240 W. N. THAYER 
Structurally the Great Plains are a unit from Texas to Alaska. 
- Although the strata of the divisions within the United States lie 
sensibly flat for short distances, the structure of the bedrock 
(Paleozoic or Mesozoic sediments) has the general character of a 
great geosyncline which rises sharply on approach to the Rocky 
Mountains, where the steeply inclined eastward dipping strata 
appear in the form of ‘“‘hogback”’ ridges fringing the Front Ranges 
of the Rocky Mountains. A regional westward dip of the plains 
strata is general for the province, but it is modified locally by 
structural deformations of some importance. ‘The eastern outcrop 
of the westward dipping strata is marked by the escarpment and 
coteaus previously referred to. 
Similar structural features exist in Canada’ and on the whole in 
Alaska, though on the Anatuvuk Plateau the Mesozoic strata 
appear to have been thrown into a series of broad, open folds, and 
even the early Tertiary deposits have been subjected to minor 
deformation.’ 
In early Tertiary time the Great Plains of the United States 
were raised out of the sea and have remained a land surface ever 
since. The Tertiary period was one of active erosion, and during 
that time the region was base-leveled, excepting the residuals 
previously cited. The western part of the province had the greater 
initial uplift, consequently it was susceptible to greater erosion. 
All strata were beveled regardless of changing dip and hardness. 
Another great uplift in late Tertiary time began the present 
erosion cycle. In both the United States and Canada this later 
erosion has caused deep dissection in the western parts of the 
Great Plains and has almost base-leveled again the low plains 
to the east. 
In Alaska the base-leveling of the Anatuvuk Plateau is assigned 
to the late Eocene or the Miocene epoch,’ and the unconsolidated 
deposits which overlie the eroded strata are supposed to be of 
Pliocene age. If this time is correct these deposits correlate closely 
in age with the terrestrial deposits that are so widely scattered over 
the Great Plains of the United States. 
1¢. A. Young, op. cié., pp. 108-13. 
2 A. H. Brooks, op. cit., pp. 278-80. 3 A. H. Brooks, 7bzd. 
