366- 
The Amebic a?i Geologist. 
June, 1900 
About the beginning of the Dakota epoch the land again 
subsided. The sand and cla\\s, washed down probably from 
the northeast, filled the hollows in the Carboniferous floor 
and finall}' smoothed over the sea bottom. (Fig. 3.) If the 
' I ' i' I ' I ' '^^. ' ' '-' 
Fig. 3. Dakota Sedimentation. 
Dakota be considered as an estuarine or beach deposit, it 
will be readily conceived that at the close of this epoch the 
surface would still be irregular. The numerous lagoons and 
bayous, interspersed with islands, would suffice to give con- 
siderable of configuration to the surface. Those, however, are 
minor considerations and most, perhaps all, traces of them 
have disappeared. The first nonconformity is then found be- 
tween the unevenly eroded surface of the Carboniferous and 
the base of the Dakota. (A. B. Fig. 3.) 
As the shore line of the Cretaceous sea retreated west- 
ward, the first deposits were elevated and the work of ero- 
sion began anew. In the soft sandstones and shales this 
was comparatively rapid and during late Cretaceous and 
Tertiary time the surface reliefs were probably as great as at 
the beginning of the Dakota epoch. (Fig. 4.) These con- 
FiG. 4. Post-Dakota Erosion. 
