Erosion on the Great Flams. — Upliain. 35 
EROSION ON THE GREAT PLAINS AND ON THE 
CORDILLERAN MOUNTAIN BELT. 
By Warren ITpham, St. Paul. Minn. 
The subaerial sculpture of great land areas is not less wor- 
thy of attention than marine sedimentation, upheaval, and vol- 
canic action, by which the lands were originally formed. It is 
also very interesting to follow the. great courses of drainage, 
and to note the marine and lacustrine deposits that have been 
derived from the wear and ^vaste of continents. 
In the region here considered, namely, the north part of 
our Great Plains and the part of our Cordilleran belt where it 
is crossed by the Northern Pacific, Great Northern, and Can- 
adian Pacific railways, the physiographic history is comprised 
in the Tertiary and Quaternary eras. During the much longer 
ages of Paleozoic and Mesozoic time, from the Cambrian peri- 
od to the Cretaceous, inclusive, the site of the Yellowstone 
National Park and a vast region to the north and west were 
covered by the sea, with practically continuous and conform- 
able sedimentation, sometimes at abyssal depths where little 
deposition took place through long periods, and sometimes 
in shallower water receiving plentiful tribute from adjoining 
eroded lands. 
The Cretaceous sea of that region, in which its latest sedi- 
ments were laid down, stretched eastward over Manitoba and 
the greater part or all of Minnesota, to the area now occupied 
by the west end of lake Superior. Though the strata then 
formed have been mostly or wholly eroded and removed from 
a tract 100 to 200 miles wide along the eastern margin of the 
Cretaceous marine area, its fossiliferous beds are found in place 
by H. \'. W'inchell so far east as on the Little fork of Rainy 
river''-' and on the Mesabi range. t Thence west to the Rocky 
mountains, an expanse of deep Cretaceous strata, mostly 
shales, was uncovered from the sea at the end of that period, 
and has since been subject to erosion. 
At first a vast flat and monotonous plain, Uiis expanse has 
lost hundreds of feet at the east and thousands of feet at the 
♦ Geol. and Nat. Hist. Survey of Minnesota, Sixteenth Annual Report, for 
1887, pp. 403-9. 431, 434. 
+ A.\i. Geologist, vol. xii, pp. 22(>-223, Oct., 1893. 
