LESSONS IN GEOLOGY. 365 



ser ones farther east. During the time of most extensive glacia- 

 tion these doubtless were all united into one broad sheet of ice 

 that extended nearly or quite to the line already traced across 

 the country ; but for the most part they were separate bodies of 

 ice. The moraines in many places are very distinct; they are 

 readily traced on Long Island, across Pennsylvania, Ohio and 

 Indiana. The moraine bounding the Lake Michigan glacier has 

 been traced, the common portion between the Lake Michigan and 

 the Saginaw glaciers being well marked, extending north and 

 south just west of the center of Michigan. And the one between 

 the Saginaw and Maumee glaciers is also well marked in the 

 eastern part of Michigan. And the peculiar relation of the St. 

 Mary's and St. Joseph rivers to the Maumee is due to a terminal 

 moraine of the Maumee glacier along which these rivers flow to 

 a common opening through the ridge. In Minnesota and Dakota 

 there are numerous moraines, forming two systems of lobes or 

 loops, the arrangement being more complicated than farther east. 



At places over this area where bed rocks are exposed, they 

 show marks of glaciation, as at Put-in-Bay in Northern Ohio, Ni- 

 agara Falls, and other places, the striae indicating the direction 

 of ice flow ; sometimes two or more systems of striae occur on the 

 same surface, indicating two or -more invasions of ice flowing in 

 different directions. 



Boulders from the hard pan are often glaciated ; sometimes 

 specimens are found that have been striated in a dozen different 

 directions, as they have turned in the loosening grasp of the ice. 

 Many of the boulders and much other glacial material came from 

 limestone and sandstone rocks whose original bed is not far 

 away, but for the most part glacial debris came from the Lauren- 

 tian tableland. 



It is interesting to trace these boulders back to the native 

 quarry, selecting well-marked varieties, so that there shall be no 

 doubt as to the identity of specimen and quarry or ledge. In 

 this way it is shown that the "drift" material of Iowa, in 

 general, came from the northeast, of Indiana and Ohio from the 

 north, of New England from the northwest. In Indiana, Ohio 



