256 GEOLOGICAL SURVEY OF THE TEEEITORIES. 



ALLUYIUM. 



IS'ext to the Loess deposits, in an economical point of view, the Allu- 

 vium formations are the most important. The valleys and flood-plains 

 of the rivers and smaller streams, where these deposits are found, are a 

 prominent feature of the surface geology of the State. All the rivers of 

 the interior, such as the Platte, the Republican, the Niobrara, the Bow, the 

 Elkhoru, the Blues, the Kemahas, and their tributaries, have broad 

 bottoms, in the center or on one side of which the streams have their 

 beds. The width of these bottoms seems to be dependent on the char- 

 acter of the underlying rock-formation. Where this is soft or yielding 

 the bottoms are broad, but where it is hard and compact they contract. 

 This is, no doubt, one reason why the bottoms on the middle or upper 

 courses of some of the rivers are wider than farther down.* These broad 

 bottoms, as we have already seen, represent the ancient river-beds 

 toward the close of the Lacustrine age. It required many ages to drain 

 this mighty ancient lake-bed ; and when the present rivers were first 

 outlined, the greater part of it was yet a vast swamp or bog. But, 

 gradually, as the continent rose to a higher level, the rivers cut deeper 

 and deeper, filling the whole flood-plain from bluff to bluff. Not until 

 the drainage of this region was completed and the continent had reached 

 nearly iis present level was the volume of water so much diminished 

 that the rivers contracted their currents and cut new beds somewhere 

 through the present bottoms. The terraces, which are so numerous 

 along many of the river-bottoms, indicate the slowness with which the 

 land assumed its present form. They mark those stages of elevation 

 when the land was stationary. 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 excavated during sub-glacial times, 

 and afterward were filled up with debris when the continent had reached 

 its lowest level. The great depth of sand and mud at the bottom of the 

 Missouri, being from forty to one hundred feet below low water along the 

 Nebraska line before solid rock is reached, indicates an elevation of this 

 region, when this was accomplished, far greater than it reached at any 

 period during 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 Eocky 

 Mountain system continues to rise, as it is believed 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 distant 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 valley is from 

 three to fifteen miles wide in Nebraska, and over five hundred miles long. 

 All the materials that once filled up this trough, from the top of 

 the highest hills on each side, have been, since the present rivers were 

 outlined toward the close of the Lacustrine age, transported by the 

 agency of water to the Missouri and the Gulf.f Here, then, are several 

 thousand miles in area of surface entirely 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 thou- 



* See on this subject Hayden's Report for 1870. 

 t Hayden's Report for J 870. * 



