46 A NATURALIST IN THE GREAT LAKES REGION 



With these brief hints of the sort of evidence on which the 

 geologist relies to reconstruct the physiographic features of past 

 ages we may sketch in bare outline the history of the Chicago 

 region and adjacent territory during the time the rocks here- 

 abouts wxre depositing. During the Acadian period (Fig. 30) the 

 Potsdam formation, largely sandstone with shales, was laid 

 down as sand and mud washed by erosive agencies from the 

 adjacent land and spread out on the sea bottom and transformed 

 into rock. Then followed the Canadian period (Fig. 31), during 

 which limestones, sandstones, and shales were formed. The 

 conditions must, of course, have frequently changed to permit 

 of such varied t}'pes of deposits. First there were laid down 

 thick beds of magnesian limestone interrupted by layers of shale 

 and marl when evidently the sea became shallower and muddy 

 or possibly even so shallow as to allow abundant vegetation that 

 was instrumental in forming the marl. Later sandstone formed 

 and then more limestone. 



There was next (Ordovician period) a change to a shallow 

 sea with swiftly flowing currents and a nearby shore. The sea 

 contracted and its margins transformed to dry land. Possibly 

 much of the time the sea completely disappeared from this 

 locality, its shores lying farther south, and the drifting sand 

 blew inland in great dunes covering wide areas. This sand in 

 time solidified to form St. Peter sandstone. Again the land sank 

 and the interior sea reformed, inundating the entire area about us 

 as well as areas considerably to the north. The sea bottom was 

 now deep enough along shore and free enough from mud to allow 

 the abundant growth of animal and plant life. Here where 

 Chicago stands were forming great beds of limestone, the 

 Platteville-Galena (Trenton) made of the wave-worn fragments 

 or even of the perfect shells or skeletons of animals then living 

 along the shores, the nearest of which were somewhere up in 

 what is now mid-Wisconsin. Animal life was abundant. There 

 were corals, brachiopods, pelecepods, and trilobites. There were 

 also extensive beds of seaweed growing luxuriantly just as 



