IOWA CAADEMY OF SCIENCES. 
107 
The facts disclosed in this exposure seem to indicate a 
succession of events somewhat as follows: There is first 
recorded an invasion of the region by the pre-Kansan ice 
sheet and the deposit of the bed of bowlder clay which 
constitutes the first member of the section. This drift 
buried the preglacial forests and concealed all traces of 
pre-Pleistocene life. Upon the withdrawal of the ice sheet 
the surface of this area was subjected to the agencies of 
leaching and oxidation as land surfaces are today. During 
this time there was developed the brown colored zone in 
the superficial portion of this bed of drift. 
After a long period, and possibly well towards the mid- 
dle of the Aftonian age, some cause or causes resulted in 
the formation of a shallow basin over a portion of the 
area which formerly existed as a land surface. This new 
basin may have resulted from a land slip or cut off, or it 
may have been formed by a beaver dam or by the closing 
of the outlet of a stream which drained this particular 
region. In this forest-encircled depression waters col- 
lected. Small streams began at once to carry into it fine 
detritus. 
In this pool water-loving mosses became established, and 
semi-aquatic vegetation flourished along its borders. Twigs 
and small branchlets that were broken from the trees dur- 
ing violent wind-storms came to rest in the water, and 
contributed their substance to the filling of the basin. 
During a long series of dry seasons a layer of almost pure 
vegetable matter would accumulate over the bottom of the 
bog. During a succession of seasons of greater rainfall a 
bed of sandy sediments w r ould be spread over the floor. In 
some of these layers of sand there are thin, brown colored 
laminae so numerous that, in places, as many as twenty- 
five can be counted in the thickness of one inch. Each of 
these laminae probably represents the carbonaceous sub- 
stance of a single season’s growth of vegetation during the 
periods when the deposition of mechanical sediments pre- 
dominated over the accumulation of vegetable matter on 
the floor of the marsh. 
