Age of the Missouri River. — II pliant. 83 
consisting- partly or wholly of the nearly horizontal strata, 
spared from erosion while all the surrounding country was 
being baseleveled. Such are the Turtle mountain and the 
Crazy and Ilighwood moimtains descrihed in my last fore- 
going paper. 
The Hand hills, in eastern Alberta, and the Cypress hills, 
in southwestern Assimhoia, are similar high remnants of the 
Cretaceous strata, rising 1,500 to 2,000 feet above the sur- 
rounding Plains ; but they also show, above the Laramie and 
other Cretaceous formations, a capping of Miocene conglom- 
erate, sandstone, and sandy clays, of fluvial deposition. It 
camiot be doubted, therefore, that Miocene deposits, somewhat 
like the Miocene and Pliocene formations in Wyoming and 
Nebraska and southward, once had a considerable development 
throughout the Canadian part of the Plains ; and it is also evi- 
dent that, since so comparatively late a stage in the Tertiary 
era, a vast amount of denudation 1 as taken place there. It thus 
seems very sure that likewise much of the denudation on the 
adjacent region of the Plains drained by the Missouri river 
belonged to the late part of Tertiary time. It progressed prob- 
ably through nearly the whole of that long era, completing the 
baseleveling, so far as that was attained, near the end of the 
Pliocene period, the Plains through most or all of their extent 
being then worn down to only a slight elevation above the 
sea.* 
Stream deposition had been very abundant, widelv ex- 
tended and deep, during the early part of the time since the 
original uplifting of the mountain ranges and plains drained 
by the [Missouri river and its tributaries. During the latter 
part of that time, until the end of the Tertiary era, the Plains 
underwent great denudation and a general baseleveling. which 
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, t 
• U. S Geol. Survey, Monograph XXV, 1895, "The Glacial Lake Agassiz," 
pp. 81-107. 
+ National Geographic Magrazine, vol. i. 1889, pp. 183-253; toI. ii, 1890, 
pp. 81-110. Proceedings, Boston Society of Natural History, vol. xxiv, 1889, 
pp. 365-423. 
