Ross: The Dunesland Heritage of Illinois 



11 



Cenozoic Era — Modern life 



Quaternary Period 



Pleistocene Epoch 1 



Tertiary Period 



Pliocene Epoch 13 



Miocene Epoch 25 



Oligocene Epoch 36 



Eocene Epoch 58 



Paleocene Epoch 63 



Mesozoic Era — Medieval life 



Cretaceous Period 135 



Jurassic Period 181 



Triassic Period 230 



Paleozoic Era — Ancient life 



Permian Period 280 



Pennsylvanian Period 310 



Mississippian Period 345 



Devonian Period 405 



Silurian Period 425 



Ordovician Period 500 



Cambrian Period 600 



Includes the Ice Age 



Dominance of flowering plants; 

 diversification of mammals 



Dominance of cycads, tree ferns, and 

 conifers; era of dinosaurs; origin 

 of mammals 



Evolution of modern insect orders 



The Carboniferous; first great tropical 



forests 

 Beginnings of land animals 



Predominance of marine invertebrates; 

 rise of land plants 



Geologic timetable. Names in the left column designate the various time periods for which there 

 ore relatively satisfactory fossil records. Each number in the middle column refers to the number 

 of millions of years ago that a particular time period began. Comments in the right column 

 indicate some of the notable events that occurred during the last 600 million years. 



geologists have divided these 600 million years into a series of eras, 

 periods, and epochs, which are listed in the accompanying table and 

 are the principal time periods referred to in this account. 



LANDSCAPES OF THE PAST 



During the first half of the Paleozoic Era, the central and eastern 

 parts of North America were covered by shallow continental seas 

 (called epeiric seas), fig. 9. Many of the animals living in these seas 

 had shells of calcium carbonate, and some of the very simple plants, 

 especially algae, contained calcium carbonate. When these organisms 

 died, their calcareous parts accumulated on the bottom of the seas, 

 became compacted and rocklike, and now form the limestone beds 

 that are quarried at Thornton, Joliet, and many other localities in 

 Illinois. 



Much later, during the Pennsylvanian period, most of the area 

 that is now Illinois and a large area to the south and east, fig. 9, were 

 covered by a luxuriant forest. Preserved remains of this forest form 



