STRATIGRAPHY OX SAND COULEE. 



313 



which the Sandcoulee mines open, where the coal appears beneath the 

 massive sandstone ledge traced up the valley. At the mines it is about 

 20 feet thick and is a hard white quartz rock. The coulee slopes are 

 generally drift-covered, but a natural section is exposed where the wagon 

 road ascends the plateau, showing the following beds : 



Fee t 



Sandstone 30 



Shaly beds 25 



Sandstone, square-jointed, buff; really a pebbly grit 3 



Shaly beds, purple and red, crumbling (?) 



Shaly beds, pebbly, gray and green, crumbling (?) 



Sandstone ledge, same as that over coal ; no coal seen. 



Where the wagon road descends into Straight coulee, near the coal 

 banks, the following strata are exposed : 



Feel 



y,j.': 



Sandstone, forming summit of plateau. 



Sandstone and sandy shales, red and gray, alternating beds. 50 

 Shales and clays, red and buff 50 to 75 



Sandstone, forming bed above coal seam 20 



dial seam 12 



Figure 4. — Section 

 on Sand Coulee. 



The plateau summits are quite gently undulating surfaces, well grassed 

 but bare of trees or shrubs, with a covering of glacial drift not of sufficient 

 thickness to produce a, marked drift topography but filling preglacial hol- 

 lows. Bowldersare not common but include a variety of rocks granite, 

 limestone, etc — found in the Belt mountains. 



Sandcoulee Basin. 



Structure. — That portion of the Great Falls coal field adjacenl to the 

 Sandcoulee mines can best be alluded to as the Sandcoulee basin. In 

 prospecting the field it has been found thai the seam thins out toward 

 both the north and the south from the mines. Toward the west the seam 

 splits into two beds, separated by 25 feet of shale, but probably is con- 

 tinuous, if not workable, to the bluff's of Smith river, though the prospect- 

 ing indicates a shallow basin. 



