294 GEOLOGY. 



the slowness with which the land assumed its present form. The 

 upper terraces were dry bottom when all the rest of the valley was 

 yet a river-bed. It is probable that some of these bottoms were ex- 

 cavated during sub-glacial times, and afterward were filled up with 

 debris when the continent had reached the lowest level. The great 

 depth of sand and mud at the bottom of the Missouri, being from 

 forty to one hundred feet below water along the Nebraska line be- 

 fore solid rock is reached, indicates an elevation of this region, when 

 this was accomplished, far greater than it reached at any period 

 during or immediately after Loess times. When this great lake 

 commenced to be drained, the waters naturally took the direction 

 and place of least resistance, which was the original bed of the 

 river. If the Rocky Mountain system continues to rise, as it is be- 

 lieved to be doing, at the rate of a few feet to the century, although 

 degradation may be equal to elevation, a time must come in the dis- 

 tant future when the Missouri will again roll over solid rock at its 

 bottom. 



As typical of the river-bottoms, let us look at the formation of 

 the Platte Valley. The general direction of this great highway 

 from the mountains to the Missouri is from west to east. This val- 

 ley is from three to twenty miles wide in Nebraska, and over five 

 hundred miles long. All the materials that once filled up this 

 trough, from the tops of the highest hills on each side, have been r 

 since the present rivers were outlined, toward the close of the Lo- 

 ess age, transported by the agency of water to the Missouri and the 

 Gulf.* Here, then, are several thousand miles in area of surface en- 

 tirely removed by denudation. Now the Platte comprises only a 

 fraction of the river-bottoms of Nebraska. The Republican, alone,. 

 for two hundred miles has a bottom ranging from three to eight 

 miles in breadth. The combined length of the main bottoms of the 

 Blues, Elkhorns, and the Loups, would be over a thousand miles, 

 and their breadth ranges from one to ten miles. The Nemahas 

 and the Bows, and portions of the Niobrara, also acid a great deal 

 to the area of bottom lands. All these rivers have numerous 

 tributaries, which have valleys in size proportionate to the main 

 rivers, and these more than double the areas of bottom-land. The 

 Missouri has, also, in some counties, like Dakota and Burt, con- 

 tributed large areas of bottom-land to the soil of the State. These 

 Missouri bottoms in Nebraska are exceptionally high, so that few 



*See Hayden's Report for 1870. 



