234 GEOLOGY. 



far separated, isolated spots. The top of all these rocky cliffs, 

 whose strata are horizontal, represent where the general level of 

 the Pliocene once was. Perhaps the most remarkable monument 

 of the original level of the Pliocene in Nebraska, is at Scott's 

 Bluffs, and at Chimney Rock, on the North Platte. These have 

 long been noted landmarks. The country is here eroded into many 

 forms, exhibiting some of the peculiar natural architecture of the 

 Bad Lands. Chimney Rock is about 150 feet high. The strata 

 here and at Scott's Bluffs are horizontal, and therefore the general 

 level of the country must have been as elevated, at least, as the top 

 of these crags. No doubt much material has also been removed 

 from the top of the highest of these old monuments, as they have 

 been subjected to erosive agencies ever since the commencement of 

 the Glacial Age. From two to four hundred feet, therefore, must 

 have been removed from the general surface of the Pliocene de- 

 posits of the plains. Notwithstanding the immensity of this erosion, 

 a considerable thickness of these deposits still remain. In Ne- 

 braska they range from 10 to 700 feet. King has remarked that 

 at the mountains, where they are lofty and form powerful con- 

 densers of moisture, the resultant streams have carried away in 

 front of them all the Tertiary and exposed the Cretaceous. 



Elevation of the Pliocene. At Chalk Bluffs, the line of separation 

 between the Miocene and Pliocene is 6,000 feet above the sea leveL 

 Near 41 30' the Pliocene reaches an altitude of over 7,000 feet. 

 In the valley of the Loup Fork the contact plane between the Mi- 

 ocene and Pliocene approximates to 3,000 feet. There is, there- 

 fore, a gradual sinking eastward of the contact plane between the 

 Miocene and Pliocene. 



Eastward Barrier of thr Pliocene Lake It has been a question 

 what barriers on the east held in the waters of the Pliocene lake 

 of the plains. Two theories have been suggested. One is that the 

 whole western shore line, with the mountain chain against which 

 it abuts, and the present incline towards the east, was low enough, 

 during Pliocene times, to hold the waters of the lake. This theory, 

 however, is irreconcilable with the known facts concerning the ele- 

 vation of the Rocky Mountain system during the Tertiary epochs*. 

 Evidently this region near the eastern shores of the lake, and on 

 the south, was once elevated into a rim, and it was the sinking of 

 this border, towards the close of the Pliocene, and the transference 



*See Clarence King's Systematic Geology of the 40th., Parallel, Chapter VI. on Stratigraphi- 

 cal Geology. 



