THE STORY OF OHIO'S ROCKS 



11 



Some igneous rocks contain radioactive minerals and these are important because of 

 their peculiar properties. They decay slowly but at a uniform rate into other materials. For 

 example, uranium changes slowly into lead and helium. The rate of this transformation has 

 been measured by the physicists and chemists and it was found to be unaffected by heat, pres- 

 sure, or the presence of chemicals. In undisturbed rocks, where neither the original material nor 

 the end products (except helium) have escaped, it is possible to calculate the original amount of 

 uranium present and the amount of lead and helium it has produced through the ages. With 

 these data, it is possible to calculate the amount of time required for the observed transforma- 

 tion. The ages in years given in the geologic time-table (fig. 4) are based on these calculations. 



For more recent fossils, those found in the glacial deposits in Ohio, for example, another 

 method can yield ages in years. Living animals and plants absorb carbon and excrete it. But 

 the physicists have found that there are at least two kinds of carbon atoms, one, the "ordinary" 

 kind, has an atomic weight of 12; the other, radioactive carbon, has an atomic weight of 14. 

 The proportions of carbon 12 and carbon 14 in a living animal or plant remain constant as long 

 as they are living. When they die, no more carbon is taken in and the carbon 14 gradually 

 decreases in amount as it changes to carbon 12. The rate of change appears to be constant so 

 that again, knowing the usual amount of carbon 14 in a living animal and its rate of change, it 

 is possible to calculate how long it has been since the animal or plant died. This, in turn, gives 

 us an age for the sediments in which the animal or plant is buried. The method is useful only for 

 comparatively recent materials like those of the Pleistocene. With these facts in mind, we can 

 reconstruct the history of Ohio from the earliest times to the present. 





OHIO OIVISCN OF GEOLOGICAL SUSvEY 



Fig. 8 



