178 



SCIENCE 



[Vol. LVI, No. 1442 



history. It is timed to the march of the ages. 



Even that slow movement of events is be- 

 yond our power to present in its continuity. 

 We can at best represent widely separated con- 

 ditions. And so, in attempting to sketch the 

 history of the Colorado Plateau region, I can 

 give you detached pictures only, some of them 

 so unlike that imagination alone can link them 

 together. It will be simplest to roll the broken 

 iilm backward from the later, better known 

 events to the earlier scenes, till knowledge be- 

 comes guess and guess fades into surmise in 

 the mists of antiquity. 



The Colorado Eiver developed during a 

 period before the present plateaus were ele- 

 vated. So great a river system, like a great 

 empire, is the result of many territorial con- 

 quests. The force by which it conquers is due 

 to its fall, for by its fall it carves its canyons 

 and extends its tributaries. Thus the earlier 

 history of the river corresponded' Avith an 

 earlier ancient uplift of the plateau country. 

 But that uplifted mass was first gashed by 

 canyons, then became a land of broad valleys 

 and mesas, and finally was eroded to a low 

 plain. A later uplift has raised that plain to 

 its present position, 7,000 feet above the sea, 

 where it is the surface of the plateau. 



The cycle of erosion just referred to is called 

 the "Great Denudation." The time of its dura- 

 tion corresponded with the so-called early Ter- 

 tiary. The early mammals drank from the 

 growing Colorado. North America then, as 

 now, stretched from the Atlantic to the Pacific, 

 an undivided continent. 



Stepping back a million years or so, we see 

 North America divided. A broad, though 

 shallow, strait stretched from the Gulf of 

 Mexico to the Arctic Ocean along the Great 

 Plains of to-day, dividing the continent into 

 an eastern and a western land. The climate 

 was mild and equable. Vegetation flourished 

 in the warm humid atmosphere. It was the 

 Coal period of Colorado and New Mexico. 

 Great saurians dominated the life of the period, 

 yet became extinct, apparently rapidly. Dull 

 brutes, they were incapable of adaptation to 

 changes of environment such as closed the 

 period of their dynasty. 



During this period, the so-called Cretaceous, 

 the region of the Colorado plateaus was a 



watered land with rivers flowing from eastern 

 coast ranges, on the site of the Rocky Moun- 

 tains, toward the Pacific. Among them may 

 have been the stream which eventually grew to 

 the Colorado, but we can not identify it. 



Still retreating down the aisles of time, we 

 come upon the panorama of a wide North 

 America, united from east to west, but sub- 

 merged along the western margin even to 

 Idaho. This, the Triassic and Permian periods 

 of geologists, might well be called the period 

 of the "Great American Desert." A red wind- 

 swept delta plain covered the Rocky Mountain 

 states from Montana to Arizona and extended 

 southeastward over Oklahoma and Texas. 

 Bleak and arid, it was like the plains of north- 

 ern Siberia. Similar cold, barren lands existed 

 widely throughout the continents. It was a 

 time of stress for all living things and led to 

 the evolution of higher forms than had pre- 

 viously existed when conditions bettered, just 

 as the severe environment of life during the 

 Glacial Period later led to the evolution of 

 man from his ape-like ancestors. 



The red muds and sands of the desert time 

 reached far into the Colorado Plateau country 

 and, in so far as they were not eroded during 

 the "Great Denudation," they give the dom- 

 inant color note to the upper gorges of the 

 river. 



Thus far in our retrospect we have found no 

 epoch during which the plateau country was 

 submerged beneath sea waters. Yet there is 

 written in the strata of the canyon walls a very 

 long record of marine conditions. Whoever 

 has been down the Bright Angel Trail has 

 seen it. The cliffs of sandstone shale and 

 limestone demonstrate by their long horizontal 

 lines of bedding, as well as by the fossils they 

 contain, that they were laid down beneath the 

 sea. It was never a deep sea, yet there gath- 

 ered in the basin more than 4,000 feet of strata. 

 Evidently the bottom sank gradually and the 

 sediments gathered as the basin deepened. Far 

 more impressive evidence of subsidence is 

 found near Salt Lake. There the strata ag- 

 gregate more than 40,000 feet in thickness and 

 indicate a corresponding subsidence of the an- 

 cient foundation rocks. 



It is clear from the great difference between 

 4,000 and 40,000 feet that we should not re- 



