Pembina Region of North Dakota — Berkey. 143 
boulders are an abundant constituent, contrasting strongly 
with the clayey local facies or the still less common gravel, 
sand, and clay mixture. As a rule the soils are very clayey. 
Structure af the Escarpments — There are two escarp- 
ments, one behind the other, in the Pembina region. The 
lower one is known as the "first mountain" and is best 
developed in the vicinity of Walhalla, North Dakota, 
where the Pembina river debouches upon the plain of the 
Red river valley. Here there is a shelf 5 or 6 miles wide, 
200 feet above the plain at its outer margin, which grad- 
ually rises toward the west through 100 feet or more to the 
foot of a second escarpment known as the "second moun- 
tain." At this point another ascent of 200 to 300 feet 
reaches the general level of the highland country which as- 
cends slowly toward the Turtle mountains beyond. The 
entire margin of this area was described by Mr. Upham* 
in his study of lake Agassiz. The "first mountain" noted 
above is Mr. Upham's Pembina delta deposit. The whole 
shelf was considered by him of this origin and separated 
from the shales below by a sheet of till. 
As will be seen from the accompanying map, the Little 
Pembina river after leaving the higher plateau runs along 
the base of the second escarpment to its junction with the 
main stream. Great quantities of boulders and some till 
have slid into this part of the gorge as undercutting of the 
blufifs has proceeded. The presence of such material in this 
transverse gorge led as the writer believes to an erroneous 
idea of the true structure of the whole first mountain. 
Ordinarily the weathering process is so uniform on the 
soft formations of the bluflfs and the creep of the clays so 
gradual that none of the real ledges might be seen on the 
east side. But the past season has been excessively wet. 
The severe rain storms started numerous land slides, un- 
covering many points that had formerly been completely 
covered with mixed residuary and alluvial accumulations. 
A recent examination therefore made clear that Cretaceous 
shales occur to within 15 or 20 feet of the top of the bluflfs 
on both sides of the Little Pembina to its mouth. The 
same is true of the Pembina from this point still farther 
east for probably two m^les. The capping of the shelf 
• U. S. G. S., Monograph xxv. p. 357. 
