — This section is on the south side of the ridge 

 south of the north fork of Dearborn river and 35 miles southeast of the 

 Camp Creek section. It is 10 miles north of the Lewis and Clark pass. 



The section begins about 2 miles up the slope below the base of the 

 Flathead Cambrian sandstone, and extends eastward to the gravels at the 

 foot of the mountain slope, a short distance west of the ranch house of 

 Steinbach and Alt. 



Cambrian, Flathead sandstones. — The Cambrian Flathead sandstone is 

 massive, coarse grained, Avith small, white quartz pebbles, and cross- 

 bedded near base. Two hundred and fifty-five feet from the base it is 

 overlain by thin bedded sandstones and shales, with numerous annelid 

 trails and fragments of trilobites. The Flathead sandstone rests un- 

 conformably upon the Algonkian (see page 16). 



Algonkian, Belt terrane. — The following is the Algonkian section ex- 

 posed on the north fork of the Dearborn river: 



Feet Feet 

 la. Gray, buff weathering, arenaceous, thin bedded layers, 



passing into greenish gray beds 65 feet from the top. , . 225 



6. Cryptozoan, siliceous limestone 4 



c. Thin bedded, hard, gray sandstone weathering buff and 



gray, with greenish tints. 205 



d. Buff colored, arenaceous shale 76 



510 



2. Buff weathering, gray, slightly siliceous limestone in shales 

 and layers up to a foot in thickness. Massive layers of 

 interformational conglomerate, composed of broken up, 

 shaly, and thin bedded, bluish gray limestone, occur 

 180 feet from the top and bluish gray layers extend 

 below for 25 feet. Oolitic layers occur at several 

 horizons 435 



3a. Greenish, arenaceous shales, with thin, interbedded sand- 

 stones at irregular intervals. At about 500 feet from 

 the top alternating bands of greenish and purplish, 

 arenaceous shale begin, and continue for some distance 

 before giving way to purple colored, siliceous layers, 

 and thin bedded, fine grained sandstone and purple 

 shales 1,150 



Strike of strata near base, north 40 degrees west, 

 magnetic ; dip 25 degrees southwest, near. About 1,000 

 feet from base, same strike, dip 30 degrees southwest. 



