Erosion on the Great Plains. — Uphani. 37 
as the Eurojx-an Alps, the Caucasus, and the Himalayas. But 
with those latest formed mountains, which to<Tcther may be 
named the Eurasian mountain belt, should be classed, in tj- 
same first rank as to hight, and of similar late Tertiary and 
Quaternary time of uplifts, other parts of this very long and 
wide Cordilleran belt, such being Mt. St. Elias and its neigh- 
bors, the Sierra Nevada of California, anil the high Andes. 
Attending and following the great folcis. faults, and uplifts 
of mountain ranges through the western side of our continent, 
which closed the Cretaceous period and began the Eocene, so 
bridging the transition between the Mesozoic and Tertiary 
eras, volcanic intrusive and eruptive rocks added greatly to the 
mountain masses of some tracts, as in tiie Yellowstone Park 
and in the Cascade range, and spread over very large plain 
areas in the basins of the Snake and Columbia rivers. 
During the Tertiary and Quaternary eras, this western half 
of our country and of Canada, newly raised from oceanic 
depths into plains and mountains, has undergone much eros- 
ion ; and the rivers have borne thence the detritus from this 
vast area, depositing it mostly beyond their mouths in the sea. 
Quantitative estimates of the amount of this erosion, and conse- 
quentlv of the ofifshore sedimentation, are afforded from the 
Plains by the Turtle mountain, on the international boundary 
of North Dakota and ]Vlanitoba, and by the Crazy and High- 
wood mountains in Montana. Farther to the west, such esti- 
mates may be taken from the valleys and canyons of the wide 
Cordilleran belt, and from the fiords of Puget Sound and the 
coast northward. 
Turtle mountain. 40 miles long from east to west and about 
25 miles wide, rises 300 to 800 feet above the surrounding east- 
ern part of the Plains, the tops of its highest hills licing about 
2,500 feet above the sea. Under a veneering of the glacial 
drift, which probably averages 50 to 75 feet in thickness, this 
wooded highland consists of nearly horizontally bedded Lara- 
mie strata, chiefly shales, with thin seams of lignite. Tt testi- 
fies that a thickness of 500 feet, or more, of Laramie and Mon- 
tana (Fox Hills and Ft. Pierre) strata has been eroded from 
the surrounding region.* 
• "The Glacial Lake Agassiz," V. S. Geol. Surv., Monograph XXV, 1895, 
pp. 85, 173. 
