TERTIARY AND EARLY QUATERNARY BASELEVELING. 19 



Westward the depth of the Tertiary baseleveUng was greater. Around the High- 

 wood and CrazN-^ mountains, in central Montana, aceoiding to Professor W. M. 

 Davis* and Dr J. E. Wolff,t the erosion of the plains has a vertical extent of 3,000 

 to 5,000 feet. Perhaps the most striking evidence of this great erosion is afforded 

 by the range of the Crazy mountains, which lies immediately north of the Yellow- 

 stone river near Livingston and is conspicuously seen from the Northern Pacilic 

 railroad. These mountains trend slightly west of north, and extend about 40 miles 

 with a width of 15 miles, attaining an elevation of 11,178 feet above the sea and 

 5,(X10 to (5,000 feet above the prairies at their base. Their structure has been thor 

 oughly studied by Wolff", who finds that they consist of late Cretaceous strata, soft 

 sandstones, nearly horizontal in stratification, intersected by a network of eruptive 

 dikes. The more enduring igneous rocks have preserved this range, while an 

 average denudation of not less than one mile in vertical amount reduced all the 

 adjoining region to a baselevel of erosion. The Plighwood mountains, about 25 

 miles east of Great Falls, having a height of 7,(500 feet above the sea or about 3,000 

 feet above their base, are described by Davis as displaying the same structure, and 

 therefore similarly testifying of great denudation. 



The uplift at the beginning of the Tertiary era appears to have raised this portion 

 of the plains to a height above the sea as great as the vertical extent of their 

 Tertiary erosion — that is, to a height of at least 1,000 to 5,000 feet, increasing from 

 east to west. Toward the end of this era the baseleveling had reduced the country 

 mostly to a plain, which was probably only a few' hundred feet above the sea, lying 

 much below its present altitude. 



Renewed Elevation and partial Baseleveling at the Close of the Tertiary 



AND DURING THE EARLY PaRT OF THE QUATERNARY ErA. 



Between the general Tertiary cycle of baseleveling and the Glacial period there 

 ntervened a second great epeirogenic uplift, as shown by a return of the conditions 

 of vigorous stream erosion and a new cycle of partial baseleveling, by which wide 

 flat valleys were cut in the eastern part of these Cretaceous plains. In ^Manitoba 

 the northeastern border of the formerly baseleveled expanse was removed, the 

 Cretaceous beds being ei'oded to the underlying Archean and Paleozoic rocks upon 

 a large area l)ounded on the west by the escarpment before mentioned as forming 

 the eastern limit of the plains. The duration of the earlier baseleveling apparently 

 coincided as to both beginning and end with the Tertiary or Somerville cycle of 

 l>artial baseleveling which Davis and Wood have studied in Pennsylvania and 

 northern New Jersey and believe to have aff'ected a large area of the other eastern 

 states.; 



East from the f(j()t of the Pembina, Riding and Duck mountains and the hills 

 farther north, together called the Manitoba escarpment by ]Mr J. B. Tyrrell, of the 

 Canadian (iecjlogical Survey, Cretaceous strata have not been found, so far as I 

 have learnerl, in Manitoba, nor in the region nortii and northeast from lake Win- 

 nipeg to Hudson bay. It seems cjuite certain, however, that Cretaceous beds con- 

 tinuous from this escarpment extende<l eastward at the end of the Tertiary base- 



* Mining industries of the United States, Tenth Censu?, vol. xv, pp. 710, 737, 745. 

 t Bulletin, Geo!. Society of Ameri(;.i, vol. lii, 1892, pp. 44.')-4.')2. 



t Proceedings Boston Society of Nutiinil Ili.story, vol. x.\iv, 18,sr). i)p. :jt;5-4j;5 ; National Gougrai)hic 

 -Muguzinf, vtil. i, \hh<j, pp l«:j-2.j:5; vol. ii, IkImj, pp. 81-llu. 



