82 The American Geologist. August, i904. 
the mountain ranges on the west, and secondarily from erosion 
and redeposition of the^very thick beds first so spread out on 
the western borders of the Plains, adjoining the mountains. 
Estuarine conditions are indicated here and there, though not 
generally, in the Laramie formations, terminating Cretaceous 
and Mesozoic time ; but the greater part of the Laramie and 
all of the Tertiary formations are freshwater deposits. 
Many geologists have ascribed these very extensive fresh- 
water beds to sedimentation in vast lakes. Prof. \V. ]\L Davis 
has shown, however, that the physical characteristics of these 
formations, as well as their fossils, indicate instead that rivers, 
meandering in variable courses, and in their seasons of flood 
spreading far and wide, were the agents of deposition, bring- 
ing the sediments from the ever progressing erosion of the 
mountains.* 
The alternations of gravels, sands, and clays, with frequent 
cross-bedding and local unconformities, in the deposits that 
have been supposed to belong to the central parts of large 
lakes, are evidently more accordant with the explanation of 
these areas as flood-plains of rivers. Shallow playa lakes of 
moderate extent undoubtedly existed temporarily in many in- 
closed basins, and on some areas occasionally large and deep 
lakes may also for some time have received sediments from in- 
flowing rivers and from shore erosion ; but the view presented 
by Davis, that fluvial deposition accounts for the far greater 
part of these late Cretaceous and Tertiary beds, seems most 
acceptable. 
Followmg the deposition of the extensive and thick Lara- 
mie strata on the Plains through which the ]\Iissouri river 
flows, a depth varying from 500 to 5,000 feet (increasing 
from east to west) of these and the underlying Cretaceous 
beds was eroded by rains, rills, brooks, and widely wandering 
rivers, reducing this region mostly to a peneplain. The ver- 
tical uplift alDovc the sea level, and the concomitant and subse- 
quent depth of denudation, wearing down the uplifted land 
until its surface was again mostly near that ultimate baselevel, 
are measured from occasional groups of mountains and liills 
* Proceedings, Am. Academy of Arts and Sciences, vol. xxxv, pp. 34.5-373. 
March, 1900. Am. Geologist, vol. xxv.p. 313, May, 1900. See also an earlier 
important paper bearing on this question, by W.D yiATTHUW, Am. Maturalist, 
vol. xxxiii, pp. 403-408, Maj', 1899; Am. Geologist, vol. xxiv, pp. 250-251, 
Oct., 1899. 
