THE AGE OF THE MAMMALIAN FOSSIL FAUNA 509 
of the lobe. Hence, from this standpoint, it seems better to credit 
the striz to an ice sheet coming from the northeast, and to inquire 
into this possibility. 
Pre-Illinoian drift in the Labrador field has been identified in 
surface exposures in the western part of the La Salle quadrangle 
and the eastern part of the Hennepin quadrangle. Drift below the 
Illinoian has also been reported in a recent well in the Springfield 
region.” Such drift is far enough east of the known limits of the 
Keewatin field to be best referred to the work of a glacier in the 
Labrador field having a general southwesterly movement in Illinois. 
But, as pointed out above, the materials collected from the Alton 
deposits do not show anything distinctively Labradorean in origin. 
More data must be awaited to settle definitely the ne of the 
drift concerned. 
The loess deposits —As given in the section described above 
there are two deposits of loess in this region, separable at least on 
the basis of color. The lower loess is characterized by its reddish 
tinge, the upper by its buff color. At all of the fresh exposures 
of the several quarries at Alton, this difference is conspicuous, and 
this is found to hold not only for street cuts in other parts of the city 
but for more distant localities. Buff loess overlying reddish loess 
is shown at the quarries at Grafton, 18 miles up the river, and at 
Edgewood, about 25 miles down the river. At Alton the reddish 
_ loess is somewhat thicker than the buff, the former being about 
30 feet thick, the latter about 22 feet. Approximately the same 
ratio holds true at Grafton, but at Edgewood the conditions of 
slope do not favor a good estimate. Both are fossiliferous, but on 
_ the whole the lower seems to contain more shells than the upper. 
The fossil content will be discussed later. In texture, both seem 
to be typical loess, with no notable quantity of sand or suggestion 
of stratification. 
Both are doubtless aeolian in origin, their relations to the 
Mississippi Valley indicating its flood plain as the chief source of 
t Gilbert H. Cady, ‘‘Geology and Mineral Resources of the Hennepin and La Salle 
Quadrangles,” Bulletin 37 (1919), p. 71. 
2K. W. Shaw, and T. E. Savage, ‘‘The Eainieoererdd Quadrangles,” U.S. 
Geol. Survey, Folio No. 188 (1913), p. 7. 
