18 INTRODUCTION. 



south at least as the Ohio river in our own country, was cov- 

 ered thickly with ice, as a large part of Greenland is at the 

 present time. The ice is believed to have existed as an almost 

 continuous glacier, which moved with an infinitely slow but 

 constant motion to the southward, grinding up in its passage 

 the surface portions of the formations upon which it rested 

 and over which it moved, into the fine material that now con- 

 stitutes the soil and subsoil, and which accumulation was left 

 by the receding ice to cover those formations like a mantle. 

 The granite boulders and pebbles so .common in the surface 

 deposits in all parts of the State,, were brought down from 

 the north by the same means and at the same time. This is 

 known from the fact that granite exists in regular ledges at 

 the surface in Minnesota, and does not so exist in Iowa. This 

 mantle-like surface deposit, which everywhere covers the 

 formations of stratified rocks except where they have since 

 been exposed by the wearing away of it by the streams, com- 

 posed of sand, clay, gravel, and boulders, in a heterogeneous 

 mixture, is called " drift" by geologists, and the same desig- 

 nation is used for it throughout this report. 



The accompanying map-model of Iowa will, in a good 

 degree, illustrate the geological structure of the State in ac- 

 cordance with the principles thus far explained. It is not 

 claimed that this model gives all the accurate details of the 

 geological formations of Iowa. It is only intended to show 

 at a glance the relations of the important formations to each 

 other, and the relative extent of surface occupied by each. 

 The geological map accompanying this report will show the 

 details more fully. If it were possible to strip off completely 

 from the whole State its mantle of drift,, the stratified rock- 

 formations which form its foundations would present an 

 appearance ill relation to each other, such as is represented 

 by the different sheets in the map-model. The illustration 

 would be still more complete if a separate sheet was devoted 

 to each one of the formations that occur in the State, but this 

 would make the model too bulky for introduction into the 

 volume. However, as the principal object now sought to be 



