62 



Captain Fremont's on the south, and thus give us the connected levels 

 over a very large area. 



The ohservations upon the great tertiary formation have developed 

 the fact, that since the close of the pliocene period the eastern base of 

 the mountains, which is the western limit of this formation, has been 

 elevated from 2,000 to 3,000 feet above the eastern, and this without 

 there being anywhere visible signs of upheaval, such as inclination 

 of the strata. The only direct evidence is in the immense denudation 

 which the tertiary has undergone probably while this elevation was 

 in progress, and which causes of denudation must have been gradu- 

 ally extinguished, as there is, at the present time, no force at work 

 sufficient to have affected them. The evidence goes to show that the 

 elevation which lias taken place since the close of the pliocene period 

 has been in Nebraska remarkably uniform, and along a line in a 

 general direction northwest and southeast, and nearly coincident with 

 the ranges of mountains previously upheaved. 



The Black Hills received their last violent upheaval at the same 

 period as the Laramie mountains, that is, at the close of the creta- 

 ceous. The geological evidence goes to show that the j)liocene and 

 miocene tertiary, south of the Shyenne, are fresh water formations; 

 yet there are no ridges now standing to mark the northern boundary 

 to this basin. In the present relative position of the different parts 

 of these plains, the elevation of the pliocene tertiary formation is now 

 so great that much of the Black Hills and the cretaceous on the Shy- 

 enne should have been covered with it. This might, however, have 

 been the case, and since have all been denuded away. North of the 

 Shyenne the cretaceous ridges are probably sufficiently high to have 

 separated the tertiary beds south of it from the lignite tertiary to 

 the north. But still it is necessary to suppose that this last elevation 

 of the tertiary has been somewhat greater near the 4 2d parallel than 

 to the north of it. 



A most interesting problem could be solved in regard to these 

 changes of level, if a locality could be found where the lignite tertiary 

 north of the Shyenne would be in contact with the pliocene or miocene 

 beds to the south of it, as well as the more important one of the age 

 of the first relative to the two latter. 



During the time of these changes since the formation of the pliocene 

 tertiary, the soft sandy material of which it was composed has been 

 crushed and separated by denuding forces, and an area of no less than 

 20,000 square miles, called the Sand Hills, has been covered with 

 barren sand, which, blown by the wind into high hills, renders this 

 section not only barren, but in a measure impracticable for travel. 

 The Niobrara river, lying on a most desirable line of communication, 

 and direct in its general course, has 100 miles of its banks obstructed 

 with these difficult hills, and the communication between this stream 

 and the Platte greatly obstructed and in some places entirely cut off. 

 The subject of routes and communications I shall hereafter take up in 

 detail. 



For nearly all of the knowledge of the age of the geological for- 

 mations of the portions of Nebraska developed by my explorations, 1 

 am indebted to the services of Dr. F. V. Hayden and Mr. F. B. Meek 



