468 KEMP AND BILLINGSLEY— SWEET GRASS HILLS, MONTANA 



the general line, Gardiner-Lewistown, there are no red beds between the 

 eroded surface of the Quadrant limestone and the Ellis marine deposits. 

 The unconformity found in the Sweet Grass Hills section thus repre- 

 sents an erosion period which becomes greater from Wyoming to Alberta, 

 uplift and denudation to the north being contemporaneous with continued 

 deposition — first, marine (Quadrant) ; then under arid continental con- 

 ditions (Chugwater) — to the south. As to the former extent of Quad- 

 rant and Chugwater deposits over the northern region, the Sweet Grass 

 Hills section throws no light. 



JURASSIO-ELLIS FORMATION 



The sea, which finally covered the Jurassic erosion surface, deposited 

 in the Sweet Grass Hills region about 200 feet of soft green shales, with 

 some impure shaly limestones and calcareous sandstone layers. These 

 deposits do not differ markedly, either in character or thickness, from 

 the Ellis else^^■here. They suggest a shallow sea, but one with no con- 

 tiguous high lands to supply coarse detritus. The Rocky Mountain uplift 

 to the Avest had not yet commenced. 



LOWER CRETACEOUS — KOOTENAI FORMATION 



The Kootenai deposits, which rest conformably upon the Ellis, mark 

 the first pronounced oscillation of level in the series of uplifts and sub- 

 sidences which distinguish the Cretaceous age in the Eocky Mountain 

 region. The record of these oscillations is nearly complete in the Sweet 

 Grass Hills section and is the only such record within a radius of nearly 

 100 miles. For this reason the Cretaceous sections of the Sweet Grass 

 -Hills deserve a rather detailed study. 



Three carefully measured sections are shown on the accompanying 

 figure 8. 



The general sequence from the base of the Kootenai is seen to be as 

 follows : 



1. Coarse sandstones deposited on the Ellis, becoming finer grained and inter- 

 bedded with red and green sliales (top of Kootenai). 



'2. Gray sandstones, sandy shales, and thin beds of dark shale, in the lower 

 400 feet of the Colorado. 



3. Limy shales, shales witli b'entonite, and tliin sandstone bands, ending with 



sandy shale and "shells" at 800 feet above the base of the Colorado. 



4. Brown and blue shales, locally black shales, generally calcareous toward 



the top — the upper 1,000 feet of the Colorado. 

 ?>. Massive sandstone — the Eagle; generally has 2 to 4 feet coal bed on top. 

 6. Soft shale with sandstone layers, most numerous toward tlie west — Claggett. 



