Siour City Academy of Science and Letters. 143 
nowhere near the Nebraska escarpment again until it 
reaches the south line of the county. This escarpment 
after continuing to the southeast for 6$ miles bends back 
just north of the town of Jackson, leaving a steep pro- 
monotory pointing out into the flood plain and makes a 
wide embayment, in which is built the town of Jackson. 
The escarpment then makes a long, graceful curve to- 
ward the southeast and once more meets the river near 
the south line of the county. This leaves the northeast 
quarter of Dakota County entirely in the river bottoms, 
which are extremely flat and unbroken save by a few low 
ridges of sand and by a few cut-off lakes that mark pre- 
vious channels of the river. These lakes are still filled 
with water, probably supplied by the underflow from the 
river and by seepage from the underlying water-bearing 
sandstone. Crystal and Jackson lakes are the largest 
examples. 
Besides shifting from one side of its broad valley to 
the other the river has doubtless during the past time 
removed the rock material from the whole width of its 
valley to a depth of in places 80 to 100 feet below the 
present water level, and possibly to a greater depth, 
especially when the region was more elevated than at 
present. The entire trough has been refilled to the level 
of the flood plain with the exception of the irregular, 
sandy channel through which the river shifts and eddies 
from pool to pool over bars of silt and quicksand. A 
heavy underflow is present in this filled valley as is evi- 
denced by the abundance of water in many shallow wells 
which persistently maintain a constant level relative to 
that of the river. One farmer is reported to have driven 
a sand point down 125 feet without encountering any 
solid material, but such records are scarce because of 
there being no necessity for deep drilling to find a water 
supply. In building the two bridges at Sioux City it was 
found that the rock underlies the river at a depth of from 
35 to 80 feet and that river deposits extend to a depth of 
more than 120 feet in places. The rock lies nearly par- 
allel to the bottom of the water, but at the west bank 
begins sloping toward the west. 
Mr. L. E. Cooley’ has shown that the trough of the 
Missouri may be still deepening during periods of heavy 
cutting and scour. 
2Cited by J. E. Todd, Bull, 158, U. S. Geol. Survey, p, 150. 
