12 



NEW YORK STATE MUSEUM 



ejects rock fragments, dust etc., or more quietly pours out molten 

 rock, while in many cases great masses of molten rock have been 

 forced upward into the earth's crust without reaching the surface 

 and hence have cooled at greater or lesser depths below the surface. 

 Such deep-seated (plutonic) igneous rocks have become exposed to 

 view only by subsequent profound erosion of the region. 



Still another process which has been very influential in the pro- 

 duction of certain important physical features of the Adirondacks 

 is glaciation. During the great Ice Age, comparatively recently, 

 a vast sheet of ice slowly moved across the whole region and then 

 gradually melted away. Its principal work was erosion and depo- 

 sition of rock materials, the latter having taken place chiefly 

 during the ice retreat. Most of the soil and rotten rock and some 

 fresh rock were scraped off by the mighty ice plow, and the 

 irregular deposition of the glacial debris brought into existence 

 many minor topographic features. The Adirondack lakes are prac- 

 tically all due to the glaciation. 



Geologic time scale 



LENGTH IN 

 YEARS 



ERA PERIOD 



1 



DOMINANT LIFE 



3 to s million. . ' 



Cenozoic I 



Mesozoic ■ 



Paleozoic ■ 



Proterozoic . . . ■ 

 Archeozoic. . . . '. 



Quaternary 



Tertiary 



Cretaceous 



Age of man 



Age of mammals 



Age of reptiles; first birds in the Jurassic; 



15 to 25 million. 



Triassic 



Permian ......... 



Pennsylvanian . . . 

 Mississippian .... 



Devonian 



Silurian . , 



Ordovician 



Cambrian 



Keweenawan 



Animikean 



Huronian 



Al^oman 



Sudburian 



jLaurentian 



1 Grenville 



Age of amphibians and cycad plants 

 1 Great coal age. with large, simple non- 



> flowering pi nts 



Age of fishes 



> Age of invertebrate animals 



■ Simple forms of life; records imperfect 



> Beginnings of life 







In order to understand the physical history of the Adirondacks, 

 it is necessary to know that the history of the earth has been 

 divided into great eras and lesser periods and epochs, and that these 

 constitute what is called the geologic time scale. The epoch names 

 for eastern North America are omitted from the above table. So 

 far as they are recorded, the principal events in the history of the 

 Adirondacks will be taken up in the regular order of geologic time 

 in the following pages. 



