GLACIAL GEOLOGT OF SOUTHERN MICHIGAN. 101 



the maiii agent of transportation was recognized. Abont sixty years 

 ago the great naturalist, Louis Agassiz. who was then living in Swit- 

 zerland, found that the transported material of that region had a simi- 

 larity in composition and structure to the material which the glaciers 

 of the Alps were depositing. T^pon journeying widely over northern 

 Europe and America the great extent of glaciation in past ages was 

 recognized and the glacial theory brought out. It took many years for 

 this theory to be tested suflfieiently to satisfy geologists of its wide appli- 

 cation. But now every geologist at all familiar with the evidence recog- 

 nizes that glacier ice has been the chief agent in transporting the drift. 

 There are places in which floating ice has been operative, and, too, 

 some of the material included in the drift has been transported by water 

 and some by wind. But these are all found to be minor agencies. 



Agassiz supposed that the glaciation of northern Europe and Amer- 

 ica was produced by a polar ice cap. but it is now known that the ice 

 fields bordered the north Atlantic and left much of the polar regions, 

 Siberia and Alaska, uncovered. There were several centers of ice 

 accumulation, some of them in latitudes thirty degrees to forty degrees 

 from the north pole. The ice spread from these centers toward the pole 

 as well as away from it. 



There was an early center of glaciation in the province of Keewatin, 

 west of Hudson Bay, another in the Labrador Peninsula, and a third 

 west of the Eocky Mountains in Canada, while Greenland is still largely 

 covered by an ice sheet. However, it is only with the Keewatin and 

 Labrador ice fields that Michigan glaciology is concerned. From the 

 Keewatin center the ice extended as far south as northeastern Kansas 

 and central Missouri, and it may possibly have spread southeastward 

 over Michigan to Indiana and Ohio, and then withdrawn in an early part 

 of the glacial epoch. 



From the Labrador center the ice extended as far southwest as south- 

 eastern Iowa and southern Illinois, and as far south as northern Ken- 

 tucky. If the Keewatin ice sheet invaded Michigan it nmst have been 

 prior to the Labrador invasion. 



The material deposited by the Keewatin ice field at its greatest ex- 

 tension has been called the Kansan drift sheet, because its extreme lim- 

 its are in the State of Kansas. There is another drift sheet completely 

 buried under it, — the pre-Kansan. The material deposited by the Labra- 

 dor ice field at its greatest extension has been termed the Illinoian drift 

 sheet, since it is well displayed and reaches its farthest limits in Illinois. 



The history of glaciation is not confined to a single advance and 

 melting away of each of these ice fields, but to repeated tidvances separ 

 ated by intervals more or less prolonged, in which the ice was either 

 restricted to a small area or had entirely disappeared. The Labrador 

 ice field is found to have made three such advances and formed three 

 drift sheets in the eastern part of the Mississippi basin, which named 

 in order from first to last are the Illinoian, the lowan, and the Wiscon- 

 sin. There are drift sheets farther east which are not yet fully corre- 

 lated with these. The lowan is well displayed in eastern Iowa and 

 northern Illinois, but farther east the Wisconsin" extends beyond its 

 limits.* Michigan was entirely covered by the Wisconsin drift sheet. 



♦A silt deposit called loess occurs extensively over the region outside the limits of the lowan in 

 the Mississippi basin, and appears to be in large part a correlative, and to some extent a dependency 

 of that drift s-hset. It occurs outside the Wisconsin drift in the states south of Michigan. 



