22 
IOWA ACADEMY OP SCIENCES. 
thin relative to its serial dimensions. The loess is well exposed, 
both above and below the till bed, and is in each case of a 
character indistinguishable from that so frequently seen in the 
city street cuttings and other excavations. The surface above 
shows the usual flowing lines which so well characterize the 
topography of a loess covered land. 
The intermixture of till and loess seen at this locality cannot 
be explained by the theories mentioned in the other cases. The 
till found here is many feet above any known drift found in the 
vicinity, and is well up in the loess. The most rational theory 
seems to be that, while the loess was being deposited, a mass of 
floating ice laden with debris from the adjacent ice sheet, 
stranded and gradually unloaded its burden or upset as it was 
floating and dumped into the water the material carried. 
Such an explanation for certain tile deposits has been sug- 
gested, but there seems hitherto to have been no such clear 
case observed. 
It remains to consider the bearing of this fact upon some 
general problems concerning the drift. The relation of these 
deposits to the outer or Altamont moraine is of interest. This 
moraine, as it has been traced, presents a gap about nine 
miles wide, in the northern part of Clay county, S. D. , about 
thirty miles northwest. It is also known that through this gap 
an ice tongue nearly that breadth (nine miles) extended down 
the valley occupied by the Vermillion river on the west and 
Brush creek on the east. This came within, perhaps, twenty 
miles of the typical exposure just described. The next gap of 
the moriane north of the one just described, is where the Big 
Sioux comes through, south of Canton, near Pairview. The 
drift in western Plymouth county, Iowa, near which the expos- 
ure described is situated, is thin and patchy, being usually not 
over fifteen feet in thickness. That the region has not been 
covered by the heavy land ice would seem to be indicated, not 
only by this, but also by the general presence of beds of fine 
sand and clay under the drift, and showing no signs of disturb- 
ance. 
The deposition of till in the loess indicates their contempora- 
neous origin, and therefore throws light upon the age of both. 
It indicates that some of the till outside the moraine is as late 
as the loess, and argues strongly in favor of all being not long 
antecedent and of probable similar origin. 
