176 



GEOLOGICAL SURVEY OF THE TERRITORIES. 



of any importance in the surface of the western continent. We suppose 

 that the channels of all the streams on the eastern slope of the Rocky 

 Mountains were at one time occupied with water from hill to hill, and 

 that the drainage was toward the sea* But in the Great Basin, which, 

 so far as we know, has no outlet, the drainage must have been by evap- 

 oration, for the evidence points to the conclusion that it was entirely 

 filled with water high up on the sides of the mountains. There is 

 greater uniformity in the terraces in the Great Basin than in the valley 

 of the Missouri, which indicates a far more equable drainage. Still, 

 those along the flanks of the Wasatch Mountains number two or three 

 principal ones, but these formations separate into five or six ; and Stans- 

 bury mentions one locality where there are ten or twelve of them. In 

 the Missouri Valley, and along the eastern slope generally, the terraces 

 vary much in height and importance. 



Fig. 18 shows the peculiar form of the main terrace as shown on the 

 Missouri River, just above Omaha. 



» 



Fig. 18. 



The distant hills are composed of the yellow marl or loess, and the 

 surface has befm weathered into the rounded, conical hills. This por- 

 tion is often covered with the drift or stray rocks, or what I have called 

 in a former report the erratic block deposit. On the terraces these erratic 

 masses are scarcely ever found, and in the broad bottoms of the Missouri 

 River seldom if ever. This fact strengthens the opinion that the terraces 

 are really one of the latest features, and that they were formed during 

 the drainage of the waters toward the sea after the temperature had 

 reached nearly its present state. Oscillations of level may have contrib- 

 uted somewhat to the formation of the terraces, but I am inclined to 

 believe that the drainage or the contraction of the waters is the main 

 cause. This is an important point, and I hope hereafter to treat it more 

 fully when I have accumulated a greater number of facts. It has been 

 my belief for years, that not only the Missouri River, but all the 

 branches, from the largest river, like the Yellowstone or Platte, that 

 flowed into it, to the smallest creek, that has cut its canon deep into 

 the sides of the mountains, were once filled with water from side to 

 side, but have gradually shrunk to their present diminutive proportions. 

 All over the West are large, dry beds which must have at one time 

 given passage to vast bodies of water. The flanks of the mountains, 

 from the north line to Mexico, are gashed with gullies or canons, many 



