106 
IOWA ACADEMY OF SCIENCES. 
Number 4 is a sbil mantle of fine-grained material that 
has been developed upon the surface of the Kansan drift 
during the long interval that has elapsed since the disap- 
pearance of the Kansan glacier. This pebbleless soil band 
and the underlying bed of Kansan drift are so familiar to 
all of you that they require no special discussion. Iowa 
geologists have encountered the drift of pre-Kansan age at 
various points in the State and are well acquainted with 
its characteristics. It is to the time of accumulation, the 
composition, and the manner of deposit of the materials 
which compose the second member, the bed of organic 
matter, to which I would request your consideration. 
It is evident that we have here to do with an accumula- 
tion that has slowly taken place in the basin of a lake 
or pond. The large amount of moss in many of the veg- 
etable layers of the member testifies to the slow rate of 
growth of the deposit, and to the long time during which 
the basin was in process of filling. 
The oxidized, iron-stained band which occurs in the 
superficial portion of the lower drift, and which forms the 
floor of the basin, would indicate that the surface of the 
pre-Kansan drift had been exposed to the atmosphere for 
a long period previous to its existence as a lake bed. Also, 
the presence of numerous branches and fragments of wood 
in the lowermost layer of organic matter would be strong 
evidence that the deposit did not begin to accumulate im- 
mediately after the withdrawal of the pre-Kansan ice 
sheet. If this basin was one of the numerous depressions 
that were left over the surface w r hen the pre-Kansan ice 
melted, the first vegetation to become established in this 
pool would have been species of algae, aquatic mosses, and 
other lowly types of water-loving plants. Not until hun- 
dreds of years after the withdrawal of the ice sheet w 7 ould 
the drift surface become clothed with forests. Not until 
long centuries had elapsed after the disappearance of the 
glacier would there be developed conditions which would 
result in the accumulation of such a layer of wood frag- 
ments oh the floor of a newly formed lake or pond. 
