Sioux City Academy of Science and Letters. 159 
The position of this lens of till well up in the loess 
and separate from any known drift precludes its being 
the deposit from an ice sheet advancing over loess 
already deposited around its margin. The theory that it 
was deposited from a floating iceberg has been suggested 
by the authors as offering the most rational explanation 
for the occurrence. This involves the suggestion of the 
contemporaneous origin of this till and loess, and indi- 
cates that some of the till in this region and to the south 
is not older than the loess, and that the basal till may 
not be of much earlier data. The higher bed of till is 
very similar to the lower one, suggesting a probable com- 
mon origin of both deposits. The presence of the lower 
till some 20 feet above the present river level suggests 
that the river trough existed in early Glacial times much 
as at present. Probably the greater volume of water car- 
ried by the river from the melting ice accomplished an 
enlargement of the channel both in depth and breadth. 
If the iceberg theory be correct the flood at one time 
passed over the tops of the bluffs, and this fact may 
account for the thinness of the deposit of bowlder clay in 
this region and its wide distribution farther down the 
Missouri. Some interloessal till may possibly exist in 
buried valleys, such as that of Elk Creek. (See fig. 4.) 
The presence of a very large granite bowlder about 
midway up the bluff in Dakota County north of Jack- 
son, affords further evidence of the probable presence 
of drift-laden icebergs floating over the region. 
If the loess of this region were deposited as a fine 
silt in the quiet waters of the expanded Missouri, as sug- 
gested by Bain, it is necessary that these waters should 
have been quiet. Such a condition seems somewhat 
doubtful in view of the fact that the stream was at this 
time carrying off the waters from the melting glacier to 
the north and was evidently not ponded by any obstruc- 
tion in its lower course. The fall of the river was greater 
at that time than it is now and yet the current flows at 
a rate of over 7 miles an hour at low water. However, 
the waters may have been so heavily overloaded with silt 
and fine material that deposition resulted notwithstand- 
ing the rapidity of the current. But these deposits show 
no evidence of stratification nor of cross-bedding. 
