GRAND CANON OF THE COLORADO 



421 



formations belonging to the four great rock systems. At the top is the 

 bluish limestone which weathers into fantastic buttresses and pinnacles, 

 many of them so dizzily perched as to give a constant challenge to the 

 winds or to man to topple them from their precarious position and 

 hurl them a thousand feet below. My companion could hardly find 

 time to sleep, so great was his delight in prying off great blocks of 

 stone and listening to their terrible crunch and roar as they fell hun- 

 dreds of feet, striking a ledge here and there and finally crushed to 

 powder, or, still in gigantic mass, they rested a thousand or more feet 

 below. This top formation, which Powell called the ' Fortification 

 limestone/ is the ' Upper Aubrey ' limestone. It is about five hundred 

 feet thick. 



Next below it, can be seen the great white four hundred feet wide 

 band of Upper Aubrey sandstone which stretches like a ribbon in sight 

 for fifty miles and more to the east and west. Its walls are even more 

 precipitous than those of the Aubrey limestone. 



Below it are a thousand feet of shelving red sandstone which form 

 the ' Lower Aubrey.' These three formations — the Upper Aubrey lime- 

 stone and sandstone, and the Lower Aubrey sandstone — constitute the 

 Upper Carboniferous system which is so familiar to the inhabitants of 

 the Mississippi Valley states as the source of coal, but which in this 

 region is not coal bearing. 



Following the Upper Carboniferous a shelf of shales leads out to 

 the precipitous wall of the great ' Red Wall ' limestone, which, with 



Fig. 4. Geological Section at Bright Angel Trail. 



