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EVOLUTION 



May. 1932 



The Story of the Grand Canyon 



By HUGH F. MUNRO 



JOHN Burroughs tells us that the West is full of Geology. must be at the bottom where the Colorado river is busy with 



So is the East for that matter, but in the West we see the continuation of it. At the river level we are standing 

 Mother, Nature in her youth, unclothed, raw, angular. In on the very cellar floor of the earth, so far as the geologist 

 the East she has taken on a mature rotundity and clothed has been able to read the story, the Archeozoic, 

 herself with a becoming mantle of green. In the eastern No recognizable fossils have been found in the Archeozoic 



landscape Hit predominates while the West is stark and bare, rocks, although there is indirect evidence that life already 

 less modified by the softening influences of plant and animal 

 life. The West is the paradise of the geologist, for in it the 

 great earth book has been stripped of its covers and its pages 

 lie open for all who care to read and take the trouble to 

 understand its story. More pages of the earth's history are 

 exposed in the Grand Canyon of Arizona than in any other 

 part of the world, yet half of the chapters are missing. 



To the tourist the Grand Canyon is merely a great gash in 

 the earth's surface about a mile and a quarter deep, from ten 

 to twelve miles wide and over two hundred miles long. The 

 south rim at El Tovaris is 6866 ft. above sea level, the north 

 rim 1000 ft. higher. Resisting the temptation to dwell on 

 the sublimity of the awe-inspirmg spectacle which is felt as 

 well as seen we turn at once to the study of its history. 



Aided by rain, frost, wind and chemical action, the Colo- 

 rado river with its tributaries has been at work for ages cut- 

 ting away the rock and carrying the debris toward the sea. 

 In the absence of the retarding effect of vegetation, rain cuts 

 its own channels which become deeper year after year, a pro- 

 cess that can be seen on any uncovered country road. In the 

 West it is not unusual to cross the dry bed of a river that 

 after a few hours of rain will become a turbid torrent flow- 

 ing swiftly in narrow channels and carrying a heavy load 

 of scouring material. The rivers all over this region cut out 

 and flow in gorges with almost perpendicular walls. 



Descending into the Canyon by the Bright Angel trail. 

 even the most casual observer can see in the varied colors 



of the rock layers that the Canyon has a history, and little Ph<iia by u. S. Ceohsicl Survey 



reflection is needed to show that the beginning of that history Grand Canyon of Colorado River in Arizona. 



existed. Where the strata have re- 

 mained undisturbed, between Grand 

 View and the mouth of the little Colo- 

 rado river, the Proterozoic rocks have a 

 thickness of 12,000 ft. Within that 

 range, which perhaps covers a greater 

 length of time than all of the subse- 

 quent formations put together, the first 

 fossils appear in the form of Algae and 

 primitive invertebrates. Ascending, the 

 upper layers of the Paleozoic are 

 reached, with fossil forms appearing 

 successively as primitive invertebrates, 

 higher invertebrates and fishes. In 

 depth this covers over 4000 ft. and ,^ 

 brings us to the Permian which is the 

 highest of the Paleozoic rocks. Here 

 Dr. Chas. W. Gilmore, curator of ver- 

 tebrate paleontology of the United 

 Columnar Seaion, showing position and structural relations of Grand Canyon rocks. States National Museum, has collected 



