UI1AXD CAXOX OF Till-: COLORADO 



423 



largely due to the difference in the kinds of rocks which the river en- 

 counters in its journey from the mountains to the sea. 



Leaving the immediate vicinity of the canon and going to the north, 

 the tourist would come upon various younger geological formations 

 which at one time buried the whole region. Although from ten to 

 fifteen thousand feet thick, they no longer remain in the vicinity of the 

 canon, having been washed away by flowing water after the elevation 

 of the land in early Eocene times. When that was accomplished and 

 the country for miles around was comparatively level, the land was 

 again gradually raised and the river began in Xeocene times to cut 

 the outer canon. When it had cut down to the Tonto terrace — ' The 



Fig. 6. Im the Heart of the CaSon. Archaean Rocks capped by Tonto Strata, which dip 



towards the River. 



Esplanade * — elevation ceased and the river wandered back and forth 

 in a lazy manner, widening instead of deepening its channel. Since 

 in that arid region there was not enough rainfall to wash down the 

 side walls, they have remained precipitous, while the main stream has 

 had time to push them back until they are a dozen miles apart. Then, 

 in the Pliocene, slow elevation of the plateau began again, causing the 

 river to run swiftly and renew its carving. This action is proceeding 

 at the present time with the result of deepening the inner canon. 



Standing on the canon's rim and looking down upon the immense 

 gulf, one marvels that so small a river could have accomplished so 

 gigantic a work. But if he laboriously descends to the brink of the 

 roaring river below and puts his hand into the rushing water and feels 



