14 
lOVv^A ACADEMY OF SCIENCE 
Morals, intimately related stratigrapliically to the lithographic beds, and of no 
small economic importance, has a geographic distribution equally extensive. 
Other well defined zones of the Devonian system are easily recognizable. To 
Norton we are indebted for the recognition and definition of the lower members 
of the Devonian, the Otis and the Coggan beds, which, however, are found only 
in the southern part of the Devonian area. Similar zones, definite and con- 
stant as to position and having great geological significance, have been deter- 
mined for the Niagaran limestones, for the various members of the Ordovician, 
for all of our geologic systems ranging from the Cambrian to the Upper Crea- 
taceous. 
The Iowa Geological Survey may be credited with some of the most important 
contributions yet made to our knowledge of Pleistocene geology. Before the 
Survey came into existence Chamberlin, Salisbury and McGee, by a series of 
masterly investigations, had determined the criteria which might be used in 
the differentiation of successive drift sheets and had pointed the way to the 
correct solution of Pleistocene problems. Three episodes of glaciation were 
recognized in a general way, and had been named by Chamberlin the Kansan, 
Iowan and Wisconsin, but the limits of these were only vaguely known. Bain, 
at that time Assistant State Geologist, demonstrated that the gravel beds at 
Afton Junction lay beneath the Kansan of Chamberlin, and that, below the 
gravels, there was an older drift sheet whose true geological position had been 
misunderstood. In this way a fourth glacial deposit took its true place in the 
Pleistocene series. This, although the fourth in the order of recognition, was 
in reality, so far as now known, the first in the order of time. It has not yet 
received its proper distinguishing geographic name, but is simply known as 
the pre-Kansan or sub-Aftonian. It has been exposed by the erosion of the 
stream in the valley of the Grand river a mile or two below Afton Junction, 
in the valley of the Little Sioux in the northern part of Harrison county, at 
the base of the Missouri river bluffs in Fremont and Mills counties and at other 
points in the southwestern part of the state; while eastward it shows in the 
Mississippi bluffs and lateral ravines near Muscatine. It has been seen in arti- 
ficial excavations as in the great Oelwein cut in Payette county; it is known 
to occur in many well sections; but there is no locality at present known where 
it is found as the surface drift. At all its exposures we see only the edge of 
the deposit, everywhere it is overlain by younger formations. We are indebted 
to Leverett for the last of the drift sheets to be recognized, the Illinoian. This, 
the fifth in order of recognition, is the third in order of chronological suces- 
sion. The Iowa Survey has worked out the margins of the several drift sheets, 
so far as the margins fall within the borders of the state; it has studied and 
described the differential structural Characters of the deposits ; it has noted 
and recorded the relative amounts of erosion, weathering and general altera- 
tion in the surface of the Kansan, Illinoian, Iowan and Wisconsin respectively, 
and, with proper caution, has used these as indices of the relative age of the 
formations; it has studied the interglacial deposits and has drawn conclusions 
concerning the interglacial faunas, floras and climates. The soils of Iowa con- 
stitute her most valuable geological endowment; over extended areas the fer- 
tility and adaptability of the soil varies with the nature of the underlying 
drift; there are alluvial soils, lacustrine soils, loess soils as well as soils of 
glacial origin; studies of the Pleistocene deposits are, in the highest sense, 
icontributions to Economic Geology. 
