Nonconformities at the Platte River. — (joiild. 
367 
ditions obtained until the beginning of the Glacial period, 
when the ice-sheet swept down from the northeast, "bearing 
its load of sediment. Boulders, pebbles, .sand, silt, and clay 
were deposited in the depressions of the Dakota rocks and 
finall)' covered the whole of the strata to the depth of seve- 
ral hundred feet. (Fig. 5.) The coarser material, known 
Fig. .5. Glacial SedimentatioQ. 
as the drift, was at first deposited and consequentU' filled 
up the hollows while the finer loess is found at higher levels. 
This is the second nonconformity; between the top of the 
eroded Carboniferous, or ordinarily the Dakota, and the 
base of the superjacent Glacial deposits. (C. D. Fig. 5.) 
After the retreat of the ice sheet, and the consequent 
cessation of deposition, erosion and denudation once more 
took place. The streams began to cut the channels they 
now occupy. This operation is still in progress. The Platte 
has reached its base level and the small creeks tributary to 
it are slowly cutting their way back into the hills. In many 
places they have cut through the various deposits, far down 
into the Carboniferous. (Fig. 6.) In not a few instances 
\ 
\ 
i^ 
y 
)„ 
'A 
A 
c 
^:^- 
^ ^ 
^ 
-1 1 L 
— L i \ 

r^ — : — r 
-. — ' 1- 
— i — 1 1 — 
V-r. 
1 1 1 1 i 1 1 1 1 1 1 ! 1 
1 1 ' 1 1 1 ' 1 \ 1 I 1 t 1 
' ' ' 1 1 1 1 1 1 I i 1 1 
Fig. 6. Post-Glacial Erosion. 
