DENUDATION OF THE WESTERN PLAINS. 103 



denudation over all the surrounding country. The plains have been 

 formed by the erosion of this vast area to s^ uniform baselevel, excepting 

 only the isolated hilly tracts of comparatively small extent, which serve to 

 show that on the eastern part of the plains, in North Dakota and south- 

 western Manitoba, a thickness of not less than 500 to 1,000 feet of the 

 Lai'amie, Fox Hills, and Fort Pien-e foraiations has been carried away. 

 Around the Highwood and Crazy mountains, in central Montana, accord- 

 ing to Prof. W. M. Davis' and Dr. J. E. WoUf,^ the corresponding erosion 

 of the plains in horizontally bedded Cretaceous fonuations has been 3,000 

 to 5,000 feet. 



When the depth and great extent of this denudation are compared with 

 those of the subsequent erosion which formed the Red River Valley and 

 the lowland adjoining the Manitoba lakes by the removal of the fonner 

 eastern part of the Cretaceous plains to the limit of the great escarpment 

 west of Lake Agassiz, the early baseleveling seems probably to have 

 occupied the Eocene and Miocene periods, with nearly all of the Pliocene, 

 comprising nine-tenths or a longer portion of the whole Tertiary era. Its 

 duration apparently coincided, as to both beginning and end, with the 

 Tertiary or Somerville cycle of partial baseleveling which Davis and 

 Wood have studied in Pennsylvania and northern New Jersey and believe 

 to have affected a large area of the other Eastern States.^ The tennination 

 of the denudation forming the plains of the Cretaceous area, and their uplift 

 to undergo the erosion of the Red River Valley and of the present Assini- 

 boine and Saskatchewan valleys, were probably also contemporaneous with 

 the great epeirogenic* movement which in California, according to Mr. J. S. 

 Diller, ended a long cycle of baseleveling that had extended through the 

 whole of Cretaceous and Tertiary time, and raised a part of that base- 

 leveled district at the beginning of the Quaternary era to form the lofty 



'Mining Industries of the United States, Tenth Census, Vol. XV, pp. 710, 737, 745. 



-Notes on the Petrography of the Crazy Mountains and other localities in Montana Territory, 

 p. 16. Bull. Geol. Soc. Am., Vol. Ill, 1892, pp. +15-452. 



3 National Geographic Magazine, Vol. 1, 1889, pp. 183-253; Vol. II, 1890, pp. 81-110. Proceedings, 

 Boston Society of Natural History, Vol. XXIV, 1889, pp. 365-123. 



'A term proposed by Gilbert, equivalent with continent-making. "The process of mountain 

 formation is orogeny ; the process of continent formation is epmrogeny." U. S. Geol. Survey, Monograph 

 I, Lake Bonneville, 1890, p. 340. 



